  
  [1X3 [33X[0;0YLists and Records[133X[101X
  
  [33X[0;0YModern  mathematics,  especially  algebra, is based on set theory. When sets
  are  represented  in  a computer, they inadvertently turn into lists. That's
  why  we  start  our  survey  of  the  various  objects [5XGAP[105X can handle with a
  description  of  lists and their manipulation. [5XGAP[105X regards sets as a special
  kind of lists, namely as lists without holes or duplicates whose entries are
  ordered with respect to the precedence relation [10X<[110X.[133X
  
  [33X[0;0YAfter  the  introduction  of the basic manipulations with lists in [14X3.1[114X, some
  difficulties  concerning  identity  and  mutability  of  lists are discussed
  in [14X3.2[114X  and [14X3.3[114X.  Sets,  ranges, row vectors, and matrices are introduced as
  special  kinds  of  lists  in [14X3.4[114X, [14X3.5[114X, [14X3.8[114X. Handy list operations are shown
  in [14X3.7[114X. Finally we explain how to use records in [14X3.9[114X.[133X
  
  
  [1X3.1 [33X[0;0YPlain Lists[133X[101X
  
  [33X[0;0YA  [13Xlist[113X  is  a  collection  of  objects  separated by commas and enclosed in
  brackets.  Let  us  for  example  construct the list [10Xprimes[110X of the first ten
  prime numbers.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xprimes:= [2, 3, 5, 7, 11, 13, 17, 19, 23, 29];[127X[104X
    [4X[28X[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YThe next two primes are 31 and 37. They may be appended to the existing list
  by  the  function  [10XAppend[110X  which  takes  the  existing list as its first and
  another  list  as  a second argument. The second argument is appended to the
  list  [10Xprimes[110X  and  no value is returned. Note that by appending another list
  the object [10Xprimes[110X is changed.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XAppend(primes, [31, 37]);[127X[104X
    [4X[25Xgap>[125X [27Xprimes;[127X[104X
    [4X[28X[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YYou  can  as  well add single new elements to existing lists by the function
  [10XAdd[110X which takes the existing list as its first argument and a new element as
  its  second  argument. The new element is added to the list [10Xprimes[110X and again
  no value is returned but the list [10Xprimes[110X is changed.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XAdd(primes, 41);[127X[104X
    [4X[25Xgap>[125X [27Xprimes;[127X[104X
    [4X[28X[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YSingle  elements of a list are referred to by their position in the list. To
  get  the  value  of the seventh prime, that is the seventh entry in our list
  [10Xprimes[110X, you simply type[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xprimes[7];[127X[104X
    [4X[28X17[128X[104X
  [4X[32X[104X
  
  [33X[0;0YThis  value can be handled like any other value, for example multiplied by 2
  or  assigned  to  a variable. On the other hand this mechanism allows one to
  assign a value to a position in a list. So the next prime 43 may be inserted
  in  the  list directly after the last occupied position of [10Xprimes[110X. This last
  occupied position is returned by the function [10XLength[110X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XLength(primes);[127X[104X
    [4X[28X13[128X[104X
    [4X[25Xgap>[125X [27Xprimes[14]:= 43;[127X[104X
    [4X[28X43[128X[104X
    [4X[25Xgap>[125X [27Xprimes;[127X[104X
    [4X[28X[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YNote  that  this  operation  again  has  changed the object [10Xprimes[110X. The next
  position  after the end of a list is not the only position capable of taking
  a  new  value. If you know that 71 is the 20th prime, you can enter it right
  now  in  the  20th position of [10Xprimes[110X. This will result in a list with holes
  which is however still a list and now has length 20.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xprimes[20]:= 71;[127X[104X
    [4X[28X71[128X[104X
    [4X[25Xgap>[125X [27Xprimes;[127X[104X
    [4X[28X[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43,,,,,, 71 ][128X[104X
    [4X[25Xgap>[125X [27XLength(primes);[127X[104X
    [4X[28X20[128X[104X
  [4X[32X[104X
  
  [33X[0;0YThe  list  itself  however  must  exist  before a value can be assigned to a
  position of the list. This list may be the empty list [10X[ ][110X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xlll[1]:= 2;[127X[104X
    [4X[28XError, Variable: 'lll' must have a value[128X[104X
    [4X[28X[128X[104X
    [4X[28X[128X[104X
    [4X[25Xgap>[125X [27Xlll:= []; lll[1]:= 2;[127X[104X
    [4X[28X[  ][128X[104X
    [4X[28X2[128X[104X
  [4X[32X[104X
  
  [33X[0;0YOf  course existing entries of a list can be changed by this mechanism, too.
  We  will  not  do  it  here  because  [10Xprimes[110X then may no longer be a list of
  primes. Try for yourself to change the 17 in the list into a 9.[133X
  
  [33X[0;0YTo  get  the  position  of  17  in the list [10Xprimes[110X use the function [2XPosition[102X
  ([14XReference:  Position[114X)  which  takes  the list as its first argument and the
  element  as  its  second  argument  and  returns  the  position of the first
  occurrence  of  the  element  17  in  the list [10Xprimes[110X. If the element is not
  contained  in  the  list then [2XPosition[102X ([14XReference: Position[114X) will return the
  special object [9Xfail[109X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XPosition(primes, 17);[127X[104X
    [4X[28X7[128X[104X
    [4X[25Xgap>[125X [27XPosition(primes, 20);[127X[104X
    [4X[28Xfail[128X[104X
  [4X[32X[104X
  
  [33X[0;0YIn  all  of  the  above  changes  to  the  list  [10Xprimes[110X,  the  list has been
  automatically resized. There is no need for you to tell [5XGAP[105X how big you want
  a list to be. This is all done dynamically.[133X
  
  [33X[0;0YIt  is  not  necessary for the objects collected in a list to be of the same
  type.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xlll:= [true, "This is a String",,, 3];[127X[104X
    [4X[28X[ true, "This is a String",,, 3 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YIn the same way a list may be part of another list.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xlll[3]:= [4,5,6];; lll;[127X[104X
    [4X[28X[ true, "This is a String", [ 4, 5, 6 ],, 3 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YA list may even be part of itself.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xlll[4]:= lll;[127X[104X
    [4X[28X[ true, "This is a String", [ 4, 5, 6 ], ~, 3 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YNow  the  tilde  in  the  fourth  position of [10Xlll[110X denotes the object that is
  currently  printed. Note that the result of the last operation is the actual
  value of the object [10Xlll[110X on the right hand side of the assignment. In fact it
  is identical to the value of the whole list [10Xlll[110X on the left hand side of the
  assignment.[133X
  
  [33X[0;0YA [13Xstring[113X is a special type of list, namely a dense list of [13Xcharacters[113X, where
  [13Xdense[113X  means  that  the  list has no holes. Here, [13Xcharacters[113X are special [5XGAP[105X
  objects  representing  an  element  of  the  character  set of the operating
  system.  The  input  of  printable characters is by enclosing them in single
  quotes [10X'[110X.  A  string literal can either be entered as the list of characters
  or  by  writing  the  characters between doublequotes [10X"[110X. Strings are handled
  specially by [2XPrint[102X ([14XReference: Print[114X). You can learn much more about strings
  in the reference manual.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xs1 := ['H','a','l','l','o',' ','w','o','r','l','d','.'];[127X[104X
    [4X[28X"Hallo world."[128X[104X
    [4X[25Xgap>[125X [27Xs1 = "Hallo world.";[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xs1[7];[127X[104X
    [4X[28X'w'[128X[104X
  [4X[32X[104X
  
  [33X[0;0YSublists  of  lists  can easily be extracted and assigned using the operator
  [10X[3Xlist[103X[10X{ [3Xpositions[103X[10X }[110X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xsl := lll{ [ 1, 2, 3 ] };[127X[104X
    [4X[28X[ true, "This is a String", [ 4, 5, 6 ] ][128X[104X
    [4X[25Xgap>[125X [27Xsl{ [ 2, 3 ] } := [ "New String", false ];[127X[104X
    [4X[28X[ "New String", false ][128X[104X
    [4X[25Xgap>[125X [27Xsl;[127X[104X
    [4X[28X[ true, "New String", false ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YThis way you get a new list whose [22Xi[122X-th entry is that element of the original
  list whose position is the [22Xi[122X-th entry of the argument in the curly braces.[133X
  
  
  [1X3.2 [33X[0;0YIdentical Lists[133X[101X
  
  [33X[0;0YThis  second  section  about  lists  is  dedicated  to the subtle difference
  between [13Xequality[113X and [13Xidentity[113X of lists. It is really important to understand
  this  difference  in  order  to  understand  how complex data structures are
  realized  in  [5XGAP[105X.  This  section  applies  to  all  [5XGAP[105X  objects  that have
  subobjects,  e.g.,  to  lists  and to records. After reading the section [14X3.9[114X
  about  records  you  should return to this section and translate it into the
  record context.[133X
  
  [33X[0;0YTwo  lists  are  [13Xequal[113X  if  all their entries are equal. This means that the
  equality operator [10X=[110X returns [9Xtrue[109X for the comparison of two lists if and only
  if  these  two lists are of the same length and for each position the values
  in the respective lists are equal.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xnumbers := primes;; numbers = primes;[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  [33X[0;0YWe  assigned the list [10Xprimes[110X to the variable [10Xnumbers[110X and, of course they are
  equal  as  they  have both the same length and the same entries. Now we will
  change the third number to 4 and compare the result again with [10Xprimes[110X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xnumbers[3]:= 4;; numbers = primes;[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  [33X[0;0YYou  see that [10Xnumbers[110X and [10Xprimes[110X are still equal, check this by printing the
  value  of  [10Xprimes[110X.  The  list [10Xprimes[110X is no longer a list of primes! What has
  happened?  The truth is that the lists [10Xprimes[110X and [10Xnumbers[110X are not only equal
  but  they  are also [13Xidentical[113X. [10Xprimes[110X and [10Xnumbers[110X are two variables pointing
  to  the  same  list.  If you change the value of the subobject [10Xnumbers[3][110X of
  [10Xnumbers[110X  this  will  also change [10Xprimes[110X. Variables do [13Xnot[113X point to a certain
  block of storage memory but they do point to an object that occupies storage
  memory.  So  the assignment [10Xnumbers := primes[110X did [13Xnot[113X create a new list in a
  different place of memory but only created the new name [10Xnumbers[110X for the same
  old list of primes.[133X
  
  [33X[0;0YFrom this we see that [13Xthe same object can have several names.[113X[133X
  
  [33X[0;0YIf  you want to change a list with the contents of [10Xprimes[110X independently from
  [10Xprimes[110X  you  will  have to make a copy of [10Xprimes[110X by the function [10XShallowCopy[110X
  which  takes  an  object as its argument and returns a copy of the argument.
  (We will first restore the old value of [10Xprimes[110X.)[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xprimes[3]:= 5;; primes;[127X[104X
    [4X[28X[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43,,,,,, 71 ][128X[104X
    [4X[25Xgap>[125X [27Xnumbers:= ShallowCopy(primes);; numbers = primes;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xnumbers[3]:= 4;; numbers = primes;[127X[104X
    [4X[28Xfalse[128X[104X
  [4X[32X[104X
  
  [33X[0;0YNow  [10Xnumbers[110X  is  no  longer  equal  to [10Xprimes[110X and [10Xprimes[110X still is a list of
  primes. Check this by printing the values of [10Xnumbers[110X and [10Xprimes[110X.[133X
  
  [33X[0;0YLists and records can be changed this way because [5XGAP[105X objects of these types
  have   subobjects.   To   clarify  this  statement  consider  the  following
  assignments.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xi:= 1;; j:= i;; i:= i+1;; [127X[104X
  [4X[32X[104X
  
  [33X[0;0YBy  adding  1  to [10Xi[110X the value of [10Xi[110X has changed. What happens to [10Xj[110X? After the
  second  statement [10Xj[110X points to the same object as [10Xi[110X, namely to the integer 1.
  The addition does [13Xnot[113X change the object [10X1[110X but creates a new object according
  to the instruction [10Xi+1[110X. It is actually the assignment that changes the value
  of  [10Xi[110X. Therefore [10Xj[110X still points to the object [10X1[110X. Integers (like permutations
  and  booleans)  have no subobjects. Objects of these types cannot be changed
  but can only be replaced by other objects. And a replacement does not change
  the  values  of other variables. In the above example an assignment of a new
  value to the variable [10Xnumbers[110X would also not change the value of [10Xprimes[110X.[133X
  
  [33X[0;0YFinally try the following examples and explain the results.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xl:= [];; l:= [l];[127X[104X
    [4X[28X[ [  ] ][128X[104X
    [4X[25Xgap>[125X [27Xl[1]:= l;[127X[104X
    [4X[28X[ ~ ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YNow return to Section [14X3.1[114X and find out whether the functions [2XAdd[102X ([14XReference:
  Add[114X) and [2XAppend[102X ([14XReference: Append[114X) change their arguments.[133X
  
  
  [1X3.3 [33X[0;0YImmutability[133X[101X
  
  [33X[0;0Y[5XGAP[105X  has  a mechanism that protects lists against changes like the ones that
  have   bothered  us  in  Section [14X3.2[114X.  The  function  [2XImmutable[102X  ([14XReference:
  Immutable[114X)  takes  as  argument  a list and returns an immutable copy of it,
  i.e.,  a  list  which  looks  exactly  like  the  old one, but has two extra
  properties:  (1) The  new  list  is immutable, i.e., the list itself and its
  subobjects  cannot  be  changed. (2) In constructing the copy, every part of
  the  list  that  can  be changed has been copied, so that changes to the old
  list  will  not  affect  the  new  one.  In other words, the new list has no
  mutable subobjects in common with the old list.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xlist := [ 1, 2, "three", [ 4 ] ];; copy := Immutable( list );;[127X[104X
    [4X[25Xgap>[125X [27Xlist[3][5] := 'w';; list; copy;[127X[104X
    [4X[28X[ 1, 2, "threw", [ 4 ] ][128X[104X
    [4X[28X[ 1, 2, "three", [ 4 ] ][128X[104X
    [4X[25Xgap>[125X [27Xcopy[3][5] := 'w';[127X[104X
    [4X[28XList Assignment: <list> must be a mutable list[128X[104X
    [4X[28Xnot in any function[128X[104X
    [4X[28XEntering break read-eval-print loop ...[128X[104X
    [4X[28Xyou can 'quit;' to quit to outer loop, or[128X[104X
    [4X[28Xyou can 'return;' and ignore the assignment to continue[128X[104X
    [4X[26Xbrk>[126X [27Xquit;[127X[104X
  [4X[32X[104X
  
  [33X[0;0YAs  a  consequence  of  these  rules,  in the immutable copy of a list which
  contains  an  already  immutable list as subobject, this immutable subobject
  need  not  be copied, because it is unchangeable. Immutable lists are useful
  in  many  complex  [5XGAP[105X objects, for example as generator lists of groups. By
  making  them  immutable,  [5XGAP[105X ensures that no generators can be added to the
  list,  removed  or  exchanged.  Such changes would of course lead to serious
  inconsistencies  with  other knowledge that may already have been calculated
  for the group.[133X
  
  [33X[0;0YA  converse  function  to  [2XImmutable[102X  ([14XReference:  Immutable[114X) is [2XShallowCopy[102X
  ([14XReference: ShallowCopy[114X), which produces a new mutable list whose [22Xi[122X-th entry
  is  the  [22Xi[122X-th entry of the old list. The single entries are not copied, they
  are just placed in the new list. If the old list is immutable, and hence the
  list entries are immutable themselves, the result of [2XShallowCopy[102X ([14XReference:
  ShallowCopy[114X) is mutable only on the top level.[133X
  
  [33X[0;0YIt  should be noted that also other objects than lists can appear in mutable
  or immutable form. Records (see Section [14X3.9[114X) provide another example.[133X
  
  
  [1X3.4 [33X[0;0YSets[133X[101X
  
  [33X[0;0Y[5XGAP[105X  knows  several  special  kinds  of  lists.  A [13Xset[113X in [5XGAP[105X is a list that
  contains  no  holes  (such  a  list  is called [13Xdense[113X) and whose elements are
  strictly  sorted  w.r.t. [10X<[110X;  in particular, a set cannot contain duplicates.
  (More  precisely,  the  elements  of a set in [5XGAP[105X are required to lie in the
  same  [13Xfamily[113X,  but  roughly this means that they can be compared using the [10X<[110X
  operator.)[133X
  
  [33X[0;0YThis provides a natural model for mathematical sets whose elements are given
  by an explicit enumeration.[133X
  
  [33X[0;0Y[5XGAP[105X  also calls a set a [13Xstrictly sorted list[113X, and the function [2XIsSSortedList[102X
  ([14XReference: IsSSortedList[114X) tests whether a given list is a set. It returns a
  boolean  value. For almost any list whose elements are contained in the same
  family,  there  exists  a  corresponding set. This set is constructed by the
  function  [2XSet[102X  ([14XReference:  Set[114X)  which  takes  the list as its argument and
  returns  a  set obtained from this list by ignoring holes and duplicates and
  by sorting the elements.[133X
  
  [33X[0;0YThe elements of the sets used in the examples of this section are strings.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xfruits:= ["apple", "strawberry", "cherry", "plum"];[127X[104X
    [4X[28X[ "apple", "strawberry", "cherry", "plum" ][128X[104X
    [4X[25Xgap>[125X [27XIsSSortedList(fruits);[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27Xfruits:= Set(fruits);[127X[104X
    [4X[28X[ "apple", "cherry", "plum", "strawberry" ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YNote  that  the  original  list  [10Xfruits[110X  is  not changed by the function [2XSet[102X
  ([14XReference: Set[114X). We have to make a new assignment to the variable [10Xfruits[110X in
  order to make it a set.[133X
  
  [33X[0;0YThe operator [9Xin[109X is used to test whether an object is an element of a set. It
  returns a boolean value [9Xtrue[109X or [9Xfalse[109X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27X"apple" in fruits;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27X"banana" in fruits;[127X[104X
    [4X[28Xfalse[128X[104X
  [4X[32X[104X
  
  [33X[0;0YThe  operator  [9Xin[109X  can also be applied to ordinary lists. It is however much
  faster  to  perform  a membership test for sets since sets are always sorted
  and a binary search can be used instead of a linear search. New elements may
  be added to a set by the function [2XAddSet[102X ([14XReference: AddSet[114X) which takes the
  set  [10Xfruits[110X  as its first argument and an element as its second argument and
  adds the element to the set if it wasn't already there. Note that the object
  [10Xfruits[110X is changed.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XAddSet(fruits, "banana");[127X[104X
    [4X[25Xgap>[125X [27Xfruits;  #  The banana is inserted in the right place.[127X[104X
    [4X[28X[ "apple", "banana", "cherry", "plum", "strawberry" ][128X[104X
    [4X[25Xgap>[125X [27XAddSet(fruits, "apple");[127X[104X
    [4X[25Xgap>[125X [27Xfruits;  #  fruits has not changed.[127X[104X
    [4X[28X[ "apple", "banana", "cherry", "plum", "strawberry" ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YNote  that inserting new elements into a set with [2XAddSet[102X ([14XReference: AddSet[114X)
  is  usually  more  expensive than simply adding new elements at the end of a
  list.[133X
  
  [33X[0;0YSets   can   be   intersected   by  the  function  [2XIntersection[102X  ([14XReference:
  Intersection[114X) and united by the function [2XUnion[102X ([14XReference: Union[114X) which both
  take  two sets as their arguments and return the intersection resp. union of
  the two sets as a new object.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xbreakfast:= ["tea", "apple", "egg"];[127X[104X
    [4X[28X[ "tea", "apple", "egg" ][128X[104X
    [4X[25Xgap>[125X [27XIntersection(breakfast, fruits);[127X[104X
    [4X[28X[ "apple" ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YThe  arguments  of  the functions [2XIntersection[102X ([14XReference: Intersection[114X) and
  [2XUnion[102X  ([14XReference:  Union[114X)  could  be  ordinary lists, while their result is
  always  a  set.  Note that in the preceding example at least one argument of
  [2XIntersection[102X   ([14XReference:  Intersection[114X)  was  not  a  set.  The  functions
  [2XIntersectSet[102X  ([14XReference:  IntersectSet[114X)  and [2XUniteSet[102X ([14XReference: UniteSet[114X)
  also  form  the  intersection resp. union of two sets. They will however not
  return the result but change their first argument to be the result. Try them
  carefully.[133X
  
  
  [1X3.5 [33X[0;0YRanges[133X[101X
  
  [33X[0;0YA  [13Xrange[113X  is  a  finite  arithmetic progression of integers. This is another
  special  kind  of list. A range is described by the first two values and the
  last  value  of  the  arithmetic  progression  which  are  given in the form
  [10X[[3Xfirst[103X[10X,[3Xsecond[103X[10X..[3Xlast[103X[10X][110X.  In the usual case of an ascending list of consecutive
  integers the second entry may be omitted.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27X[1..999999];     #  a range of almost a million numbers[127X[104X
    [4X[28X[ 1 .. 999999 ][128X[104X
    [4X[25Xgap>[125X [27X[1, 2..999999];  #  this is equivalent[127X[104X
    [4X[28X[ 1 .. 999999 ][128X[104X
    [4X[25Xgap>[125X [27X[1, 3..999999];  #  here the step is 2[127X[104X
    [4X[28X[ 1, 3 .. 999999 ][128X[104X
    [4X[25Xgap>[125X [27XLength( last );[127X[104X
    [4X[28X500000[128X[104X
    [4X[25Xgap>[125X [27X[ 999999, 999997 .. 1 ];[127X[104X
    [4X[28X[ 999999, 999997 .. 1 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YThis  compact  printed representation of a fairly long list corresponds to a
  compact  internal  representation. The function [2XIsRange[102X ([14XReference: IsRange[114X)
  tests   whether  an  object  is  a  range,  the  function  [2XConvertToRangeRep[102X
  ([14XReference:  ConvertToRangeRep[114X) changes the representation of a list that is
  in fact a range to this compact internal representation.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xa:= [-2,-1,0,1,2,3,4,5];[127X[104X
    [4X[28X[ -2, -1, 0, 1, 2, 3, 4, 5 ][128X[104X
    [4X[25Xgap>[125X [27XIsRange( a );[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XConvertToRangeRep( a );;  a;[127X[104X
    [4X[28X[ -2 .. 5 ][128X[104X
    [4X[25Xgap>[125X [27Xa[1]:= 0;; IsRange( a );[127X[104X
    [4X[28Xfalse[128X[104X
  [4X[32X[104X
  
  [33X[0;0YNote  that  this  change  of representation does [13Xnot[113X change the value of the
  list  [10Xa[110X. The list [10Xa[110X still behaves in any context in the same way as it would
  have in the long representation.[133X
  
  
  [1X3.6 [33X[0;0YFor and While Loops[133X[101X
  
  [33X[0;0YGiven  a list [10Xpp[110X of permutations we can form their product by means of a [9Xfor[109X
  loop instead of writing down the product explicitly.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xpp:= [ (1,3,2,6,8)(4,5,9), (1,6)(2,7,8), (1,5,7)(2,3,8,6),[127X[104X
    [4X[25X>[125X [27X          (1,8,9)(2,3,5,6,4), (1,9,8,6,3,4,7,2)];;[127X[104X
    [4X[25Xgap>[125X [27Xprod:= ();        [127X[104X
    [4X[28X()[128X[104X
    [4X[25Xgap>[125X [27Xfor p in pp do[127X[104X
    [4X[25X>[125X [27X      prod:= prod*p;    [127X[104X
    [4X[25X>[125X [27X   od;[127X[104X
    [4X[25Xgap>[125X [27Xprod;        [127X[104X
    [4X[28X(1,8,4,2,3,6,5,9)[128X[104X
  [4X[32X[104X
  
  [33X[0;0YFirst  a  new  variable  [10Xprod[110X is initialized to the identity permutation [10X()[110X.
  Then  the loop variable [10Xp[110X takes as its value one permutation after the other
  from  the list [10Xpp[110X and is multiplied with the present value of [10Xprod[110X resulting
  in a new value which is then assigned to [10Xprod[110X.[133X
  
  [33X[0;0YThe [9Xfor[109X loop has the following syntax[133X
  
  [33X[0;0Y[9Xfor[109X [3Xvar[103X [9Xin[109X [3Xlist[103X [9Xdo[109X [3Xstatements[103X [9Xod[109X[10X;[110X[133X
  
  [33X[0;0YThe effect of the [9Xfor[109X loop is to execute the [3Xstatements[103X for every element of
  the [3Xlist[103X. A [9Xfor[109X loop is a statement and therefore terminated by a semicolon.
  The list of [3Xstatements[103X is enclosed by the keywords [9Xdo[109X and [9Xod[109X (reverse [9Xdo[109X). A
  [9Xfor[109X  loop returns no value. Therefore we had to ask explicitly for the value
  of [10Xprod[110X in the preceding example.[133X
  
  [33X[0;0YThe [9Xfor[109X loop can loop over any kind of list, even a list with holes. In many
  programming languages the [9Xfor[109X loop has the form[133X
  
  [33X[0;0Y[10Xfor [3Xvar[103X[10X from [3Xfirst[103X[10X to [3Xlast[103X[10X do [3Xstatements[103X[10X od;[110X[133X
  
  [33X[0;0YIn  [5XGAP[105X  this  is  merely  a special case of the general [9Xfor[109X loop as defined
  above where the [3Xlist[103X in the loop body is a range (see [14X3.5[114X):[133X
  
  [33X[0;0Y[9Xfor[109X [3Xvar[103X [9Xin[109X [10X[[3Xfirst[103X[10X..[3Xlast[103X[10X][110X [9Xdo[109X [3Xstatements[103X [9Xod[109X[10X;[110X[133X
  
  [33X[0;0YYou  can  for instance loop over a range to compute the factorial [22X15![122X of the
  number [22X15[122X in the following way.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xff:= 1;[127X[104X
    [4X[28X1[128X[104X
    [4X[25Xgap>[125X [27Xfor i in [1..15] do[127X[104X
    [4X[25X>[125X [27X      ff:= ff * i;[127X[104X
    [4X[25X>[125X [27X   od;[127X[104X
    [4X[25Xgap>[125X [27Xff;[127X[104X
    [4X[28X1307674368000[128X[104X
  [4X[32X[104X
  
  [33X[0;0YThe [9Xwhile[109X loop has the following syntax[133X
  
  [33X[0;0Y[9Xwhile[109X [3Xcondition[103X [9Xdo[109X [3Xstatements[103X [9Xod[109X[10X;[110X[133X
  
  [33X[0;0YThe  [9Xwhile[109X loop loops over the [3Xstatements[103X as long as the [3Xcondition[103X evaluates
  to  [9Xtrue[109X.  Like  the [9Xfor[109X loop the [9Xwhile[109X loop is terminated by the keyword [9Xod[109X
  followed by a semicolon.[133X
  
  [33X[0;0YWe  can use our list [10Xprimes[110X to perform a very simple factorization. We begin
  by  initializing  a  list [10Xfactors[110X to the empty list. In this list we want to
  collect  the  prime  factors of the number 1333. Remember that a list has to
  exist  before  any  values can be assigned to positions of the list. Then we
  will  loop  over  the list [10Xprimes[110X and test for each prime whether it divides
  the  number.  If  it does we will divide the number by that prime, add it to
  the list [10Xfactors[110X and continue.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xn:= 1333;;[127X[104X
    [4X[25Xgap>[125X [27Xfactors:= [];;[127X[104X
    [4X[25Xgap>[125X [27Xfor p in primes do[127X[104X
    [4X[25X>[125X [27X      while n mod p = 0 do[127X[104X
    [4X[25X>[125X [27X         n:= n/p;[127X[104X
    [4X[25X>[125X [27X         Add(factors, p);[127X[104X
    [4X[25X>[125X [27X      od;[127X[104X
    [4X[25X>[125X [27X   od;[127X[104X
    [4X[25Xgap>[125X [27Xfactors;[127X[104X
    [4X[28X[ 31, 43 ][128X[104X
    [4X[25Xgap>[125X [27Xn;[127X[104X
    [4X[28X1[128X[104X
  [4X[32X[104X
  
  [33X[0;0YAs  [10Xn[110X  now  has  the  value  1 all prime factors of 1333 have been found and
  [10Xfactors[110X  contains  a  complete  factorization of 1333. This can of course be
  verified by multiplying 31 and 43.[133X
  
  [33X[0;0YThis  loop  may  be  applied  to  arbitrary  numbers  in order to find prime
  factors.  But  as  [10Xprimes[110X is not a complete list of all primes this loop may
  fail  to  find all prime factors of a number greater than 2000, say. You can
  try to improve it in such a way that new primes are added to the list [10Xprimes[110X
  if needed.[133X
  
  [33X[0;0YYou  have already seen that list objects may be changed. This of course also
  holds  for the list in a loop body. In most cases you have to be careful not
  to  change  this  list, but there are situations where this is quite useful.
  The following example shows a quick way to determine the primes smaller than
  1000  by  a  sieve  method.  Here we will make use of the function [10XUnbind[110X to
  delete entries from a list, and the [10Xif[110X statement covered in [14X4.2[114X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xprimes:= [];;[127X[104X
    [4X[25Xgap>[125X [27Xnumbers:= [2..1000];;[127X[104X
    [4X[25Xgap>[125X [27Xfor p in numbers do[127X[104X
    [4X[25X>[125X [27X      Add(primes, p);[127X[104X
    [4X[25X>[125X [27X      for n in numbers do[127X[104X
    [4X[25X>[125X [27X         if n mod p = 0 then[127X[104X
    [4X[25X>[125X [27X            Unbind(numbers[n-1]);[127X[104X
    [4X[25X>[125X [27X         fi;[127X[104X
    [4X[25X>[125X [27X      od;[127X[104X
    [4X[25X>[125X [27X   od;[127X[104X
  [4X[32X[104X
  
  [33X[0;0YThe  inner  loop  removes all entries from [10Xnumbers[110X that are divisible by the
  last detected prime [10Xp[110X. This is done by the function [10XUnbind[110X which deletes the
  binding  of the list position [10Xnumbers[n-1][110X to the value [10Xn[110X so that afterwards
  [10Xnumbers[n-1][110X  no  longer has an assigned value. The next element encountered
  in [10Xnumbers[110X by the outer loop necessarily is the next prime.[133X
  
  [33X[0;0YIn  a  similar  way it is possible to enlarge the list which is looped over.
  This  yields a nice and short orbit algorithm for the action of a group, for
  example.[133X
  
  [33X[0;0YMore  about  [9Xfor[109X  and  [9Xwhile[109X  loops can be found in the sections [14X'Reference:
  While'[114X and [14X'Reference: For'[114X.[133X
  
  
  [1X3.7 [33X[0;0YList Operations[133X[101X
  
  [33X[0;0YThere  is  a more comfortable way than that given in the previous section to
  compute the product of a list of numbers or permutations.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XProduct([1..15]);[127X[104X
    [4X[28X1307674368000[128X[104X
    [4X[25Xgap>[125X [27XProduct(pp);[127X[104X
    [4X[28X(1,8,4,2,3,6,5,9)[128X[104X
  [4X[32X[104X
  
  [33X[0;0YThe  function  [2XProduct[102X ([14XReference: Product[114X) takes a list as its argument and
  computes  the product of the elements of the list. This is possible whenever
  a  multiplication  of  the  elements  of  the  list  is  defined. So [2XProduct[102X
  ([14XReference: Product[114X) executes a loop over all elements of the list.[133X
  
  [33X[0;0YThere  are  other  often  used  loops available as functions. Guess what the
  function  [2XSum[102X  ([14XReference:  Sum[114X)  does.  The  function [2XList[102X ([14XReference: list
  element  operation[114X) may take a list and a function as its arguments. It will
  then  apply  the  function  to  each  element  of  the  list  and return the
  corresponding  list  of results. A list of cubes is produced as follows with
  the function [10Xcubed[110X from Section [14X4[114X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xcubed:= x -> x^3;;[127X[104X
    [4X[25Xgap>[125X [27XList([2..10], cubed);[127X[104X
    [4X[28X[ 8, 27, 64, 125, 216, 343, 512, 729, 1000 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YTo  add  all these cubes we might apply the function [2XSum[102X ([14XReference: Sum[114X) to
  the last list. But we may as well give the function [10Xcubed[110X to [2XSum[102X ([14XReference:
  Sum[114X) as an additional argument.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XSum(last) = Sum([2..10], cubed);[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  [33X[0;0YThe  primes  less  than  30  can  be  retrieved  out of the list [10Xprimes[110X from
  Section [14X3.1[114X  by  the  function [2XFiltered[102X ([14XReference: Filtered[114X). This function
  takes  the  list  [10Xprimes[110X and a property as its arguments and will return the
  list  of  those elements of [10Xprimes[110X which have this property. Such a property
  will  be  represented  by  a  function that returns a boolean value. In this
  example  the  property  of  being  less  than  30  can be represented by the
  function  [10Xx  ->  x < 30[110X since [10Xx < 30[110X will evaluate to [9Xtrue[109X for values [10Xx[110X less
  than 30 and to [9Xfalse[109X otherwise.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XFiltered(primes, x -> x < 30);[127X[104X
    [4X[28X[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YWe  have  already mentioned the operator [10X{ }[110X that forms sublists. It takes a
  list  of positions as its argument and will return the list of elements from
  the original list corresponding to these positions.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xprimes{ [1 .. 10] };[127X[104X
    [4X[28X[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YFinally  we  mention  the  function  [2XForAll[102X  ([14XReference: ForAll[114X) that checks
  whether  a  property  holds  for  all  elements  of  a list. It takes as its
  arguments  a  list  and  a  function  that  returns  a boolean value. [2XForAll[102X
  ([14XReference:  ForAll[114X)  checks  whether  the  function  returns  [9Xtrue[109X  for all
  elements of the list.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xlist:= [ 1, 2, 3, 4 ];;[127X[104X
    [4X[25Xgap>[125X [27XForAll( list, x -> x > 0 );[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XForAll( list, x -> x in primes );[127X[104X
    [4X[28Xfalse[128X[104X
  [4X[32X[104X
  
  [33X[0;0YYou will find more predefined [9Xfor[109X loops in chapter [14X'Reference: Lists'[114X.[133X
  
  
  [1X3.8 [33X[0;0YVectors and Matrices[133X[101X
  
  [33X[0;0YThis  section  describes  how  [5XGAP[105X  uses  lists to represent row vectors and
  matrices.  A  [13Xrow  vector[113X is a dense list of elements from a common field. A
  [13Xmatrix[113X  is  a  dense  list  of  row vectors over a common field and of equal
  length.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xv:= [3, 6, 2, 5/2];;  IsRowVector(v);[127X[104X
    [4X[28Xtrue[128X[104X
  [4X[32X[104X
  
  [33X[0;0YRow  vectors  may  be  added  and  multiplied  by  scalars from their field.
  Multiplication  of  row  vectors  of  equal  length  results in their scalar
  product.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27X2 * v;  v * 1/3;[127X[104X
    [4X[28X[ 6, 12, 4, 5 ][128X[104X
    [4X[28X[ 1, 2, 2/3, 5/6 ][128X[104X
    [4X[25Xgap>[125X [27Xv * v;   # the scalar product of `v' with itself[127X[104X
    [4X[28X221/4[128X[104X
  [4X[32X[104X
  
  [33X[0;0YNote  that the expression [10Xv * 1/3[110X is actually evaluated by first multiplying
  [10Xv[110X  by  1  (which  yields again [10Xv[110X) and by then dividing by 3. This is also an
  allowed scalar operation. The expression [10Xv/3[110X would result in the same value.[133X
  
  [33X[0;0YSuch  arithmetical  operations  (if the results are again vectors) result in
  [13Xmutable[113X  vectors  except  if  the  operation is binary and both operands are
  immutable; thus the vectors shown in the examples above are all mutable.[133X
  
  [33X[0;0YSo  if  you want to produce a mutable list with 100 entries equal to 25, you
  can  simply  say  [10X25  +  0 * [ 1 .. 100 ][110X. Note that ranges are also vectors
  (over the rationals), and that [10X[ 1 .. 100 ][110X is mutable.[133X
  
  [33X[0;0YA matrix is a dense list of row vectors of equal length.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xm:= [[1,-1, 1],[127X[104X
    [4X[25X>[125X [27X        [2, 0,-1],[127X[104X
    [4X[25X>[125X [27X        [1, 1, 1]];[127X[104X
    [4X[28X[ [ 1, -1, 1 ], [ 2, 0, -1 ], [ 1, 1, 1 ] ][128X[104X
    [4X[25Xgap>[125X [27Xm[2][1];[127X[104X
    [4X[28X2[128X[104X
  [4X[32X[104X
  
  [33X[0;0YSyntactically a matrix is a list of lists. So the number 2 in the second row
  and  the first column of the matrix [10Xm[110X is referred to as the first element of
  the second element of the list [10Xm[110X via [10Xm[2][1][110X.[133X
  
  [33X[0;0YA  matrix  may be multiplied by scalars, row vectors and other matrices. (If
  the  row  vectors and matrices involved in such a multiplication do not have
  suitable  dimensions  then  the [21Xmissing[121X entries are treated as zeros, so the
  results may look unexpectedly in such cases.)[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27X[1, 0, 0] * m;[127X[104X
    [4X[28X[ 1, -1, 1 ][128X[104X
    [4X[25Xgap>[125X [27X[1, 0, 0, 2] * m;[127X[104X
    [4X[28X[ 1, -1, 1 ][128X[104X
    [4X[25Xgap>[125X [27Xm * [1, 0, 0];[127X[104X
    [4X[28X[ 1, 2, 1 ][128X[104X
    [4X[25Xgap>[125X [27Xm * [1, 0, 0, 2];[127X[104X
    [4X[28X[ 1, 2, 1 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YNote  that  multiplication  of  a  row vector with a matrix will result in a
  linear  combination  of  the  rows  of the matrix, while multiplication of a
  matrix  with  a row vector results in a linear combination of the columns of
  the  matrix.  In  the  latter  case the row vector is considered as a column
  vector.[133X
  
  [33X[0;0YA  vector  or  matrix of integers can also be multiplied with a finite field
  scalar  and  vice  versa.  Such  products result in a matrix over the finite
  field  with  the  integers  mapped into the finite field in the obvious way.
  Finite  field matrices are nicer to read when they are [10XDisplay[110Xed rather than
  [10XPrint[110Xed.  (Here we write [10XZ(q)[110X to denote a primitive root of the finite field
  with [10Xq[110X elements.)[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XDisplay( m * One( GF(5) ) );[127X[104X
    [4X[28X 1 4 1[128X[104X
    [4X[28X 2 . 4[128X[104X
    [4X[28X 1 1 1[128X[104X
    [4X[25Xgap>[125X [27XDisplay( m^2 * Z(2) + m * Z(4) );[127X[104X
    [4X[28Xz = Z(4)[128X[104X
    [4X[28X z^1 z^1 z^2[128X[104X
    [4X[28X   1   1 z^2[128X[104X
    [4X[28X z^1 z^1 z^2[128X[104X
  [4X[32X[104X
  
  [33X[0;0YSubmatrices can easily be extracted using the expression [10X[3Xmat[103X[10X{[3Xrows[103X[10X}{[3Xcolumns[103X[10X}[110X.
  They  can  also be assigned to, provided the big matrix is mutable (which it
  is not if it is the result of an arithmetical operation, see above).[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xsm := m{ [ 1, 2 ] }{ [ 2, 3 ] };[127X[104X
    [4X[28X[ [ -1, 1 ], [ 0, -1 ] ][128X[104X
    [4X[25Xgap>[125X [27Xsm{ [ 1, 2 ] }{ [2] } := [[-2],[0]];;  sm;[127X[104X
    [4X[28X[ [ -1, -2 ], [ 0, 0 ] ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YThe  first  curly brackets contain the selection of rows, the second that of
  columns.[133X
  
  [33X[0;0YMatrices  appear  not  only  in  linear algebra, but also as group elements,
  provided  they  are  invertible.  Here  we  have  the  opportunity to meet a
  group-theoretical  function, namely [2XOrder[102X ([14XReference: Order[114X), which computes
  the order of a group element.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XOrder( m * One( GF(5) ) );[127X[104X
    [4X[28X8[128X[104X
    [4X[25Xgap>[125X [27XOrder( m );[127X[104X
    [4X[28Xinfinity[128X[104X
  [4X[32X[104X
  
  [33X[0;0YFor  matrices  whose  entries are more complex objects, for example rational
  functions, [5XGAP[105X's [2XOrder[102X ([14XReference: Order[114X) methods might not be able to prove
  that the matrix has infinite order, and one gets the following warning.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[28X#I  Order: warning, order of <mat> might be infinite[128X[104X
  [4X[32X[104X
  
  [33X[0;0YIn such a case, if the order of the matrix really is infinite, you will have
  to  interrupt [5XGAP[105X by pressing [10X[3Xctl[103X[10X-C[110X (followed by [10X[3Xctl[103X[10X-D[110X or [10Xquit;[110X to leave the
  break loop).[133X
  
  [33X[0;0YTo  prove that the order of [10Xm[110X is infinite, we also could look at the minimal
  polynomial of [10Xm[110X over the rationals.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xf:= MinimalPolynomial( Rationals, m );;  Factors( f );[127X[104X
    [4X[28X[ x_1-2, x_1^2+3 ][128X[104X
  [4X[32X[104X
  
  [33X[0;0Y[2XFactors[102X  ([14XReference:  Factors[114X)  returns a list of irreducible factors of the
  polynomial  [10Xf[110X.  The  first  irreducible  factor  [22XX-2[122X  reveals  that  2 is an
  eigenvalue of [10Xm[110X, hence its order cannot be finite.[133X
  
  
  [1X3.9 [33X[0;0YPlain Records[133X[101X
  
  [33X[0;0YA  record  provides  another way to build new data structures. Like a list a
  record  contains  subobjects. In a record the elements, the so-called [13Xrecord
  components[113X, are not indexed by numbers but by names.[133X
  
  [33X[0;0YIn  this  section you will see how to define and how to use records. Records
  are  changed  by  assignments  to  record  components or by unbinding record
  components.[133X
  
  [33X[0;0YInitially  a  record  is defined as a comma separated list of assignments to
  its record components.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xdate:= rec(year:= 1997,[127X[104X
    [4X[25X>[125X [27X              month:= "Jul",[127X[104X
    [4X[25X>[125X [27X              day:= 14);[127X[104X
    [4X[28Xrec( day := 14, month := "Jul", year := 1997 )[128X[104X
  [4X[32X[104X
  
  [33X[0;0YThe  value  of  a  record component is accessible by the record name and the
  record component name separated by one dot as the record component selector.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xdate.year;[127X[104X
    [4X[28X1997[128X[104X
  [4X[32X[104X
  
  [33X[0;0YAssignments  to  new  record  components  are  possible in the same way. The
  record is automatically resized to hold the new component.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xdate.time:= rec(hour:= 19, minute:= 23, second:= 12);[127X[104X
    [4X[28Xrec( hour := 19, minute := 23, second := 12 )[128X[104X
    [4X[25Xgap>[125X [27Xdate;[127X[104X
    [4X[28Xrec( day := 14, month := "Jul", [128X[104X
    [4X[28X  time := rec( hour := 19, minute := 23, second := 12 ), year := 1997 )[128X[104X
  [4X[32X[104X
  
  [33X[0;0YRecords are objects that may be changed. An assignment to a record component
  changes  the original object. The remarks made in Sections [14X3.2[114X and [14X3.3[114X about
  identity and mutability of lists are also true for records.[133X
  
  [33X[0;0YSometimes it is interesting to know which components of a certain record are
  bound.  This information is available from the function [2XRecNames[102X ([14XReference:
  RecNames[114X),  which takes a record as its argument and returns a list of names
  of all bound components of this record as a list of strings.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XRecNames(date);[127X[104X
    [4X[28X[ "time", "year", "month", "day" ][128X[104X
  [4X[32X[104X
  
  [33X[0;0YNow return to Sections [14X3.2[114X and [14X3.3[114X and find out what these sections mean for
  records.[133X
  
  
  [1X3.10 [33X[0;0YFurther Information about Lists[133X[101X
  
  [33X[0;0Y(The following cross-references point to the [5XGAP[105X Reference Manual.)[133X
  
  [33X[0;0YYou  will  find  more  about  lists, sets, and ranges in Chapter [14X'Reference:
  Lists'[114X,  in  particular  more  about  identical lists in Section [14X'Reference:
  Identical  Lists'[114X.  A  more  detailed description of strings is contained in
  Chapter [14X'Reference:   Strings  and  Characters'[114X.  Fields  are  described  in
  Chapter [14X'Reference: Fields and Division Rings'[114X, some known fields in [5XGAP[105X are
  described  in  Chapters [14X'Reference:  Rational  Numbers'[114X, [14X'Reference: Abelian
  Number Fields'[114X, and [14X'Reference: Finite Fields'[114X. Row vectors and matrices are
  described    in   more   detail   in   Chapters [14X'Reference:   Row   Vectors'[114X
  and [14X'Reference:  Matrices'[114X;  note  that [5XGAP[105X supports also linear algebra for
  objects  which  are  [13Xnot[113X  lists,  see  Chapter [14X'Reference: Vector and Matrix
  Objects'[114X. Vector spaces are described in Chapter [14X'Reference: Vector Spaces'[114X,
  further  matrix  related  structures  are  described in Chapters [14X'Reference:
  Matrix Groups'[114X, [14X'Reference: Algebras'[114X, and [14X'Reference: Lie Algebras'[114X.[133X
  
  [33X[0;0YYou will find more list operations in Chapter [14X'Reference: Lists'[114X.[133X
  
  [33X[0;0YRecords   and   functions   for   records   are   described   in  detail  in
  Chapter [14X'Reference: Records'[114X.[133X
  
