isfield checks if the given fieldname exists in a structure or mlist or tlist

## Syntax

    bool = isfield(s, fieldname)

## Arguments

s

  a struct array, a tlist or a mlist.

fieldname

  a matrix of strings: names of fields to be detected

bool

  a matrix of boolean (same format as `fieldname>`)

## Description

This function returns `%t` if the specified structure or tlist or mlist `s` includes the field `fieldname`, regardless of the corresponding value. For a structure, the hidden field `dims` is ignored.

## Examples

    s = struct("field_1",123,"field_2",456,"field_4",789)

    // Single Fieldname Syntax
    isfield( s , "field_1"  )

    // Multiple Fieldname Syntax
    isfield( s , [ "field_1" "field_2" ; "field_3" "field_4" ] )

    // with a tlist:
    s = tlist(["test" "t" "n","","p"], "Hello", %pi, "anonymous", (1-%z)^3)
    isfield(s, ["b" "t" ""])  // =>  F  T  T  (anonymous fields can be detected)

## See also

- struct
- tlist
- mlist
- getfield
- definedfields

## History

5.5 Extension to any mlist and tlist.