GwyExpr: Gwyddion Library Reference Manual

## GwyExpr GwyExpr — Arithmetic expression parser and evaluator |

## Functions

GQuark | gwy_expr_error_quark ()
GwyExpr * | gwy_expr_new ()
void | gwy_expr_free ()
gboolean | gwy_expr_evaluate ()
gboolean | gwy_expr_compile ()
guint | gwy_expr_resolve_variables ()
gint | gwy_expr_get_variables ()
gdouble | gwy_expr_execute ()
void | gwy_expr_vector_execute ()
gboolean | gwy_expr_define_constant ()
gboolean | gwy_expr_undefine_constant ()
const gchar * | gwy_expr_get_expression ()

## Types and Values

#define | GWY_EXPR_ERROR
enum | GwyExprError
| GwyExpr

## Includes

    #include <libgwyddion/gwyddion.h>

## Description

GwyExpr is an expression evaluator, more precisely parser, compiler, and evaluator. A new GwyExpr can be created with `gwy_expr_new()`, then it can be used to evaluate any number of expressions; when it's no longer needed, it should be destroyed with `gwy_expr_free()`.

Expression syntax is described in

Gwyddion user guide.

Simple arithmetic expressions without variables can be directly evaluated with `gwy_expr_evaluate()`.

Expression with variables have to be compiled first with `gwy_expr_compile()`. Then either `gwy_expr_resolve_variables()` or `gwy_expr_get_variables()` can be used to obtain information what variables are present in the expression and what integer index they correspond to (variables are identified by an integer index, not name, during final evaluation for effieiency). Subsequent evaluation with variable value substitution is performed by `gwy_expr_execute()`. It is also possible to evaluate expression on each item of value arrays with `gwy_expr_vector_execute()`.

One-shot evaluation of expressions without variables is easy:

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    GwyExpr *expr;
    GError *err = NULL;
    gdouble result;

    expr = gwy_expr_new();
    if (!gwy_expr_evaluate(expr, "1+2", &result, &err)) {
        /* Handle compilation error */
    }
    g_print("The result: %g\n", result);
    gwy_expr_free(expr);

One-shot evaluation of expressions with known variables can be done by defining them as constants beforehand:

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    GwyExpr *expr;
    GError *err = NULL;
    gdouble result;

    /* Create expression and define variables as constants */
    expr = gwy_expr_new();
    gwy_expr_define_constant(expr, "x", 3.0, NULL);
    gwy_expr_define_constant(expr, "y", 4.0, NULL);

    /* Evaluate expression */
    if (!gwy_expr_evaluate(expr, "hypot x,y", &result, &err)) {
        /* Handle compilation error */
    }
    g_print("The result: %g\n", result);

    gwy_expr_free(expr);

When the same expression is evaluated multiple times with different variable values and the variables are again from some known set, the `gwy_expr_resolve_variables()` should be used (if the repeated evaluation happens over items of some arrays, `gwy_expr_vector_execute()` is more efficient and usually simplier to use too):

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    GwyExpr *expr;
    GError *err = NULL;
    const gchar *const var_names[] = { "lambda", "theta", "psi", "d", "k" };
    const gdouble var_values1[] = { 300.0, 0.09, 2.0, 52.3, 1.43 };
    const gdouble var_values2[] = { 400.0, 0.12, 2.0, 54.1, 1.44 };
    guint var_positions[G_N_ELEMENTS(var_names)];
    gdouble vars[G_N_ELEMENTS(var_names) + 1];
    guint i;

    /* Create expression and define constant pi */
    expr = gwy_expr_new();
    gwy_expr_define_constant(expr, "pi", G_PI, NULL);

    /* Compile expression */
    if (!gwy_expr_compile(expr, "2*pi/lambda*d*sin theta", &err)) {
        /* Handle compilation error */
    }

    /* Resolve variables */
    if (gwy_expr_resolve_variables(expr, G_N_ELEMENTS(var_names),
                                   var_names, var_positions)) {
        /* Expression contains unknown variables */
    }

    /* Evaluate first set */
    for (i = 0; i < G_N_ELEMENTS(var_names); i++) {
        vars[var_positions[i]] = var_values1[i];
    }
    g_print("First result: %g\n", gwy_expr_execute(expr, vars));

    /* Evaluate second set */
    for (i = 0; i < G_N_ELEMENTS(var_names); i++)
        vars[var_positions[i]] = var_values2[i];
    g_print("Second result: %g\n", gwy_expr_execute(expr, vars));

    gwy_expr_free(expr);

The most general case is when the variables are from a large set (or completely arbitrary). Then it is best to get the list of variables with `gwy_expr_get_variables()` and supply only values of variables that are actually present in the expression.

## Functions

### gwy_expr_error_quark()

    GQuark
    gwy_expr_error_quark (void);

Returns error domain for expression parsing and evaluation.

See and use `GWY_EXPR_ERROR`.

#### Returns

The error domain.

### gwy_expr_new()

    GwyExpr *
    gwy_expr_new (void);

Creates a new expression evaluator.

#### Returns

A newly created expression evaluator.

### gwy_expr_free()

    void
    gwy_expr_free (GwyExpr *expr);

Frees all memory used by an expression evaluator.

#### Parameters

expr | An expression evaluator. |

### gwy_expr_evaluate()

    gboolean
    gwy_expr_evaluate (GwyExpr *expr,
                       const gchar *text,
                       gdouble *result,
                       GError **err);

Evaulates an arithmetic expression.

#### Parameters

expr | An expression evaluator. |
text | String containing the expression to evaluate. |
result | Location to store result to. |
err | Location to store compilation or evaluation error to, or `NULL`. Errors from GwyExprError domain can occur. |

#### Returns

`TRUE` on success, `FALSE` if evaluation failed.

### gwy_expr_compile()

    gboolean
    gwy_expr_compile (GwyExpr *expr,
                      const gchar *text,
                      GError **err);

Compiles an expression for later execution.

This function is useful for expressions with variables. For normal arithmetic expressions it's easier to use `gwy_expr_evaluate()`.

#### Parameters

expr | An expression evaluator. |
text | String containing the expression to compile. |
err | Location to store compilation or evaluation error to, or `NULL`. Errors from GwyExprError domain can occur. |

#### Returns

`TRUE` on success, `FALSE` if compilation failed.

### gwy_expr_resolve_variables()

    guint
    gwy_expr_resolve_variables (GwyExpr *expr,
                                guint n,
                                const gchar * const *names,
                                guint *indices);

Finds positions of variables in an expression.

#### Parameters

expr | An expression evaluator. |
n | The length of `names` and `indices` arrays. |
names | List of variable names to get positions of. |
indices | Array to store variable positions to. The positions are the same as in `gwy_expr_execute()`. Variables not present in the expression are assigned the reserved position 0. This permits assigning safely values to all variables before execution wthout caring which variables are actually present. |

#### Returns

The number of remaining, unresolved variables in `expr` .

### gwy_expr_get_variables()

    gint
    gwy_expr_get_variables (GwyExpr *expr,
                            gchar ***names);

Get the number, names, and indices of unresolved identifiers in `expr` .

It is an error to call this function after an unsuccessful compilation.

If you only care about variables from a prefedined set, that is if any unknown variable is an error, it's easier to use `gwy_expr_resolve_variables()`.

The position of each variable in `names` corresponds to the position of its value in `values` array in `gwy_expr_execute()` call. Namely, the first item in the array is always reserved and do not correspond to any variable.

#### Parameters

expr | An expression evaluator. |
names | Location to store pointer to array of variable names to (may be `NULL` to get just number of variables). The string array returned in this argument in owned by `expr` and is valid only until next `gwy_expr_compile()`, `gwy_expr_evaluate()`, or `gwy_expr_free()` call. |

#### Returns

The length of array stored to `names` . This is the number of variables plus one (for the first reserved item). On failure, 0 is returned.

### gwy_expr_execute()

    gdouble
    gwy_expr_execute (GwyExpr *expr,
                      const gdouble *values);

Executes a compiled expression with variables, substituting given values.

#### Parameters

expr | An expression evaluator. |
values | Array with variable values. Its zeroth item is always unused. Variable list can be obtained by `gwy_expr_get_variables()`. |

#### Returns

The result.

### gwy_expr_vector_execute()

    void
    gwy_expr_vector_execute (GwyExpr *expr,
                             guint n,
                             const gdouble **data,
                             gdouble *result);

Executes a compiled expression on each item of data arrays.

The result is the same as repeatedly taking one element from each array and using `gwy_expr_execute()` to evaluate the expression with this set of values. However, it is more efficient.

#### Parameters

expr | An expression evaluator. |
n | The length of `result` and of `data` member arrays, that is vector length. |
data | An array of arrays of length `n` . The arrays correspond to expression variables in `gwy_expr_execute()`. The zeroth array can be `NULL`. |
result | An array of length `n` to store computation results to. It may be one of those in `data` . |

### gwy_expr_define_constant()

    gboolean
    gwy_expr_define_constant (GwyExpr *expr,
                              const gchar *name,
                              gdouble value,
                              GError **err);

Defines a symbolic constant.

Note the definition does not affect already compiled expression, you have to recompile it (and eventually re-resolve variables).

#### Parameters

expr | An expression evaluator. |
name | Name of constant to define. |
value | Constant numeric value. |
err | Location to store error to, or `NULL`. Only the `GWY_EXPR_ERROR_CONSTANT_NAME` error from GwyExprError domain can occur. |

#### Returns

`TRUE` on success, `FALSE` if definition failed.

### gwy_expr_undefine_constant()

    gboolean
    gwy_expr_undefine_constant (GwyExpr *expr,
                                const gchar *name);

Undefines a symbolic constant.

Note the definition removal does not affect already compiled expression, you have to recompile it (and eventually re-resolve variables).

#### Parameters

expr | An expression evaluator. |
name | Name of constant to undefine. |

#### Returns

`TRUE` if there was such a constant and was removed, `FALSE` otherwise.

### gwy_expr_get_expression()

    const gchar *
    gwy_expr_get_expression (GwyExpr *expr);

Gets the expression string.

#### Parameters

expr | An expression evaluator. |

#### Returns

The last string passed to `gwy_expr_evaluate()` or `gwy_expr_compile()`. It is owned by `expr` and must not be modified or freed.

## Types and Values

### GWY_EXPR_ERROR

    #define GWY_EXPR_ERROR gwy_expr_error_quark()

Error domain for expression parsing and evaluation. Errors in this domain will be from the GwyExprError enumeration. See GError for information on error domains.

### enum GwyExprError

Error codes returned by expression parsing and execution.

#### Members

GWY_EXPR_ERROR_CLOSING_PARENTHESIS | A closing parenthesis is missing. |
GWY_EXPR_ERROR_EMPTY | Expression is empty. |
GWY_EXPR_ERROR_EMPTY_PARENTHESES | A parentheses pair contain nothing inside. |
GWY_EXPR_ERROR_GARBAGE | An symbol unexpectedly managed to survive. |
GWY_EXPR_ERROR_INVALID_ARGUMENT | Function or operator argument is not a value. |
GWY_EXPR_ERROR_INVALID_TOKEN | Expression contains an invalid token. |
GWY_EXPR_ERROR_MISSING_ARGUMENT | Function or operator arguments is missing. |
GWY_EXPR_ERROR_NOT_EXECUTABLE | Compiled stack is not executable. |
GWY_EXPR_ERROR_OPENING_PARENTHESIS | An opening parenthesis is missing. |
GWY_EXPR_ERROR_STRAY_COMMA | A comma at the start or end of list. |
GWY_EXPR_ERROR_UNRESOLVED_IDENTIFIERS | Expression contains unresolved identifiers. |
GWY_EXPR_ERROR_CONSTANT_NAME | Constant name is invalid. |

### GwyExpr

    typedef struct _GwyExpr GwyExpr;

GwyExpr is an opaque data structure and should be only manipulated with the functions below.