I need my NSTextFields be able to evaluate formulas...

24/07/03 18:55
It would be very nice for my customers to enter math formulas in text fields, how to achieve this in Cocoa ?
-(BOOL)control:(NSControl *)control textShouldEndEditing:(NSText *)fieldEditor; to know if editing could be ended: you could design such a delegate that will parse the text and replace it by the result (or an error message is something goes wrong).
But how fo you evaluate a formula would you ask ?
This is a parsing problem and the solution depends on the complexity of the formula language you want to offer to your users. If you limit yourself to +,-,*,/ operations without parenthensis, it's quite easy to parse the string token by token to calculate the result.
If you want something more sophisticated you can use /usr/bin/yacc to generate the code of the parser from a grammar definition. Here is an example supporting some basic math function like tan, sqrt and exponentiation operator ^.
%{

#include
#include

int yyparse(void);
void yyerror(char *);
int yylex(void);

#define YYTRACE
#define YYSTYPE double
#define kPI (3.1415926535897932384626433832795)

#define YYACCEPT { yyresult = yyval ; return(0); }

extern YYSTYPE yyresult ;

int last=0;
%}

%left '+' '-'
%left '*' '/'
%left '^'
%nonassoc UMINUS
%nonassoc UPLUS

%token NUMBER, SIN, COS, TAN, ASIN, ACOS, ATAN, ATAN2, POW, PI, SQRT

%%

start : /* Nothing */
{ $$ = 0.0; }
| expr
{ $$ = $1; }
;

expr : NUMBER { $$ = $1 }
| function_call { $$ = $1 }
| PI { $$ = kPI }
| '(' expr ')' { $$ = $2 }
| expr '+' expr { $$ = $1 + $3 }
| expr '-' expr { $$ = $1 - $3 }
| expr '*' expr { $$ = $1 * $3 }
| expr '/' expr { $$ = $1 / $3 }
| expr '^' expr { $$ = pow ($1, $3) }
| '-' expr %prec UMINUS { $$ = -$2 }
| '+' expr %prec UPLUS { $$ = $2 }
;

function_call : SIN '(' expr ')'
{ $$ = sin( $3 ) }
| COS '(' expr ')'
{ $$ = cos( $3 ) }
| TAN '(' expr ')'
{ $$ = tan( $3 ) }
| ASIN '(' expr ')'
{ $$ = asin( $3 ) }
| ACOS '(' expr ')'
{ $$ = acos( $3 ) }
| ATAN '(' expr ')'
{ $$ = atan( $3 ) }
| ATAN2 '(' expr ',' expr ')'
{ $$ = atan2($3,$5) }
| POW '(' expr ',' expr ')'
{ $$ = pow($3,$5) }
| SQRT '(' expr ')'
{ $$ = sqrt($3) }
;
%%
There are several solutions:
- NSTextField are validated and formated by NSFormater objects: to make your NSTextField accept math formulas, you may write a NSFormater subclass that will evaluate the formula and returns the value as the object value.
- NSTextField calls its delegate's method