Question

I’m building an app that graph a text given function using DDMathParser by Dave DeLong. I need to know (for every “x” I evaluate) if the solution exist, or it just gives me 0.00 because it couldn’t evaluate it. Maybe a Bool?

while (x <= (viewWidth - originOffsetX)/axisLenghtX) {

        NSDictionary *variableSubstitutions = [NSDictionary dictionaryWithObject: [NSNumber numberWithDouble:x] forKey:@"x"];
        NSString *solution = [NSString stringWithFormat:@"%@",[[DDMathEvaluator sharedMathEvaluator] 
                                                               evaluateString:plotterExpression withSubstitutions:variableSubstitutions]];
        numericSolution = solution.numberByEvaluatingString.doubleValue;
        NSLog(@"%f", numericSolution);
        if (newline) {
            CGContextMoveToPoint(curveContext, (x*axisLenghtX + originOffsetX), (-numericSolution * axisLenghtY + originOffsetY));
            newline = FALSE;
        } else {
            CGContextAddLineToPoint(curveContext, (x*axisLenghtX + originOffsetX), (-numericSolution * axisLenghtY + originOffsetY));
        }
        x += dx;

Answer 1

Well, since you’re using the simplest API possible, there’s no way to by notified if there’s an error. This is clearly explained in the first section of the Usage page on the wiki:

There are several ways to evaluate strings, depending on how much
customization you want to do. Most of these options require an NSError
** parameter, although some do not.

• If you use one of the options that does not accept an NSError **, then
  any tokenization, parsing, or evaluation errors will be NSLogged.
• If you use one of the options that does accept an NSError **, then you
  must supply one. Failing to do so will probably result in a crash.

So what you want to do is this:

NSDictionary *variableSubstitutions = [NSDictionary dictionaryWithObject: [NSNumber numberWithDouble:x] forKey:@"x"];
NSError *error = nil;
NSNumber *number = [[DDMathEvaluator sharedMathEvaluator] evaluateString:plotterExpression withSubstitutions:variableSubstitutions error:&error]];

if (number == nil) {
  NSLog(@"an error occurred while parsing: %@", error);
} else {
  numericSolution = [number doubleValue];
  // continue on normally
}