Question

Currently I am making a Calculator that allows the user to type out the formula they wish.

Ex. ((1+1)**9)+2)

This works just fine, I have used two methods for calculating this.

First:

    answer = [[NSExpression expressionWithFormat:typeTo.text, nil] expressionValueWithObject:nil context:nil];
    typeTo.text = [NSString stringWithFormat:@"%@", answer];
    answerLabel.text = [NSString stringWithFormat:@"ANS { %@ }", answer];

Second:

     answer = [GCMathParser evaluate:typeTo.text];

Both of these calculate the problem without difficulty. But if the user types in:

(1+1)) [two brackets]

Both ways crash. This is one example of many different syntax errors. Is there a way to easily prevent this?

.

Additional info:
This is the way the second method catches the error:

#ifdef __COCOA_IMPLEMENTATION__
[NSException raise:@"Error in expression" format:@"error = %s", errStr];
#endif

THANKS 😀

Answer 1

I haven’t used either of those but based on the additional info, it may be throwing an NSException.

If that’s the case, you can catch it and handle it. It looks like it might format a useful message telling you what’s wrong with with expressions.

@try
{
    // do work
}
@catch(NSException* ex)
{
    // handle
}

https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/Exceptions/Tasks/HandlingExceptions.html

Also, look to see if those libraries offer functions that pass in a ref to NSError. If so, you can use that.

There’s also DDMathParser which is supposed to be a modern math parser and it looks like it takes NSError. Might be worth a look.

http://github.com/davedelong/DDMathParser