Convert NSString math equation to value

Question

I would like to know how to evaluate the string representation of an equation as if it were a real equation:

if(@"15+14==23") { //True statement... } else { //False statement.... } 

I want to return false, because 15 + 14 is not 23. How can I make this work?

Answer 1

Here is an example of how to do this with NSPredicate :

 NSPredicate *p = [NSPredicate predicateWithFormat:@"1+2==3"]; NSLog(@"%d", [p evaluateWithObject:nil]); p = [NSPredicate predicateWithFormat:@"1+2==4"]; NSLog(@"%d", [p evaluateWithObject:nil]); 

The first NSLog creates 1 because 1+2==3 is true; the second produces 0 .

Answer 2

So, this is a problem that, in my opinion, is much more complicated than the related question allows (although the question is asked by a “simple” parser).

Luckily for you, I think this is a really interesting problem and has already been written for you: DDMathParser .

It has a good amount of documentation , including such things as how to add it to your project and a high overview of its capabilities . It supports all standard mathematical operators , including logical and comparative operators ( || , && , == != , <= , Etc.).

In your case, you would do something like this:

 NSNumber *result = [@"15+14 == 23" numberByEvaluatingString]; if ([result boolValue] == YES) { ....True statement.... } else { .....False statement..... } 

As a heads-up, DDMathParser is licensed under the MIT, which requires you to include copyright information and the full text of the license in everything that uses it.

Answer 3

 NSString *equation = @"15+14==29"; NSPredicate *pred = [NSPredicate predicateWithFormat:equation]; NSExpression *LeftExp = [pred leftExpression]; NSExpression *RightExp = [pred rightExpression]; NSNumber *left = [LeftExp expressionValueWithObject:nil context:nil]; NSNumber *right = [RightExp expressionValueWithObject:nil context:nil]; if ([left isEqualToNumber:right]) { NSLog(@"yes left is equal to right"); } else{ NSLog(@"yes left is NOT equal to right"); } NSLog(@"left %@", left); NSLog(@"right %@", right);