Convert NSString of a math equation to a value

Question

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

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


        

Answer 1

Here is an example how to do it 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 produces 1 because 1+2==3 is true; the second produces 0.

Answer 2

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); 

Answer 3

So, this is a problem that I believe is a lot more complicated than the linked question lets on (although the question is asking for a "simple" equation parser).

Fortunately for you, I think this is a really interesting problem and have already written one for you: DDMathParser.

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

In your case, you'd 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 made available under the MIT license, which requires you to include the copyright information and the full text of the license in anything that uses it.