Question

Why does this add M_PI as a string “3.141593” to an NSMutableArray? How can I add M_PI as a float to the array?

- (void)pushOperand:(float)operand
{
    [self.operandStack addObject:[NSNumber numberWithFloat:operand]];
}

[self pushOperand:M_PI];

Answer 1

As others have said, you can’t store a float directly in an NSMutableArray. Your code is already inserting the value as an instance of NSNumber, however, and this is what you want. Later, when you pull the object back out of the array, you can restore it to a POD type with something like this:

double value = 0;

id topOfStack = [stack popOperand];

if ([topOfStack isKindOfClass:[NSNumber class]]) 
    result = [topOfStack doubleValue];

Kudos, by the way, for doing the Stanford iOS course 🙂