Compare float value

Question

I try to compare float value but I can't do it, I use this code:

float a = [[array objectAtIndex:(i+1)] floatValue];
float b = [[array objectAtIndex:(i)] floatValue];

a = [[NSString stringWithFormat:@"%.2f",a] floatValue];
b = [[NSString stringWithFormat:@"%.2f",b] floatValue];
step = [[NSString stringWithFormat:@"%.2f",step] floatValue];

float newStep = a-b;

if (newStep != step) {
      NSLog(@"NewStep: %f Step: %f",newStep,step);
}

this is the output:

2013-04-28 19:07:57.396 Pro[24265:c07] NewStep: 0.220000 Step: 0.220000
2013-04-28 19:07:57.397 Pro[24265:c07] NewStep: 0.220000 Step: 0.220000
2013-04-28 19:07:57.397 Pro[24265:c07] NewStep: 0.220000 Step: 0.220000
2013-04-28 19:07:57.397 Pro[24265:c07] NewStep: 2.420000 Step: 0.220000
2013-04-28 19:07:57.397 Pro[24265:c07] NewStep: 0.220000 Step: 0.220000
2013-04-28 19:07:57.397 Pro[24265:c07] NewStep: 0.220000 Step: 0.220000

obviously don't work, where is the mistake? the code seems to be correct!

Where do you assign to step for the first time? Also, the easier (and possibly working) comparison would be abs(a - b - step) < 0.005. — Ry-
– Ry- ♦, Commented Apr 28, 2013 at 17:13
Try Googling around for "comparing floating point numbers"; there are lots of tricky bits. — Jesse Rusak
– Jesse Rusak, Commented Apr 28, 2013 at 17:19
step is a parameter, however thank you! I use this comparison and now it work! — francesco.venica
– francesco.venica, Commented Apr 28, 2013 at 17:29

Matthias · Accepted Answer · 2013-04-28 17:22:07Z

4

Never ever compare float variables for equality! Floats are an approximation only, due to the limited accuracy. Use

if (fabs(a-b)<0.0001) {...

or something similar, if you want to check for equality.

answered Apr 28, 2013 at 17:22

Matthias

8,2402 gold badges30 silver badges56 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Compare float value

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related