Why is Swift giving me inaccurate floating point arithmetic results?

Question

Swift floating point arithmetic seems broken compared to C (and therefore Objective-C).

Let's take a simple example. In C:

double output = 90/100.0; // Gives 0.9
float output = 90/100.0f; // Gives 0.9

In Swift:

var output = Double(90)/Double(100.0) // Gives 0.90000000000000002
var output = Float(90)/Float(100.0) // Gives 0.899999976

What's going on? Is this a bug or am I missing something?

EDIT:

#import <iostream>

int main() {
    double inter = 90/100.0;
    std::cout << inter << std::endl; // Outputs 0.9
    return 0;
}

oh anther floating arithmetic question... btw `double output = 90/100.0; // doesn't gives 0.9` — Bryan Chen, Sep 04 '14 at 05:28
Printing the results in C with the same number of decimals would probably produce the same output. — Martin R, Sep 04 '14 at 05:30
You're missing something. Lots of Qs&As here on SO about this, try starting at http://stackoverflow.com/questions/588004/is-floating-point-math-broken — High Performance Mark, Sep 04 '14 at 05:42
#import int main() { double inter = 90/100.0; std::cout << inter << std::endl; // Output 0.9 return 0; } — Gadzair, Sep 05 '14 at 01:55

bames53 · Accepted Answer · 2014-09-05T02:14:18.110

The issue is simply the different number of digits being printed out.

#include <iostream>
#include <iomanip>

int main() {
    double d = 90.0 / 100.0;
    float f = 90.0f / 100.0f;
    std::cout << d  << ' ' << f << '\n';
    std::cout << std::setprecision(20) << d << ' ' << f << '\n';
}

0.9 0.9
0.9000000000000000222 0.89999997615814208984

(I wrote this example in C++, but you will get the same results in every language that uses the hardware's floating point arithmetic and allows this formatting.)

If you want to understand why finite precision floating point math does not give you exact results then:

What Every Computer Scientist Should Know About Floating-Point Arithmetic

And:

Float

Why is Swift giving me inaccurate floating point arithmetic results?

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