C-language floating number brain buster

Question

A friend showed me this c-language code, and I'm very confused with its output. Can any one please explain why is it giving output "It was a piece of black forest cakeThis is weird", instead of "It was a piece of black forest cakeIt was a piece of black forest cake"?

# include <cstdio>

using namespace std;

int main()
{
    float a = 0.5;

    if(a < 0.5) 
        printf("This is Weird");
    else 
        printf("It was a piece of black forest cake");

    float b = 0.7;
    if(b < 0.7)
        printf("This is Weird");
    else 
        printf("It was a piece of black forest cake");

    return 0;
}

Many, many duplicates, e.g. [Floating point comparison \`a != 0.7\`](http://stackoverflow.com/questions/6883306/floating-point-comparison-a-0-7) and [strange output in comparison of float with float literal](http://stackoverflow.com/questions/1839422/strange-output-in-comparison-of-float-with-float-literal) - please do a search before posting. — Paul R, Aug 24 '13 at 15:00
Floating point precision… Computers are finite… Yadda yadda yadda. — Dave, Aug 24 '13 at 15:01
Why don't you just do a little search on StackOverflow ? Such questions are asked daily a dozen number of times. — P0W, Aug 24 '13 at 15:01
A five-question quiz in the same vein. If you get the first four right you have understood: http://blog.frama-c.com/index.php?post/2011/11/08/Floating-point-quiz — Pascal Cuoq, Aug 24 '13 at 16:01

Antti Haapala -- Слава Україні · Accepted Answer · 2013-08-24T15:07:29.990

This is because 0.5 can be presented as a float exactly, whereas 0.7 cannot. Also both of these are double constants, that is, to store them at their standard precision, they need to be stored in a variable of type double. YOu can get a float constant by suffixing the number with f. Furthermore in if (b < 0.7) comparison the float is converted to double implicitly; 0.7 can be stored as a double more precisely, and thus in this case is a larger number, because the 0.7 was rounded down when stored in a float variable.

Try also the following code:

#include <stdio.h>

int main()
{   
    if (0.7f < 0.7) {
       printf("This is Weird");
    }
    return 0;
}

Thank you so much for making it simple , i see other previous answers are very complicating — Taha Rushain, Aug 24 '13 at 15:12

C-language floating number brain buster

1 Answers1