Why a double variable in C doesn't like to store the number '7'?

Question

I tested a double variable in C to store the value '12345678901234567'. But it can't store the last digit '7' and shows it as '8'. However, if I change the last digit to '6' or '8', then it can show the same value. What is that?

Here is the program:

#include <stdio.h>

int main()

{

    double a;

   

    a = 12345678901234567;

    printf("%lf", a);

    return 0;

}

Output: 12345678901234568.000000

 

Answer 1

Hi Debojit Acharjee,

I tested your code and got the same results as you, but when I tried to output 12345678901234566 and 12345678901234568, my compiler was able to display them correctly. The compiler I'm using is VS2022, I suggest you try it.

In C language, if you assign a number to a double variable, the compiler will do its best to represent it as a double. In some cases it may round to the nearest representable number, while in other cases it may choose a close representable number.

This behavior depends on the specification and implementation details of floating-point numbers, as well as compiler and hardware differences. Therefore, even the same code may give different results when run on different compilers or platforms.

Best regards,

Elya Yao

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