C Programming

Comparison of a float with a value

Predict the output of following C program.

#include<stdio.h>
int main()
{
    float x = 0.1;
    if (x == 0.1)
        printf("IF");
    else if (x == 0.1f)
        printf("ELSE IF");
    else
        printf("ELSE");
}

The output of above program is “ELSE IF ” which means the expression “x == 0.1? returns false and expression “x == 0.1f” returns true.

Let consider the of following program to understand the reason behind the above output.

#include<stdio.h>
int main()
{
   float x = 0.1;
   printf("%d %d %d", sizeof(x), sizeof(0.1), sizeof(0.1f));
   return 0;
}

The output of above program is “4 8 4 ” on a typical C compiler. It actually prints size of float, size of double and size of float.

The values used in an expression are considered as double (double precision floating point format) unless a ‘f’ is specified at the end. So the expression “x==0.1? has a double on right side and float which are stored in a single precision floating point format on left side. In such situations float is promoted to double. The double precision format uses uses more bits for precision than single precision format.
Note that the promotion of float to double can only cause mismatch when a value (like 0.1) uses more precision bits than the bits of single precision. For example, the following C program prints “IF”.

#include<stdio.h>
int main()
{
    float x = 0.5;
    if (x == 0.5)
        printf("IF");
    else if (x == 0.5f)
        printf("ELSE IF");
    else
        printf("ELSE");
}

Output:

IF
You can refer Floating Point Representation – Basics for representation of floating point numbers.


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