Why Does My Floating-Point Comparison Produce Unexpected Results?

Floating Point Comparison Discrepancy

In the provided code snippet:

int main()
{
    float a = 0.7;
    float b = 0.5;
    if (a < 0.7)
    {
       if (b < 0.5) printf("2 are right");
       else         printf("1 is right");
    }
    else printf("0 are right");
}

the expected output of "0 are right" is not obtained. Instead, "1 is right" is printed. This discrepancy arises due to the inherent imprecision of floating-point numbers.

When performing comparisons with floating-point operands, they are automatically promoted to double-precision values. Floats, being less precise than doubles, may not represent the intended value exactly. In this case, when a (a float) is compared to 0.7 (a double), a is internally converted to a double and loses precision. This results in a being slightly less than 0.7, causing the a < 0.7 comparison to evaluate to true.

To resolve this issue, one can either:

• Change float to double:

double a = 0.7;
double b = 0.5;

• Use float literals with an 'f' suffix:

float a = 0.7f;
float b = 0.5f;

Both approaches ensure that the operands remain as floats and avoid the precision loss during promotion to double.

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