Floating-point conversion occurs when a value is converted between floating-point types, such as float, double, and long double, or between floating-point and integer types. The conversion rules determine whether the value is preserved, rounded, or truncated.

#include <stdio.h>

int main(void)
{
    float f = 3.1415926535f;
    double d = f;
    long double ld = d;

    printf("float       = %.9f\n", f);
    printf("double      = %.9f\n", d);
    printf("long double = %.9Lf\n", ld);

    return 0;
}

Example Output #

The exact output depends on the implementation and floating-point representation. A common result is:

float       = 3.141592741
double      = 3.141592741
long double = 3.141592741

Explanation #

The first assignment stores a decimal value in a float:

float f = 3.1415926535f;

A float cannot represent every decimal fraction exactly. The value is rounded to a representable floating-point value.

The next assignment converts that stored value to double:

double d = f;

This conversion preserves the value represented by f, but it does not restore precision that was already lost when the value was stored as a float.

Similarly:

long double ld = d;

converts the double value to long double. The conversion preserves the value represented by d, subject to the rules of the implementation.

Floating-Point Conversion Rules #

For example:

double value = 9.75;
int number = (int)value;

The resulting integer is 9, because conversion from floating-point to integer discards the fractional part.

If the integral part of a floating-point value cannot be represented by the destination integer type, the conversion has undefined behavior in C.