IEEE 754 Explore

ieeefloat.c

/**
 * Shows how a number is represented as IEEE 754
 *
 * @author      Pat Hawks
 * Course:      COMP B13
 * Created:     Sept 29, 2015
 * Source File: ieeefloat.c
 */

#include <stdio.h>
#include <limits.h>
#include <float.h>

union Float {
    float value;
    int ieee;
};

long calculateMantissa(int m) {
    long c = 0;
    int i = FLT_MANT_DIG;
    long pos = 5000000000000000;
    while (--i) {
        if (m & (1<<(FLT_MANT_DIG-1))) {
            c += pos;
        }
        m <<= 1;
        pos >>= 1;
    }
    if (c) {
        while (c % 10 == 0) {
            c /= 10;
        }
    }

    return c;
}

int main(void) {
    union Float num;
    int sign;
    int mantissa;
    int exponent;
    char const* const prompt = "Please enter a floating point decimal number: ";
    char const* const table = "\n            Sign Exponent Mantissa\n     Value: %3d  2^%-6d 1.%ld\nEncoded as: %3d  %-8d %d\n";

    printf("%s", prompt);
    scanf("%f", &num.value);

    sign = (num.ieee & INT_MIN)?0:1;
    mantissa = num.ieee & ((1 << (FLT_MANT_DIG-1)) - 1);
    exponent = ((num.ieee) >> (FLT_MANT_DIG - 1)) & UCHAR_MAX;

    printf(table, sign==0?1:-1, exponent-127, calculateMantissa(mantissa), sign, exponent, mantissa);

    return 0;
}