C Math modf() Function

What You’ll Learn

The modf() function splits a floating-point number into its integral (whole-number) part and fractional part. The fractional part is returned; the integral part is written through a pointer. Handy for formatting money, parsing decimals, and any task that treats whole and fractional pieces separately.

Definition and Usage

modf(value, &iptr) decomposes value into an integral part stored at *iptr and a fractional part returned by the function, such that:

value = integral + fractional

The fractional part has the same sign as value (or is zero) and its magnitude is less than 1. For 123.456, the integral part is 123.0 and the fractional part is 0.456.

📝 Syntax

Standard C declaration:

double modf(double value, double *iptr);

Related variants

float modff(float value, float *iptr);             /* <math.h> */
long double modfl(long double value, long double *iptr); /* <math.h> */

Parameters

• value — the floating-point number to decompose.
• iptr — pointer to double where the integral part is stored (must not be null).

Return Value

• The fractional part of value as double.
• Integral part written to *iptr.
• modf(123.456, &w) returns 0.456 and sets w = 123.0.

Headers and linking

• #include <math.h>
• Compile: gcc modf.c -std=c11 -o modf -lm
• Integral part is truncated toward zero (like trunc), not floored.

Examples Gallery

Compile every example with gcc file.c -std=c11 -o out -lm. modf is the standard way to peel off the decimal portion of a floating-point value in C.

#include <math.h>
#include <stdio.h>

int main(void) {
    double value = 123.456;
    double integralPart;
    double fractionalPart = modf(value, &integralPart);

    printf("Original value:  %.6f\n", value);
    printf("Integral part:   %.6f\n", integralPart);
    printf("Fractional part: %.6f\n", fractionalPart);

    return 0;
}

Original value:  123.456000
Integral part:   123.000000
Fractional part: 0.456000

#include <math.h>
#include <stdio.h>

int main(void) {
    double value = -3.75;
    double integral;
    double fractional = modf(value, &integral);

    printf("value = %.2f\n", value);
    printf("integral   = %.2f  (trunc toward zero)\n", integral);
    printf("fractional = %.2f  (same sign as value)\n", fractional);
    printf("sum        = %.2f\n", integral + fractional);

    return 0;
}

value = -3.75
integral   = -3.00  (trunc toward zero)
fractional = -0.75  (same sign as value)
sum        = -3.75

#include <math.h>
#include <stdio.h>

int main(void) {
    double values[] = { 42.0, 3.14159, -99.99, 0.0 };
    size_t i;

    printf("  value      integral  fractional   sum\n");
    for (i = 0; i < 4; i++) {
        double v = values[i];
        double integral;
        double frac = modf(v, &integral);
        printf("%9.5f  %9.2f  %10.5f  %9.5f\n",
               v, integral, frac, integral + frac);
    }

    return 0;
}

  value      integral  fractional   sum
 42.00000      42.00    0.00000   42.00000
  3.14159       3.00    0.14159    3.14159
-99.99000     -99.00   -0.99000  -99.99000
  0.00000       0.00    0.00000    0.00000

#include <math.h>
#include <stdio.h>

int main(void) {
    double price = 19.99;
    double dollars;
    double frac = modf(price, &dollars);
    int cents = (int)round(frac * 100.0);

    printf("Price: $%.2f\n", price);
    printf("Dollars: $%.0f\n", dollars);
    printf("Cents:   %d\n", cents);

    return 0;
}

Price: $19.99
Dollars: $19
Cents:   99

#include <math.h>
#include <stdio.h>

int main(void) {
    double value = 10.5;
    double integral;

    printf("value = %.1f\n\n", value);

    printf("modf: splits into parts\n");
    printf("  fractional = %.1f\n", modf(value, &integral));
    printf("  integral   = %.1f\n\n", integral);

    printf("fmod: remainder of division\n");
    printf("  fmod(%.1f, 3.2) = %.1f\n", value, fmod(value, 3.2));

    return 0;
}

value = 10.5

modf: splits into parts
  fractional = 0.5
  integral   = 10.0

fmod: remainder of division
  fmod(10.5, 3.2) = 0.9

🚀 Common Use Cases

• Number formatting — separate whole and decimal portions for display.
• Financial UI — dollars vs cents (prefer integer cents in production).
• Graphics — split texture coordinates into tile index and offset.
• Time parsing — whole hours plus fractional hour remainder.
• Custom rounding — process integral and fractional parts with different rules.

📝 Notes

• Fractional part has the same sign as value (or is zero).
• Integral part is truncated toward zero, not floored.
• iptr must point to valid writable memory (not NULL).
• NaN input → NaN returned; integral part unspecified.
• ±∞ input → returns ±0; integral part is ±∞.
• Not the same as fmod() or integer %.
• Link with -lm on GCC/Clang Unix-like builds.

⚡ Optimization

modf() is implemented in the platform math library and is typically fast. For very hot paths you might cast to integer types when you know the range fits—but modf handles negatives, large magnitudes, and IEEE edge cases correctly. Profile before replacing it.

💡 Best Practices