Rounding numbers

How to:

Rounding numbers in C can be accomplished using various functions, but the most common approach involves the floor(), ceil(), and round() functions. These functions are part of the standard math library, so you will need to include math.h in your program.

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

int main() {
    double num = 9.527;

    // Using floor() to round down
    double floorResult = floor(num);
    printf("floor(9.527) = %.0f\n", floorResult);

    // Using ceil() to round up
    double ceilResult = ceil(num);
    printf("ceil(9.527) = %.0f\n", ceilResult);

    // Using round() to round to the nearest integer
    double roundResult = round(num);
    printf("round(9.527) = %.0f\n", roundResult);

    // Rounding to a specified number of decimal places involves multiplication and division
    double twoDecimalPlaces = round(num * 100) / 100;
    printf("Rounding to two decimal places: %.2f\n", twoDecimalPlaces);

    return 0;


floor(9.527) = 9
ceil(9.527) = 10
round(9.527) = 10
Rounding to two decimal places: 9.53

Deep Dive

Rounding numbers has deep historical roots in mathematics and computation, integral to both theoretical and applied aspects. In C, while floor(), ceil(), and round() offer basic functionality, the essence of rounding floats to integers or specific decimal places is more nuanced due to the binary representation of floating-point numbers. This representation can lead to unexpected results due to how numbers that can’t be precisely represented in binary (like 0.1) are handled.

These functions are part of the C standard library, defined in <math.h>. When rounding numbers, especially for financial or precise engineering calculations, one must consider the implications of using binary floating-point numbers. Alternatives to the built-in C functions for highly accurate or decimal-specific rounding might include implementing custom rounding functions or using libraries designed for arbitrary-precision arithmetic, like GMP or MPFR, though these introduce additional complexity and dependencies.

In practice, choosing the right approach to rounding in C involves balancing the need for precision, performance, and practicality, with a keen understanding of the domain-specific requirements of the application being developed.