How Can I Implement a Rounding Function in C ?

Rounding Floating-Point Numbers in C with round()

In the absence of a dedicated round() function in the standard C library, developers often resort to other functions like ceil() or floor(). However, for precision control, a specific rounding function is often required.

Implementing a Custom round() Function

The round() function rounds a floating-point number based on the "round-half-up" rule, where numbers exactly halfway between integers are rounded up. The following C code provides an implementation of this function:

double round(double d) {
  return floor(d + 0.5);
}

This implementation essentially adds 0.5 to the number and truncates the result to the nearest integer using the floor() function.

Other Rounding Methods

While the "round-half-up" rule is commonly used, there are other rounding methods, including:

• Round-to-Even: This method rounds numbers halfway between integers to the nearest even number, reducing bias.
• Round-Downward: This method always rounds numbers towards negative infinity.
• Round-Upward: This method always rounds numbers towards positive infinity.

Implementations of these methods can be found online or created custom based on specific requirements.

Note:

The C 11 standard introduced built-in round() functions (std::round, std::lround, and std::llround) that provide similar functionality, making a custom implementation unnecessary for modern compilers and standard libraries.

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