Significance of user defined functions

A user-defined function is a function written by the programmer to do a particular job, apart from the library functions. Its significance:

• Divide and conquer: a large problem is split into smaller functions, each easy to code and test.
• Code reuse: the same function is called whenever needed instead of copying the code.
• Easier debugging and updating: a change is made in one place only.
• Better readability: meaningful names show what each part does.
• Hides detail: the caller needs to know only what the function does, not how.
• Team development: different people can write different functions.

Example:

#include <stdio.h>
float area_circle(float r) { return 3.1416f * r * r; }
float area_rect(float l, float b) { return l * b; }
int main(void)
{
    printf("Circle: %.2f\n", area_circle(3));
    printf("Rectangle: %.2f\n", area_rect(4, 5));
    printf("Circle: %.2f\n", area_circle(1.5f));   /* reuse */
    return 0;
}

Output:

Circle: 28.27
Rectangle: 20.00
Circle: 7.07

area_circle() is written once and is used twice with different data.