Let's use the example function f(x) = x^2
This should be very familiar to you.
If i gave you mathematical instructions to solve the values for a and z, could you do it?
a= f(2)
y = 5
z = f(y)
What is the value of a and z?
a = 4
z =25
You got the answer by using the above function. The function f(2) got replaced by whatever f(2) returned.(In this case it was 4 because x was replaced by the number 2)
In the case of f(y), y is equal to 5 so x in f(x) get's replaced by 5 and f(y) returned the value 25.
/* File: FirstFunction.c By: Christopher Manloloyo This Program is used to learn about functions The function that is implemented is f(x)=x^2 for integers*/#include<stdio.h>int f(int x); // This is known as the prototypeint main(){ // declare variables int a, y, z; a = f(2); // the first function call y = 5; z = f(y); // the second function call /* Printing the Results To the Screen */ printf("a = %d \n", a); printf("z = %d \n", z);}//notice the main function ends above//The function f starts hereint f(int x) // This part is the function definition{ int result; result = x*x; return result // Alternatively, you could just use the code below // return x*x;}