This program is an example of using functions. It teaches you how to declare function, define it and call. Here functions for calculating factorial and permutation/combination is used. Once you see the code you will be able to understand following

- Function declaration, definition, calls.
- Use of switch statement in program.
- How to write structured program.
- How to make menu in C

The complete source code and output is given here…

**Source Code**

//calculation of permutation and combination using function in C#include<stdio.h>#include<conio.h> //for getch() function//function declaration must be//written before main function,//function definition is optional//you can write befor or after mainint factorial(int n){int fact = 1;int i;for(i = n; i > 0; i--){fact = fact*i;}return fact;}int permutation(int n,int r){int per;per = factorial(n)/(factorial(n-r));return per;}int combination(int n,int r){int comb; //local variablecomb = factorial(n)/(factorial(n-r)*factorial(r));return comb; //return statement}int main(){int choice;int n,r;while(1){printf("\n1. Permutation\n2. Combination\n3. Factorial\n");printf("Choose option: ");scanf("%d",&choice);switch(choice){ //start of switch statementcase 1:printf("Enter value of n,r: ");scanf("%d%d",&n,&r);//calculation of combination with function callprintf("Permutation P(%d,%d) = %d",n,r,permutation(n,r));break;case 2:printf("Enter value of n,r: ");scanf("%d%d",&n,&r);printf("Combination C(%d,%d) = %d",n,r,combination(n,r));break;case 3:printf("Enter value of n: ");scanf("%d",&n);printf("The factorial %d! = %d",n,factorial(n));break;default: //if user enter other than 1 or 2printf("Invalid choice! Enter either 1 or 2");} //end of switch statementprintf("\nWant another calculation?(y/n):");if(getche()=='n')break;}return 0;}//end of main

**Output**

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