Linker Error/Undefined Reference

I know there's a million of these but I've been trying to work through it for a while and can't grasp it.

Basically have to make a calculator using functions for the basic mathematical operations.

Here's a small bit of my code, and I'm getting the same error for each of my switch statements so one answer should work for all.

Code:

#include <stdio.h>
#include <stdlib.h>

     int add(int, int); //addition function prototype
     int subtract(int, int); //subtraction function prototype
     int multiply(int, int); //multiply function prototype
     int divide(int, int); //divide function prototype
     int modulus(int, int); //modulus function prototype
     int primeNumber(int); //test prime function prototype
     int power(int, int); //exponent function prototype
     int factorial(int); //factorial function prototype

int main()
{
      
      /*Declare/Initialize Variables*/
     int iSelection = 0; //Menu selection
     int iFirstDigit = 0;  //First digit entered
     int iSecondDigit = 0; //Second digit entered
     
while (iSelection !=10)
{
     
     /*Begin Calculator*/
     printf("\n                Calculator\n\n");
     
     printf("1   Addition\n");
     printf("2   Subtraction\n");
     printf("3   Multiplication\n");
     printf("4   Division\n");
     printf("5   Modulus (Integers Only)\n");
     printf("6   Test If Prime (Integers Only)\n");
     printf("7   Factorial (Integers Only)\n");
     printf("8   Power\n");
     printf("10  Exit Calculator\n");
     
     /*Prompt User To Begin Interaction*/
     printf("\nPlease select one of the options (1-7): ");
     scanf("&#37;d", &iSelection);


     switch(iSelection) //begin switch
     {
          case 1:
                 printf("\nPlease enter first digit to be added: ");
                 scanf("%d", &iFirstDigit);
                 printf("\nPlease enter second digit to be added: ");
                 scanf("%d", &iSecondDigit);
                 printf("\nThe sum of the two numbers is %d\n\n", add(iFirstDigit, iSecondDigit));
     
          int add (int iFirstDigit, int iSecondDigit)
               {
                  return iFirstDigit + iSecondDigit;
               }
               break; //end case 1

My compile error is

[Linker error]: undefined reference to 'add'



I'm assuming it has something to do with how I've defined/prototyped my function or something along those lines, but I can't spot the error myself.

I've looked through past threads of this nature and couldn't really find the solution. Any help at all is appreciated.

Yes, you have to write those functions add(), subtract(), etc. I think you've started doing that with the add() function - but it's embedded inside the case statement. You need to move that code outside of the main(). At the top I would put them. Then they act as prototypes and you don't have a need for the prototype statements. Or you can place the functions AFTER the main. Then you DO need the prototype statements.

Code:

int add (int iFirstDigit, int iSecondDigit)
               {
                  return iFirstDigit + iSecondDigit;
               }

int subtract...

int main()
{
      
 ...

Thank you thank you

So simple -.-