C:

Code:
#include <stdio.h>

int add(int a0, int a1);
int sub(int a0, int a1);
int mult(int a0, int a1);
int div(int a0, int a1);

int selection, num1, num2;

int main(){system("cls");
    printf("Select your operation:\n");
    printf("1 - Addition\n2 - Subtraction\n3 - Mulitplication\n4 - Division\n\n");
    scanf("%d", &selection);
    system("cls");
    printf("Number one: ");
    scanf("%d", &num1);
    printf("Number two: ");
    scanf("%d", &num2);
    printf("Result: ");
    if (selection == 1){
                  printf("%d", add(num1,num2));
                  }
    if (selection == 2){
                  printf("%d", sub(num1,num2));
                  }
    if (selection == 3){
                  printf("%d", mult(num1,num2));
                  }
    if (selection == 4){
                  printf("%d", div(num1,num2));
                  }
    getch();
    return main();
}

int add(int a0, int a1){
    return a0 + a1;
}

int sub(int a0, int a1){
    return a0 - a1;
}

int mult(int a0, int a1){
    return a0 * a1;
}

int div(int a0, int a1){
    return a0 / a1;
}
C++:

Code:
#include <iostream>
using namespace std;

int add(int a0, int a1);
int sub(int a0, int a1);
int mult(int a0, int a1);
int divi(int a0, int a1);

int selection, num1, num2;

int main(){system("cls");
    cout << "Select your operation:\n";
    cout << "1 - Addition\n2 - Subtraction\n3 - Multiplication\n4 - Division\n\n";
    cin >> selection;
    system("cls");
    cout << "Number one: ";
    cin >> num1;
    cout << "Number two: ";
    cin >> num2;
    cout << "Result: ";
    if (selection == 1){
                  cout << add(num1,num2);
                  }
    if (selection == 2){
                  cout << sub(num1,num2);
                  }
    if (selection == 3){
                  cout << mult(num1,num2);
                  }
    if (selection == 4){
                  cout << divi(num1,num2);
                  }
    cout << "\n";
    system("pause");
    return main();
}

int add(int a0, int a1){
    return a0 + a1;
}

int sub(int a0, int a1){
    return a0 - a1;
}

int mult(int a0, int a1){
    return a0 * a1;
}

int divi(int a0, int a1){
    return a0 / a1;
}
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