Start by defining a function for each arithmetic operation. This modular approach helps in maintaining and understanding the code easily.

#include <stdio.h>

double add(double a, double b) { return a + b; }
double subtract(double a, double b) { return a - b; }
double multiply(double a, double b) { return a * b; }
double divide(double a, double b) { return b != 0 ? a / b : 0.0; }

int main() {
    double num1, num2;
    char operator;
    printf("Enter an operation (+, -, *, /): ");
    scanf("%c", &operator);
    printf("Enter two numbers: ");
    scanf("%lf %lf", &num1, &num2);

    switch(operator) {
        case '+': printf("Result: %.2lf", add(num1, num2)); break;
        case '-': printf("Result: %.2lf", subtract(num1, num2)); break;
        case '*': printf("Result: %.2lf", multiply(num1, num2)); break;
        case '/': printf("Result: %.2lf", divide(num1, num2)); break;
        default: printf("Invalid operation!");
    }
    return 0;
}
            

This program asks the user for an operation and two numbers, then calculates the result based on the operation using a switch-case statement.

Compile the code using GCC or your preferred C compiler:

gcc -o calculator calculator.c

Execute your program to use the calculator:

./calculator

This will allow you to perform different arithmetic calculations based on your input.