Command-Line Menu System in C

Introduction

Command-line interfaces (CLI) are simple but can be made more user-friendly by adding menus. This example demonstrates a menu-driven program in C that allows the user to select from multiple options, perform tasks, and return to the main menu.

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Full Source Code (main.c)

// ELEC2645 Unit 2 Activity 2 - Menu-Driven Program
#include <stdio.h>   // for printf(), scanf()
#include <stdlib.h>  // for exit()

// ---------- Function Prototypes ----------
void main_menu(void);
int get_user_input(void);
void select_menu_item(int input);
void print_main_menu(void);
void go_back_to_main(void);
void menu_item_1(void); // Square a number
void menu_item_2(void); // ASCII code -> character
void menu_item_3(void); // Celsius -> Fahrenheit
void menu_item_4(void); // Even/Odd

#define PI 3.14159f

// ---------- Main Function ----------
int main(void) {
    main_menu();
    return 0;
}

// ---------- Menu Functions ----------
void main_menu(void) {
    print_main_menu();
    int input = get_user_input();
    select_menu_item(input);
}

int get_user_input(void) {
    int input;
    printf("\nSelect item: ");
    scanf("%d", &input); // assume valid input for this activity
    return input;
}

void select_menu_item(int input) {
    switch (input) {
        case 1: menu_item_1(); break;
        case 2: menu_item_2(); break;
        case 3: menu_item_3(); break;
        case 4: menu_item_4(); break;
        case 6: printf("Exiting program...\n"); exit(0);
        default: printf("Invalid selection. Exiting...\n"); exit(1);
    }
}

void print_main_menu(void) {
    printf("\n----------- Main menu -----------\n");
    printf("\n\t1. Square a number\t\n");
    printf("\n\t2. ASCII code -> char\t\n");
    printf("\n\t3. Celsius -> Fahrenheit\n");
    printf("\n\t4. Even or Odd\t\t\n");
    printf("\n\t6. Exit\t\t\t\n");
    printf("---------------------------------\n");
}

void go_back_to_main(void) {
    char input;
    do {
        printf("\nEnter 'b' or 'B' to go back to main menu: ");
        scanf(" %c", &input);
    } while (input != 'b' && input != 'B');
    main_menu();
}

// ---------- Menu Item Functions ----------
void menu_item_1(void) {
    printf("\n>> Menu 1: Square a number\n");
    printf("Enter a number to square:\n");
    int num;
    scanf("%d", &num);
    printf("Result: %d\n", num * num);
    go_back_to_main();
}

void menu_item_2(void) {
    printf("\n>> Menu 2: ASCII code to character\n");
    int code;
    printf("Enter ASCII code (e.g. 65 for 'A'):\n");
    scanf("%d", &code);
    if (code < 0 || code > 127) {
        printf("Code out of ASCII range (0-127)\n");
    } else {
        printf("Character: '%c'\n", (char)code);
    }
    go_back_to_main();
}

void menu_item_3(void) {
    printf("\n>> Menu 3: Celsius to Fahrenheit\n");
    int celsius;
    printf("Enter temperature in Celsius:\n");
    scanf("%d", &celsius);
    float fahrenheit = (celsius * 9.0f / 5.0f) + 32.0f;
    printf("%d°C = %.2f°F\n", celsius, fahrenheit);
    go_back_to_main();
}

void menu_item_4(void) {
    printf("\n>> Menu 4: Even or Odd\n");
    int num;
    printf("Enter an integer:\n");
    scanf("%d", &num);
    if (num % 2 == 0) {
        printf("%d is Even\n", num);
    } else {
        printf("%d is Odd\n", num);
    }
    go_back_to_main();
}

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Detailed Explanation of Each Function

main()

• Entry point of the program.
• Calls main_menu() to start the menu loop.

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main_menu()

• Displays the menu using print_main_menu().
• Gets user input via get_user_input().
• Passes the input to select_menu_item() for processing.

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print_main_menu()

• Prints a formatted menu with options:
  • 1: Square a number
  • 2: ASCII code → character
  • 3: Celsius → Fahrenheit
  • 4: Even or Odd
  • 6: Exit program

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get_user_input()

• Prompts the user to enter an integer.
• Uses scanf() to read the input.
• Returns the integer to main_menu().

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select_menu_item(int input)

• Uses a switch statement to call the correct function based on user input.
• If input is invalid, prints an error and exits.

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go_back_to_main()

• After completing an operation, prompts the user to enter 'b' or 'B' to return to the main menu.
• Loops until valid input is given.
• Calls main_menu() again.

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Menu Item Functions

• menu_item_1(): Squares an integer entered by the user.
• menu_item_2(): Converts an ASCII code (0–127) to its corresponding character.
• menu_item_3(): Converts Celsius temperature to Fahrenheit using the formula: F = C x 9\5 + 32
• menu_item_4(): Checks if an integer is even or odd using num % 2.

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Example Output

----------- Main menu -----------
    1. Square a number
    2. ASCII code -> char
    3. Celsius -> Fahrenheit
    4. Even or Odd
    6. Exit
---------------------------------
Select item: 1
>> Menu 1: Square a number
Enter a number to square:
Result: 25
Enter 'b' or 'B' to go back to main menu:

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