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Functions គឺជាបណ្ដុំកូដ ដែលដំណើរការការងារជាក់លាក់មួយ។

យើងអាចបង្កើត ​function មួយដោយខ្លួនឯងបាន គឺអាស្រ័យដោយ function ចំនួនបី នោះគឺ៖

• Function Declaration : ជា function ដែលត្រូវបានប្រកាសជា global នៅពីលើ main function ។ យើងបង្កើត​ function នេះឡើងដើម្បីប្រាប់​ compiler ឳ្យដឹងថា តើ function ដែលយើងនឹងបង្កើត គឺជា function ប្រភេទអ្វី (function with parameter, return type) ហើយមានឈ្មោះអ្វី?
• Function Call : គឺជា function មួយដែលយើងបង្កើតវាឡើងដើម្បីហៅ function ដែលយើងបានបង្កើតយកមកប្រើ ដើម្បីឳ្យវាដំណើរការនៅកន្លែងដែលយើងបានហៅនោះ ។
• Function Definition : ជា function ដែលផ្ទុកនូវ Code ដែលយើងបានសរសេរទុកសម្រាប់ដំណើរការ។

1. Return Function: ជា function ដែលអាច return/ផ្ទេរ តម្លៃពី function definition ទៅឳ្យ function call។ នៅក្នុង ​return function យើងនឹងសិក្សាទៅលើលក្ខណៈពីរបន្ថែមទៀតគឺ Return Function with Parametersនិង​ Return Function with no parameters។
2. Non-Return Function: ជា function ដែលមិនអាច return តម្លៃចេញទៅឳ្យ function ផ្សេងបានទេ។ នៅក្នុង ​non-return function ក៏មានលក្ខណៈពីរផងដែរគឺ Non-Return Function with Parameters និង​ Non-Return Function with no parameters។ ជាទូទៅ non-return function គឺផ្ដើមដោយ keyword “void”។

In C programming, a function is defined as a self-contained block of statements that perform a coherent task of some kind. Every C program can be designed in terms of functions, and the return statement in a function is used to exit from the function and go back to the point from where it was called. The return statement can also return a value from the function to the calling function.

Here is a simple example to illustrate how functions and the return statement work in C:

Example: Function to Add Two Numbers

#include <stdio.h>

// Function declaration
int addNumbers(int a, int b);

int main() {
    int num1, num2, sum;
    
    // Input numbers from user
    printf("Enter two numbers: ");
    scanf("%d %d", &num1, &num2);
    
    // Function call
    sum = addNumbers(num1, num2);
    
    // Output the returned value
    printf("Sum = %d", sum);
    
    return 0;
}

// Function definition
int addNumbers(int a, int b) {
    int result;
    result = a + b;
    
    // Return statement
    return result;
}

Explanation:

1. Function Declaration: The function addNumbers(int a, int b) is declared before the main() function. This declaration tells the compiler about the function’s name, return type, and parameters.
2. Function Call: Inside the main() function, addNumbers is called with num1 and num2 as arguments. The function computes the sum of these numbers and returns it.
3. Return Statement: The return result; statement in addNumbers function sends the value of result back to the caller, which is then assigned to the variable sum in the main() function.
4. Output: The program prompts the user to enter two numbers, computes their sum through a function call, and prints the result.

This example demonstrates how functions can be used to organize and reuse code effectively in C programming, as well as how values can be returned from a function to the calling code using the return statement.

#include <stdio.h>

// Function to draw a horizontal line
void drawLine(int length) {
    for(int i = 0; i < length; i++) {
        printf("*");
    }
    printf("\n");
}

int main() {
    // Calling the drawLine function
    drawLine(10); // Draws a line of 10 asterisks

    return 0;
}

#include <stdio.h>

// Function declarations
void showMenu();
int add(int a, int b);
int subtract(int a, int b);
int multiply(int a, int b);
float divide(int a, int b);
void processChoice(int choice);

int main() {
    int choice;

    do {
        showMenu(); // Display menu
        printf("Enter your choice (1-6): ");
        scanf("%d", &choice);

        if (choice >= 1 && choice <= 5) {
            processChoice(choice); // Process the chosen operation
        } else if (choice == 6) {
            printf("Exiting program...\n");
        } else {
            printf("Invalid choice, please enter a number between 1 and 6.\n");
        }
    } while (choice != 6); // Continue until the user chooses to exit

    return 0;
}

// Function definitions
void showMenu() {
    printf("Menu:\n");
    printf("1. Add\n");
    printf("2. Subtract\n");
    printf("3. Multiply\n");
    printf("4. Divide\n");
    printf("5. Print Hello\n");
    printf("6. Exit\n");
}

int add(int a, int b) {
    return a + b;
}

int subtract(int a, int b) {
    return a - b;
}

int multiply(int a, int b) {
    return a * b;
}

float divide(int a, int b) {
    if (b != 0) {
        return (float)a / b;
    } else {
        printf("Error: Division by zero.\n");
        return 0;
    }
}

void processChoice(int choice) {
    int a, b;
    if (choice >= 1 && choice <= 4) {
        printf("Enter two numbers: ");
        scanf("%d %d", &a, &b);
    }

    switch (choice) {
        case 1:
            printf("Result: %d\n", add(a, b));
            break;
        case 2:
            printf("Result: %d\n", subtract(a, b));
            break;
        case 3:
            printf("Result: %d\n", multiply(a, b));
            break;
        case 4:
            printf("Result: %.2f\n", divide(a, b));
            break;
        case 5:
            printf("Hello\n");
            break;
        default:
            // This case should never be reached because of the input validation
            printf("Error: Invalid operation.\n");
    }
}

Explanation:

• The showMenu function displays the options available to the user and does not return any value (void type).
• The add, subtract, multiply, and divide functions perform basic arithmetic operations and return the result. The divide function returns a float to accommodate division results, and it handles division by zero.
• The processChoice function takes the user’s choice, prompts for input if required, performs the chosen operation by calling the appropriate function, and displays the result. This function is also of type void as it does not return any value.
• The main function displays the menu in a loop, reads the user’s choice, and calls processChoice to handle it, until the user chooses to exit the program by entering 6.

s

#include <stdio.h>

// Function declarations
void showMenu();
double kilometersToMiles(double kilometers);
double milesToKilometers(double miles);
double inchesToCentimeters(double inches);
double metersToKilometers(double meters);
double litersToKilograms(double liters); // This conversion depends on the substance. Assuming water for simplicity.
void processChoice(int choice);

int main() {
    int choice;

    do {
        showMenu(); // Display menu
        printf("Enter your choice (1-6): ");
        scanf("%d", &choice);

        if (choice >= 1 && choice <= 5) {
            processChoice(choice); // Process the chosen operation
        } else if (choice == 6) {
            printf("Exiting program...\n");
        } else {
            printf("Invalid choice, please enter a number between 1 and 6.\n");
        }
    } while (choice != 6); // Continue until the user chooses to exit

    return 0;
}

// Function to show the conversion menu
void showMenu() {
    printf("\nConversion Menu:\n");
    printf("1. Kilometers to Miles\n");
    printf("2. Miles to Kilometers\n");
    printf("3. Inches to Centimeters\n");
    printf("4. Meters to Kilometers\n");
    printf("5. Liters to Kilograms (water)\n");
    printf("6. Exit\n");
}

// Conversion functions
double kilometersToMiles(double kilometers) {
    return kilometers * 0.621371;
}

double milesToKilometers(double miles) {
    return miles / 0.621371;
}

double inchesToCentimeters(double inches) {
    return inches * 2.54;
}

double metersToKilometers(double meters) {
    return meters / 1000.0;
}

// Assuming the conversion for water: 1 liter of water is approximately 1 kilogram.
double litersToKilograms(double liters) {
    return liters; // For substances other than water, this conversion would be different.
}

// Function to process user choice
void processChoice(int choice) {
    double input, result;

    switch (choice) {
        case 1:
            printf("Enter distance in Kilometers: ");
            scanf("%lf", &input);
            result = kilometersToMiles(input);
            printf("%.2f Kilometers = %.2f Miles\n", input, result);
            break;
        case 2:
            printf("Enter distance in Miles: ");
            scanf("%lf", &input);
            result = milesToKilometers(input);
            printf("%.2f Miles = %.2f Kilometers\n", input, result);
            break;
        case 3:
            printf("Enter length in Inches: ");
            scanf("%lf", &input);
            result = inchesToCentimeters(input);
            printf("%.2f Inches = %.2f Centimeters\n", input, result);
            break;
        case 4:
            printf("Enter length in Meters: ");
            scanf("%lf", &input);
            result = metersToKilometers(input);
            printf("%.2f Meters = %.2f Kilometers\n", input, result);
            break;
        case 5:
            printf("Enter volume in Liters: ");
            scanf("%lf", &input);
            result = litersToKilograms(input);
            printf("%.2f Liters of water = %.2f Kilograms\n", input, result);
            break;
        case 6:
            printf("Exiting program...\n");
            break;
        default:
            printf("Invalid choice, please enter a number between 1 and 6.\n");
    }
}

Notes:

• This program assumes the conversion from liters to kilograms is for water, where 1 liter of water is approximately equal to 1 kilogram. For other substances, the density would need to be considered for an accurate conversion.
• The showMenu function displays the available conversions to the user.
• The processChoice function handles user input and output based on the selected conversion operation.
• Each conversion function takes an input value, performs the conversion, and returns the result.