A Very Gentle Introduction to C

Jyotiprakash MishraJyotiprakash Mishra
24 min read

Welcome to the fascinating world of C programming, where simplicity meets power. C, created by Dennis Ritchie at Bell Labs in the early 1970s, has stood the test of time as one of the most influential and widely used programming languages. Its elegant design and versatility have made it a cornerstone in the development of operating systems, embedded systems, and countless applications. Whether you're a beginner embarking on your programming journey or a seasoned developer exploring a new language, C provides a solid foundation and a rich learning experience. In this gentle introduction, we'll navigate the basics of C programming, demystifying its syntax and exploring its fundamental concepts, setting you on a path to unlock the full potential of this timeless language.

Below is a simple "Hello, World!" program written in C:

#include <stdio.h>

int main() {
    // This is a comment. Comments are ignored by the compiler.

    /*
    This is a multi-line comment.
    It is also ignored by the compiler.
    */

    // printf is a function that prints text to the console.
    printf("Hello, World!\n");

    // The return statement indicates the end of the main function.
    return 0;
}

Now, let's break down the parts of this program:

  1. #include <stdio.h>: This line is a preprocessor directive that tells the compiler to include the standard input/output library (stdio.h). This library provides functions like printf that are used for input and output operations.

  2. int main() { ... }: The main function is the entry point of every C program. It's where the program begins its execution. The int before main indicates that the function returns an integer value.

  3. // This is a comment... and /* ... */: Comments are ignored by the compiler and are used to provide explanations or notes in the code. They improve code readability and understanding.

  4. printf("Hello, World!\n");: This line uses the printf function to print the text "Hello, World!" to the console. The \n represents a newline character, which moves the cursor to the beginning of the next line after printing.

  5. return 0;: The return statement ends the main function and returns an integer value to the operating system. Conventionally, a return value of 0 indicates successful execution, while a non-zero value signals an error.

To compile and run the C program using the Bash command line and GCC (GNU Compiler Collection), follow these steps:

  1. Open a text editor and copy the "Hello, World!" C program into a new file. Save the file with a .c extension, for example, hello.c.

  2. Open a terminal or command prompt.

  3. Navigate to the directory where you saved your C file using the cd command. For example:

     cd path/to/directory

  4. Compile the C program using the gcc command:

     gcc -o hello hello.c

    The -o option is used to specify the name of the output file. In this case, it's set to hello.

  5. After successful compilation, you can run the executable:

     ./hello

    The ./ before hello is necessary to execute the compiled program in the current directory.

  6. The output on the console should be:

     Hello, World!

Explanation:

  • The gcc command is the GNU Compiler Collection and is used to compile C programs.

  • The -o option allows you to specify the name of the output (executable) file.

  • ./hello executes the compiled program named hello.

  • The program then prints "Hello, World!" to the console.

In the world of coding, think of variables as containers that hold important information. These containers have names, making it easier for us to organize and use data in our programs. Imagine you're making a recipe, and you need a place to store the quantity of ingredients or the cooking time – that's what variables do for a program. In C, understanding variables helps us manage numbers, letters, and other types of data smartly.

Now, the cool thing about C is that it lets us specify what kind of information our containers can hold. For example, a variable can be designed to store whole numbers (like 5 or 100), decimal numbers (like 3.14), or even letters and words. This idea of categorizing data is called "data types," and it helps us work with information in a precise and efficient manner.

So, as we embark on our C programming journey, diving into the world of variables and types will be like unlocking a toolbox full of possibilities. It's how we make our programs smart and adaptable, ready to handle all sorts of tasks!

In the realm of C programming, understanding data types is like mastering different tools for various tasks. One crucial tool is the integer, which is perfect for handling whole numbers. In this beginner-friendly journey, we'll explore how to use integers in C by creating a program that multiplies a number by 10. This simple exercise will lay the foundation for grasping how C manages and manipulates data.

C Program:

#include <stdio.h>

int main() {
    // We declare an integer variable named 'myNumber' and initialize it to 2.
    int myNumber = 2;

    // Multiply 'myNumber' by 10.
    myNumber = myNumber * 10;

    // Print the result.
    printf("The result is: %d\n", myNumber);

    // The return statement indicates the end of the main function.
    return 0;
}

Explanation:

  1. #include <stdio.h>: This line includes the standard input/output library, allowing us to use functions like printf for printing to the console.

  2. int main() { ... }: The main function, the starting point of our program. It returns an integer value (0 in this case, indicating successful execution).

  3. int myNumber = 2;: This line declares an integer variable named myNumber and initializes it with the value 2.

  4. myNumber = myNumber * 10;: Here, we multiply the value stored in myNumber by 10 and update its value.

  5. printf("The result is: %d\n", myNumber);: The printf function is used to print the result to the console. The %d is a format specifier for integers.

  6. return 0;: Indicates that the program has executed successfully.

When you run this program, it will output:

The result is: 20

In the world of C programming, initializing variables can be done in various ways, offering flexibility to programmers.

C Program:

#include <stdio.h>

int main() {
    // Method 1: Initializing a single integer
    int num1 = 5;

    // Method 2: Initializing multiple integers on the same line
    int num2 = 10, num3 = 15;

    // Method 3: Initializing variables separately
    int num4;
    num4 = 20;

    // Method 4: Initializing with arithmetic expressions
    int result = 2 * 3 + 7;

    // Print the initialized values
    printf("Method 1: %d\n", num1);
    printf("Method 2: %d, %d\n", num2, num3);
    printf("Method 3: %d\n", num4);
    printf("Method 4: %d\n", result);

    // The return statement indicates the end of the main function.
    return 0;
}

Explanation:

  1. Method 1: int num1 = 5;

    • Here, a single integer num1 is declared and initialized with the value 5.

  2. Method 2: int num2 = 10, num3 = 15;

    • Multiple integers, num2 and num3, are declared and initialized on the same line.

  3. Method 3:

     int num4;
 num4 = 20;
    • num4 is declared first, and later, it's assigned the value 20.

  4. Method 4: int result = 2 * 3 + 7;

    • The integer result is initialized with the result of an arithmetic expression.

  5. The printf statements print the values initialized using each method to the console.

When you run this program, the output will be:

Method 1: 5
Method 2: 10, 15
Method 3: 20
Method 4: 13

In the vast landscape of C programming, understanding data types is fundamental to effectively managing and manipulating information. C provides a diverse set of data types, each tailored for specific kinds of data. From integers to characters, floating-point numbers to arrays, these data types empower programmers to create versatile and efficient solutions.

C Program:

#include <stdio.h>

void myFunction();

int main() {
    // Integer data type
    int myInteger = 42;
    printf("Integer: %d\n", myInteger);

    // Floating-point data type
    float myFloat = 3.14;
    printf("Float: %f\n", myFloat);

    // Character data type
    char myChar = 'A';
    printf("Character: %c\n", myChar);

    // Double data type
    double myDouble = 1234.5678;
    printf("Double: %lf\n", myDouble);

    // Array data type
    int myArray[3] = {1, 2, 3};
    printf("Array: %d, %d, %d\n", myArray[0], myArray[1], myArray[2]);

    // String data type (using an array of characters)
    char myString[] = "Hello, C!";
    printf("String: %s\n", myString);

    myFunction();

    // The return statement indicates the end of the main function.
    return 0;
}

// Void data type (used with functions that return nothing
void myFunction() {
    printf("This function does not return anything.\n");
}

Explanation:

  1. Integer (int):

    • int myInteger = 42; declares an integer variable myInteger and initializes it with the value 42.

  2. Floating-point (float):

    • float myFloat = 3.14; declares a floating-point variable myFloat and initializes it with the value 3.14.

  3. Character (char):

    • char myChar = 'A'; declares a character variable myChar and initializes it with the character 'A'.

  4. Double (double):

    • double myDouble = 1234.5678; declares a double-precision floating-point variable myDouble and initializes it with the value 1234.5678.

  5. Array:

    • int myArray[3] = {1, 2, 3}; declares an array of integers myArray with three elements and initializes them with the values 1, 2, and 3.

  6. String (Array of Characters):

    • char myString[] = "Hello, C!"; declares a character array myString and initializes it with the string "Hello, C!".

  7. Void (void):

    • void myFunction() { ... } declares a function named myFunction that doesn't return any value.

    • myFunction(); calls the function.

Mastering input and output (I/O) is a crucial step. This involves getting information from users and displaying results. Two essential functions, printf and scanf, serve as the backbone of these operations.

C Program:

#include <stdio.h>

int main() {
    // Declare variables to store user input
    int userAge;
    float userHeight;

    // Prompt user for input
    printf("Enter your age: ");

    // Use scanf to read an integer, note the use of & before the variable
    scanf("%d", &userAge);

    // Prompt user for another input
    printf("Enter your height in meters: ");

    // Use scanf to read a floating-point number, note the use of & before the variable
    scanf("%f", &userHeight);

    // Display user's information using printf
    printf("You entered:\n");
    printf("Age: %d years\n", userAge);
    printf("Height: %.2f meters\n", userHeight);

    // The return statement indicates the end of the main function.
    return 0;
}

Explanation:

  1. Variable Declaration:

    • int userAge; declares an integer variable to store the user's age.

    • float userHeight; declares a floating-point variable to store the user's height.

  2. User Input using scanf:

    • printf("Enter your age: "); prompts the user to enter their age.

    • scanf("%d", &userAge); reads an integer from the user and stores it in the userAge variable. Note the use of & before userAge, which is the address-of operator required by scanf.

    • printf("Enter your height in meters: "); prompts the user to enter their height.

    • scanf("%f", &userHeight); reads a floating-point number from the user and stores it in the userHeight variable. Again, the & operator is used.

  3. User Output using printf:

    • printf("You entered:\n"); displays a message.

    • printf("Age: %d years\n", userAge); displays the user's age using %d as the format specifier for integers.

    • printf("Height: %.2f meters\n", userHeight); displays the user's height using %f as the format specifier for floating-point numbers, with .2 specifying two decimal places.

When you run this program, it prompts the user for age and height, reads the input, and then displays the entered information back to the user.

Arithmetic operations are the building blocks of any programming language, providing the means to perform fundamental mathematical calculations.

C Program:

#include <stdio.h>

int main() {
    // Declare variables for operands
    int num1 = 10, num2 = 5;

    // Addition
    int sum = num1 + num2;
    printf("Sum: %d\n", sum);

    // Subtraction
    int difference = num1 - num2;
    printf("Difference: %d\n", difference);

    // Multiplication
    int product = num1 * num2;
    printf("Product: %d\n", product);

    // Division
    float quotient = (float)num1 / num2; // Casting to float for precise division
    printf("Quotient: %.2f\n", quotient);

    // Modulo
    int remainder = num1 % num2;
    printf("Remainder: %d\n", remainder);

    // The return statement indicates the end of the main function.
    return 0;
}

Explanation:

  1. Variable Declaration:

    • int num1 = 10, num2 = 5; declares two integer variables, num1 and num2, and initializes them with values.

  2. Addition (+):

    • int sum = num1 + num2; adds num1 and num2 and stores the result in the variable sum.

  3. Subtraction (-):

    • int difference = num1 - num2; subtracts num2 from num1 and stores the result in the variable difference.

  4. Multiplication (*):

    • int product = num1 * num2; multiplies num1 by num2 and stores the result in the variable product.

  5. Division (/):

    • float quotient = (float)num1 / num2; divides num1 by num2 and stores the result in the variable quotient. Note the casting of num1 to float to ensure precise division.

  6. Modulo (%):

    • int remainder = num1 % num2; calculates the remainder when num1 is divided by num2 and stores the result in the variable remainder.

  7. Printing Results using printf:

    • printf("Sum: %d\n", sum); prints the sum using %d as the format specifier for integers.

    • Similar printf statements are used for other operations.

Decision-making is a fundamental concept that allows your programs to respond intelligently to different situations. Conditional statements, such as "if-then" and "if-then-else," are the tools that empower your programs to make choices based on certain conditions.

C Program:

#include <stdio.h>

int main() {
    // Declare a variable for user input
    int userAge;

    // Prompt user for age
    printf("Enter your age: ");
    scanf("%d", &userAge);

    // If-then statement
    if (userAge >= 18) {
        printf("You are eligible to vote!\n");
    }

    // If-then-else statement
    if (userAge >= 21) {
        printf("You are eligible to purchase alcohol.\n");
    } else {
        printf("You cannot purchase alcohol yet.\n");
    }

    // The return statement indicates the end of the main function.
    return 0;
}

Explanation:

  1. Variable Declaration:

    • int userAge; declares an integer variable to store the user's age.

  2. User Input using scanf:

    • printf("Enter your age: "); prompts the user to enter their age.

    • scanf("%d", &userAge); reads an integer from the user and stores it in the userAge variable.

  3. If-then Statement:

    • if (userAge >= 18) { ... } checks if the user's age is greater than or equal to 18.

    • If the condition is true, the code inside the curly braces {} is executed, printing a message indicating eligibility to vote.

  4. If-then-else Statement:

    • if (userAge >= 21) { ... } else { ... } checks if the user's age is greater than or equal to 21.

    • If the condition is true, the code inside the first set of curly braces is executed, printing a message about eligibility to purchase alcohol.

    • If the condition is false, the code inside the second set of curly braces (the else block) is executed, printing a message about not being able to purchase alcohol.

In C, conditions are evaluated based on whether the result is zero (false) or non-zero (true). For example, in our program, userAge >= 18 evaluates to true if the user's age is 18 or older.

Efficient decision-making often involves comparing values and performing actions based on the outcomes. Relational operators are the tools that allow us to make these comparisons, evaluating whether one value is greater than, less than, equal to, or different from another. Additionally, the ternary operator provides a concise way to express conditional statements in a single line, streamlining the code.

C Program:

#include <stdio.h>

int main() {
    // Declare two variables for user input
    int num1, num2;

    // Prompt user for input
    printf("Enter two numbers separated by a space: ");
    scanf("%d %d", &num1, &num2);

    // Relational Operators
    printf("Relational Operators:\n");
    printf("%d > %d is %d\n", num1, num2, num1 > num2);
    printf("%d < %d is %d\n", num1, num2, num1 < num2);
    printf("%d >= %d is %d\n", num1, num2, num1 >= num2);
    printf("%d <= %d is %d\n", num1, num2, num1 <= num2);
    printf("%d == %d is %d\n", num1, num2, num1 == num2);
    printf("%d != %d is %d\n", num1, num2, num1 != num2);

    // Ternary Operator
    printf("\nTernary Operator:\n");
    int result = (num1 > num2) ? num1 : num2;
    printf("The larger number is: %d\n", result);

    // The return statement indicates the end of the main function.
    return 0;
}

Explanation:

  1. Variable Declaration:

    • int num1, num2; declares two integer variables to store user input.

  2. User Input using scanf:

    • printf("Enter two numbers separated by a space: "); prompts the user to enter two numbers.

    • scanf("%d %d", &num1, &num2); reads two integers from the user and stores them in the num1 and num2 variables.

  3. Relational Operators:

    • The program uses various relational operators (>, <, >=, <=, ==, !=) to compare num1 and num2. The results (true or false) are printed to the console.

  4. Ternary Operator:

    • int result = (num1 > num2) ? num1 : num2; uses the ternary operator (? :) to determine the larger number between num1 and num2. If num1 > num2 is true, result is assigned the value of num1; otherwise, it gets the value of num2.

    • The result is then printed to the console.

Relational operators evaluate the truth of conditions, returning 1 for true and 0 for false. The ternary operator provides a concise way to express conditional assignments, enhancing code readability.

Loops are indispensable constructs that enable the repetition of tasks, allowing for efficient and concise code. Loops serve as the workhorses, executing a set of instructions repeatedly until a certain condition is met. We'll dive into three primary loop structures in C: for, while, and do-while.

C Program:

#include <stdio.h>

int main() {
    // For Loop
    printf("For Loop:\n");
    for (int i = 1; i <= 5; i++) {
        printf("%d ", i);
    }
    printf("\n");

    // While Loop
    printf("While Loop:\n");
    int j = 1;
    while (j <= 5) {
        printf("%d ", j);
        j++;
    }
    printf("\n");

    // Do-While Loop
    printf("Do-While Loop:\n");
    int k = 1;
    do {
        printf("%d ", k);
        k++;
    } while (k <= 5);
    printf("\n");

    // The return statement indicates the end of the main function.
    return 0;
}

Explanation:

  1. For Loop:

    • for (int i = 1; i <= 5; i++) { ... } is a for loop that initializes i to 1, executes the loop body as long as i is less than or equal to 5, and increments i by 1 after each iteration.

    • The loop prints the value of i on each iteration.

  2. While Loop:

    • int j = 1; while (j <= 5) { ... } is a while loop that checks the condition before executing the loop body. It initializes j to 1, and the loop continues as long as j is less than or equal to 5.

    • The loop prints the value of j on each iteration and increments j inside the loop.

  3. Do-While Loop:

    • int k = 1; do { ... } while (k <= 5); is a do-while loop that executes the loop body at least once and then checks the condition. It initializes k to 1, and the loop continues as long as k is less than or equal to 5.

    • The loop prints the value of k on each iteration and increments k inside the loop.

Arrays in C are versatile and fundamental data structures that allow the storage of multiple values of the same data type in a single container. They serve as organized sequences, making it efficient to manage and process a collection of related elements.

C Program:

#include <stdio.h>

int main() {
    // Array Initialization and Index Access
    printf("Array Initialization and Index Access:\n");
    int myArray[5] = {1, 2, 3, 4, 5};

    // Accessing elements using indices
    printf("Element at index 2: %d\n", myArray[2]);

    // Looping to Access and Print Array Elements
    printf("\nLooping to Access and Print Array Elements:\n");
    for (int i = 0; i < 5; i++) {
        printf("%d ", myArray[i]);
    }
    printf("\n");

    // Mutating Array Elements
    printf("\nMutating Array Elements:\n");
    myArray[3] = 99;
    for (int i = 0; i < 5; i++) {
        printf("%d ", myArray[i]);
    }
    printf("\n");

    // Multi-Dimensional Array
    printf("\nMulti-Dimensional Array:\n");
    int multiDimArray[2][3] = {{1, 2, 3}, {4, 5, 6}};

    // Accessing elements using indices
    printf("Element at row 1, column 2: %d\n", multiDimArray[1][2]);

    // Looping to Access and Print Multi-Dimensional Array Elements
    printf("\nLooping to Access and Print Multi-Dimensional Array Elements:\n");
    for (int i = 0; i < 2; i++) {
        for (int j = 0; j < 3; j++) {
            printf("%d ", multiDimArray[i][j]);
        }
        printf("\n");
    }

    // The return statement indicates the end of the main function.
    return 0;
}

Explanation:

  1. Array Initialization and Index Access:

    • int myArray[5] = {1, 2, 3, 4, 5}; declares and initializes an array named myArray with five elements.

    • printf("Element at index 2: %d\n", myArray[2]); accesses and prints the element at index 2.

  2. Looping to Access and Print Array Elements:

    • for (int i = 0; i < 5; i++) { printf("%d ", myArray[i]); } uses a loop to traverse and print all elements of the array.

  3. Mutating Array Elements:

    • myArray[3] = 99; modifies the value at index 3.

    • Another loop is used to print the updated array.

  4. Multi-Dimensional Array:

    • int multiDimArray[2][3] = {{1, 2, 3}, {4, 5, 6}}; declares and initializes a 2x3 multi-dimensional array.

    • printf("Element at row 1, column 2: %d\n", multiDimArray[1][2]); accesses and prints an element from the multi-dimensional array.

  5. Looping to Access and Print Multi-Dimensional Array Elements:

    • Nested loops are employed to traverse and print all elements of the multi-dimensional array.

Structures, often referred to as structs, are powerful data constructs in C that enable the grouping of different data types under a single name. They allow you to create custom data types, enhancing the organization and clarity of your code.

C Program:

#include <stdio.h>

// Define a struct named 'Person'
struct Person {
    char name[50];
    int age;
    float height;
};

// Introduce typedef for a more concise declaration
typedef struct Person Person;

int main() {
    // Declare and Initialize a struct variable
    struct Person person1;
    // Initialize struct members
    strcpy(person1.name, "John");
    person1.age = 25;
    person1.height = 1.75;

    // Accessing and Printing Struct Members
    printf("Person 1:\nName: %s\nAge: %d\nHeight: %.2f meters\n\n", person1.name, person1.age, person1.height);

    // Using Typedef for a more concise declaration
    Person person2;  // 'Person' instead of 'struct Person'
    // Initialize struct members
    strcpy(person2.name, "Alice");
    person2.age = 30;
    person2.height = 1.65;

    // Accessing and Printing Struct Members
    printf("Person 2:\nName: %s\nAge: %d\nHeight: %.2f meters\n", person2.name, person2.age, person2.height);

    // The return statement indicates the end of the main function.
    return 0;
}

Explanation:

  1. Struct Definition:

    • struct Person { char name[50]; int age; float height; }; defines a struct named Person with three members: name, age, and height.

  2. Typedef:

    • typedef struct Person Person; introduces a typedef for a more concise declaration. Now, instead of writing struct Person, you can use just Person.

  3. Struct Variable Declaration and Initialization:

    • struct Person person1; declares a variable of type struct Person named person1.

    • person1.name, person1.age, and person1.height are then initialized with values.

  4. Accessing and Printing Struct Members:

    • printf("Person 1: ...", person1.name, person1.age, person1.height); accesses and prints the values of the struct members.

  5. Using Typedef for a More Concise Declaration:

    • Person person2; declares a variable of type Person named person2. This is possible due to the introduction of typedef.

  6. Accessing and Printing Struct Members Again:

    • printf("Person 2: ...", person2.name, person2.age, person2.height); accesses and prints the values of the struct members for the second person.

Strings are an integral part of C programming, serving as sequences of characters that allow for the manipulation and representation of textual data. In C, strings are essentially arrays of characters, terminated by a special character known as the null terminator ('\0').

C Program:

#include <stdio.h>
#include <string.h>

int main() {
    // String Initialization
    printf("String Initialization:\n");
    char greeting1[] = "Hello, C!";
    char greeting2[12] = {'H', 'e', 'l', 'l', 'o', ',', ' ', 'C', '!', '\0'};

    // Print Strings
    printf("Greeting 1: %s\n", greeting1);
    printf("Greeting 2: %s\n\n", greeting2);

    // String Length
    printf("String Length:\n");
    printf("Length of Greeting 1: %zu\n", strlen(greeting1));
    printf("Length of Greeting 2: %zu\n\n", strlen(greeting2));

    // String Concatenation
    printf("String Concatenation:\n");
    char firstName[] = "John";
    char lastName[] = "Doe";
    char fullName[20];

    strcpy(fullName, firstName);  // Copy the first name
    strcat(fullName, " ");       // Concatenate a space
    strcat(fullName, lastName);   // Concatenate the last name

    printf("Full Name: %s\n\n", fullName);

    // String Comparison
    printf("String Comparison:\n");
    char word1[] = "apple";
    char word2[] = "banana";

    if (strcmp(word1, word2) == 0) {
        printf("%s and %s are equal.\n", word1, word2);
    } else {
        printf("%s and %s are not equal.\n", word1, word2);
    }

    // The return statement indicates the end of the main function.
    return 0;
}

Explanation:

  1. String Initialization:

    • char greeting1[] = "Hello, C!"; initializes a string using the shorthand syntax.

    • char greeting2[12] = {...}; initializes a string using an array of characters with a null terminator.

  2. Print Strings:

    • printf("Greeting 1: %s\n", greeting1); prints the first greeting using the %s format specifier.

    • printf("Greeting 2: %s\n\n", greeting2); prints the second greeting.

  3. String Length:

    • printf("Length of Greeting 1: %zu\n", strlen(greeting1)); calculates and prints the length of the first greeting using strlen.

    • printf("Length of Greeting 2: %zu\n\n", strlen(greeting2)); does the same for the second greeting.

  4. String Concatenation:

    • strcpy(fullName, firstName); copies the first name into fullName.

    • strcat(fullName, " "); concatenates a space.

    • strcat(fullName, lastName); concatenates the last name.

  5. String Comparison:

    • strcmp(word1, word2) == 0 compares two strings, and if they are equal, it prints a message.

Functions are essential components of structured C programming, allowing code to be organized into modular units that perform specific tasks. These units promote reusability, readability, and maintenance. In C, functions are defined with a return type, a name, and parameters.

C Program:

#include <stdio.h>

// Custom Function: String Length
int customStrlen(const char* str) {
    // Iterate through the characters until the null terminator is encountered
    int length = 0;
    while (str[length] != '\0') {
        length++;
    }
    return length;
}

// Custom Function: Check if a Number is Prime
int isPrime(int num) {
    if (num <= 1) {
        return 0;  // Numbers less than or equal to 1 are not prime
    }

    // Check for factors from 2 to the square root of the number
    for (int i = 2; i * i <= num; i++) {
        if (num % i == 0) {
            return 0;  // Found a factor, not a prime number
        }
    }

    return 1;  // No factors found, prime number
}

int main() {
    // Example: Using customStrlen
    char greeting[] = "Hello, C!";
    printf("Length of Greeting: %d\n", customStrlen(greeting));

    // Example: Using isPrime
    int number = 19;
    if (isPrime(number)) {
        printf("%d is a prime number.\n", number);
    } else {
        printf("%d is not a prime number.\n", number);
    }

    // The return statement indicates the end of the main function.
    return 0;
}

Explanation:

  1. Custom Function: String Length (customStrlen):

    • int customStrlen(const char* str) { ... } declares a function named customStrlen that takes a pointer to a character array (const char* str) and returns an integer.

    • The function iterates through the characters of the string until it encounters the null terminator ('\0'), counting the characters and returning the length.

  2. Custom Function: Check if a Number is Prime (isPrime):

    • int isPrime(int num) { ... } declares a function named isPrime that takes an integer num and returns an integer.

    • The function checks if the number is less than or equal to 1 (not prime). It then iterates through potential factors from 2 to the square root of the number, determining if it has any factors other than 1 and itself.

  3. Example Usage in main:

    • customStrlen(greeting) is used to find and print the length of the string "Hello, C!".

    • isPrime(number) is used to determine if the number 19 is prime, and the result is printed accordingly.

Pointers are a fundamental concept in C programming, offering a powerful mechanism for manipulating and managing memory addresses. They allow direct access to the memory location of variables, enabling more efficient memory usage and facilitating advanced operations. In C, understanding pointers is crucial for tasks such as dynamic memory allocation, data structures, and efficient function parameter passing.

Certainly! Let's start with a basic pointer program and then proceed to the swap function.

Basic Pointer Program:

#include <stdio.h>

int main() {
    int number = 42;
    int *ptr;  // Declare a pointer variable

    // Assign the address of 'number' to the pointer
    ptr = &number;

    // Display the value and address of 'number' using the pointer
    printf("Value of 'number': %d\n", *ptr);  // Dereferencing the pointer
    printf("Address of 'number': %p\n", ptr);

    // The return statement indicates the end of the main function.
    return 0;
}

Explanation:

  1. int *ptr;: Declares a pointer variable named ptr that can store the memory address of an integer.

  2. ptr = &number;: Assigns the address of the variable number to the pointer ptr.

  3. *ptr: Dereferences the pointer, giving us access to the value stored at the memory location it points to.

Now, let's proceed with the swap function.

Swap Function Using Pointers:

#include <stdio.h>

// Custom Function: Swap Two Values
void swapValues(int* a, int* b) {
    int temp = *a;  // Dereferencing pointer to get the value at address 'a'
    *a = *b;        // Assigning the value at address 'b' to the location 'a'
    *b = temp;      // Assigning the original value of 'a' (stored in 'temp') to the location 'b'
}

int main() {
    int num1 = 5, num2 = 10;

    // Display original values
    printf("Before Swap:\n");
    printf("num1: %d\nnum2: %d\n\n", num1, num2);

    // Swap values using the custom function
    swapValues(&num1, &num2);

    // Display values after swapping
    printf("After Swap:\n");
    printf("num1: %d\nnum2: %d\n", num1, num2);

    // The return statement indicates the end of the main function.
    return 0;
}

Explanation:

  1. Custom Function: Swap Two Values (swapValues):

    • void swapValues(int* a, int* b) { ... } declares a function named swapValues that takes two integer pointers (int* a, int* b) as parameters.

    • Inside the function, a temporary variable temp is used to store the value at the memory location pointed to by a.

    • By dereferencing the pointers (*a and *b), the values at the memory locations are manipulated, effectively swapping the values.

  2. Example Usage in main:

    • swapValues(&num1, &num2); calls the custom function, passing the addresses of num1 and num2 as arguments.

    • This results in the values of num1 and num2 being swapped directly in memory.

The use of pointers in the swapValues function is essential because it allows the manipulation of the actual values at specific memory addresses. Without pointers, the function would only work with local copies of the variables, and the original values in the main function would remain unchanged.

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Written by

Jyotiprakash Mishra
Jyotiprakash Mishra

I am Jyotiprakash, a deeply driven computer systems engineer, software developer, teacher, and philosopher. With a decade of professional experience, I have contributed to various cutting-edge software products in network security, mobile apps, and healthcare software at renowned companies like Oracle, Yahoo, and Epic. My academic journey has taken me to prestigious institutions such as the University of Wisconsin-Madison and BITS Pilani in India, where I consistently ranked among the top of my class. At my core, I am a computer enthusiast with a profound interest in understanding the intricacies of computer programming. My skills are not limited to application programming in Java; I have also delved deeply into computer hardware, learning about various architectures, low-level assembly programming, Linux kernel implementation, and writing device drivers. The contributions of Linus Torvalds, Ken Thompson, and Dennis Ritchie—who revolutionized the computer industry—inspire me. I believe that real contributions to computer science are made by mastering all levels of abstraction and understanding systems inside out. In addition to my professional pursuits, I am passionate about teaching and sharing knowledge. I have spent two years as a teaching assistant at UW Madison, where I taught complex concepts in operating systems, computer graphics, and data structures to both graduate and undergraduate students. Currently, I am an assistant professor at KIIT, Bhubaneswar, where I continue to teach computer science to undergraduate and graduate students. I am also working on writing a few free books on systems programming, as I believe in freely sharing knowledge to empower others.