What is the Program of C?

"What is the Program of C?" refers to a set of instructions written in the C language to perform specific tasks. C programs demonstrate logic through functions, variables, loops, and conditions, serving as the foundation for system-level and application development.

Varun Saharawat30 Oct, 2025

A Prime number is a number that is divisible by itself or by 1, such as 2, 3, 5, and so on. In this article, we will learn different ways to write a C program for checking whether a given number is prime or not.

When you're starting out, it's not just important to write code but also to understand how to explain a C program? Explaining helps build clarity and also prepares you for interviews and academic evaluations. In this guide, we’ll explore C programming examples with answers, structure, and the logic behind each program.

What are the functions in C?

Functions in C are the blocks of code which are designed to perform a specific task and an example is given below: int add(int a, int b) { return a + b; }

What are the four basic types of C?

The four basic datatypes in c are:

int - values that are in integer
float - the values that are having decimals
char - character
double - values that are decimal with high precision.

What are the 4 Types of Functions in C?

Function has no arguments and no return value.
Function has arguments, but no return value.
Function has arguments and return value.
Function has no arguments but returns a value.

What are the 4 principles of c?

Though not officially defined, the guiding principles are as follows:

Modularity
Simplicity
Efficiency
Portability

What are the three methods of c programming?

You can categorize C programming methods as:

Procedural - Using functions and structure.
Modular - Divide big code in smaller functions.
Structured - Readable and maintainable codes.

How to define in C program?

Constant definitions are made with the #define directive: #define PI 3.14

How to write C program format?

Looking for Generic format of how to write C programs? Lets explore. #include <stdio.h> int main() { // Your code here return0 }

Where Is C Mainly Used?

In operating systems (Linux kernel, for example)

Embedded systems
Firmware development
Game
Compilers
System software

What are the three methods of C?

Although the C programming language does not have "methods" on a par with object-oriented languages such as Java or Python, these three core ways to structure and write logic in C are what students project among themselves. For example, they help beginners know the three ways to approach the basics of problem-solving in C.

Using Functions

This method is perhaps the most well-known in C programming. It pertains to dividing the code into reusable functions to make the program cleaner and modular. Example: Writing a function with the task of computing the result of adding two numbers.

Using Pointers and Structures

C gives low-level memory access using pointers, and structures can group related variables together. One of the main approaches in dealing with dynamic memory, arrays, and file handling. Example: Passing a structure by pointer to a function.

Using File Handling

Another main method that C adopts is file handling. Using fopen(), fprintf(), and fscanf() to read from and write to external files are some of the most common, real-world examples in C programming. Example: Creating a file and having it hold student records.

Importance of These Methods

Understanding these methods will equip one with the necessary foundation for

Writing scalable programs
Handling some sort of real-world problem in C
Understanding the basics of procedural programming

These three approaches huddle together in all the basic programs in C for beginners, who can thus get from simple logic blocks to full-fledged programs. What is a Prime Number? Any natural number that can be divided by both 1 and itself is called a prime number. There are only two factors in a prime number. For example, 2, 3, 5. 7, and so on. These prime numbers can be expressed as a product of two numbers such as 3 can be written as 1*3, 5 can be written as 5*1, etc. Explore What are 10 examples of C? Below in this article.

Write a Program to Check Whether a Number Is Prime or Not in C

Here's a simple C program to check whether a number is prime or not:

#include <stdio.h>
int main() {
    int num, i, isPrime = 1;
    printf("Enter a number: ");
    scanf("%d", &num);
    if (num <= 1) {
        isPrime = 0;
    } else {
        for (i = 2; i <= num / 2; i++) {
            if (num % i == 0) {
                isPrime = 0;
                break;
            }
        }
    }
    if (isPrime)
        printf("%d is a prime number.\n", num);
    else
        printf("%d is not a prime number.\n", num);

    return 0;
}

🧪 Sample Output

Enter a number: 7  
7 is a prime number.

This program checks divisibility from 2 to num / 2. If any divisor is found, it sets isPrime to 0.

Write a C Program to Check Whether the Given Number Is Prime or Not

Here's a simple and efficient C program to check whether a given number is prime or not:

#include <stdio.h>

int main() {
    int num, i, isPrime = 1;
    printf("Enter a number: ");
    scanf("%d", &num);

    if (num <= 1) {
        isPrime = 0;
    } else {
        for (i = 2; i * i <= num; i++) {
            if (num % i == 0) {
                isPrime = 0;
                break;
            }
        }
    }

    if (isPrime)
        printf("%d is a prime number.\n", num);
    else
        printf("%d is not a prime number.\n", num);

    return 0;
}

✅ Sample Output:

Enter a number: 17  
17 is a prime number.

📌 Explanation:

A prime number has only two divisors: 1 and itself.
The loop runs till i * i <= num for efficiency.
isPrime is used as a flag to determine if any divisor was found.

Write an Algorithm to Find a Number Is Prime or Not in C

Here’s a simple algorithm to check whether a number is prime or not in C:

✅ Algorithm to Check Prime Number in C

Start
Input an integer num
If num <= 1, it's not prime → Print and stop
Loop from i = 2 to sqrt(num)
- If num % i == 0, it's not prime → Print and stop
If no divisor found in the loop, then num is prime
End

💡 C programs for Practice Example in Words:

Check if a number like 29 is divisible by any number between 2 and √29. If none divide evenly, then 29 is prime.

Flowchart or C Code

Here you go! Below is the flowchart explanation and its corresponding C code for the "Check Whether a Number is Prime" algorithm.

✅ Flowchart Steps (Text-Based Format)

[Start]
   ↓  
Input: num  
   ↓  
Is num ≤ 1?  
   ↓ Yes       ↓ No  
Print "Not Prime"    Initialize i = 2  
         ↓  
   Is i * i ≤ num?  
         ↓ No          ↓ Yes  
     Print "Prime"     Is num % i == 0?  
                           ↓ Yes         ↓ No  
                    Print "Not Prime"     i++  
                           ↓                 ↑  
                         [End] ←←←←←←←←←←←←←←

🧠 C Code Based on This Flowchart

#include <stdio.h>

int main() {
    int num, i, isPrime = 1;

    printf("Enter a number: ");
    scanf("%d", &num);

    if (num <= 1) {
        printf("%d is not a prime number.\n", num);
        return 0;
    }

    for (i = 2; i * i <= num; i++) {
        if (num % i == 0) {
            isPrime = 0;
            break;
        }
    }

    if (isPrime)
        printf("%d is a prime number.\n", num);
    else
        printf("%d is not a prime number.\n", num);

    return 0;
}

🧾 Sample Output

Enter a number: 23
23 is a prime number.

Write a C Program to Check Whether a Number Is Prime or Not

A prime number is a natural number greater than 1 that has exactly two distinct positive divisors: 1 and itself. Examples: 2, 3, 5, 7, 11, 13, etc. Numbers like 4, 6, 8, and 9 are not prime as they have additional divisors.

🧠 Logic to Check Prime Number

To determine if a number is prime:

If number ≤ 1 → Not Prime
Loop from i = 2 to √number
If number divisible by i → Not Prime
If no such divisor found → Prime

This avoids unnecessary checks beyond square root of the number, making the solution efficient.

🔁 Algorithm (Step-by-Step)

Step 1: Start  
Step 2: Read the number (num)  
Step 3: If num ≤ 1, print "Not Prime", stop  
Step 4: Set flag = 1  
Step 5: For i = 2 to sqrt(num)  
          If num % i == 0  
             flag = 0  
             break  
Step 6: If flag == 1, print "Prime"  
        Else, print "Not Prime"  
Step 7: End

📊 Flowchart (Text Version)

[Start]
   ↓
Input number → num
   ↓
Is num ≤ 1?
 → Yes → Print "Not Prime" → [End]
 → No  → i = 2, flag = 1
   ↓
While i * i ≤ num
   ↓
If num % i == 0?
 → Yes → flag = 0 → break
 → No  → i++
   ↓
Is flag == 1?
 → Yes → Print "Prime"
 → No  → Print "Not Prime"
   ↓
[End]

🧪 C Program Code

#include <stdio.h>
int main() {
    int num, i, isPrime = 1;
    printf("Enter a number: ");
    scanf("%d", &num);
    if (num <= 1) {
        printf("%d is not a prime number.\n", num);
        return 0;
    }
    for (i = 2; i * i <= num; i++) {
        if (num % i == 0) {
            isPrime = 0;
            break;
        }
    }
    if (isPrime)
        printf("%d is a prime number.\n", num);
    else
        printf("%d is not a prime number.\n", num);
    return 0;
}

🧾 Sample Output

Enter a number: 7
7 is a prime number.
Enter a number: 20
20 is not a prime number.

💡 Key Points

Prime check runs in O(√n) time.
The check i * i <= num is used instead of i <= num for better performance.
First filter is num <= 1 which handles invalid/edge input early.

C Program Algorithm to Check Prime Numbers using a Loop

Check the algorithm to write a C program for checking whether a prime number is a prime number.

Step 1: Enter a number as Input which you want to check whether a number is prime or not.
Step 2: Initialize a variable as a temp with 0.
Step 3: Use a for loop to run a loop from 2 to n/2.
Step 4: If the given number is divisible by the present iterator, then increment or continue temp.
Step 5: If temp stays 0 then return “ Given number is a prime number.”

Else,
Then return “The given number is not a prime number.”

Implementation of C Program Using Loop for Checking Prime Number

Check the C program code in the table below to check whether the given number is prime or not. Let's see C programming examples with output.

C Program to check Prime number Using Loop

#include <stdio.h> int main() { int i, num, temp = 0; // read input from user. printf("Enter the number you want to Check for Prime: "); scanf("%d", &num); // Run loop for num/2 for (i = 2; i <= num / 2; i++) { // check if given number is divisible by any number. if (num % i == 0) { temp++; break; } } // check for the value of temp and num. if (temp == 0 && num != 1) { printf("%d is a Prime number", num); } else { printf("%d is not a Prime number", num); } return 0; }

Output

C Program Algorithm to Check Prime Number Using Functions

Check the algorithm for using the function to check whether a given number is prime or not.

Step 1: Define a function which accepts an integer number as a parameter.
Step 2: First, initialize a variable temp with 0.
Step 3: Now iterate the loop from 2 to n/2.
Step 4: If the given number is divisible by a loop iterator, then increment the temp value.
Step 5: Now, check if the value of temp returned is 0. If it is 0 then return “Given number is Prime”
Else,
return “The given number is not a prime number.”

C Program Implementation to Check Prime Number Using Function

Check the code below in the table to check whether the given number is prime. Dive into c programming examples with output.

C Program Code to Check Prime Number Using Function

#include <stdio.h> // Function is used to check whether the number given is prime or Not int is_Prime(int num) { int i, temp = 0; // iterate up to num/2. for (i = 2; i <= num / 2; i++) { // if num has factors, // update temp. if (num % i == 0) { temp++; } } return temp; } int main() { int num, temp = 0; printf("Enter the number you want to check for Prime: "); scanf("%d", &num); // Calling function temp = is_Prime(num); if (temp == 0 && num != 1) { printf("\n %d is a Prime Number", num); } else { printf("\n %d is Not a Prime Number", num); } return 0; }

Output

C Program Algorithm to Check for Prime Using Recursion

This is an algorithm to check whether a given number is prime or not using recursion.

Step 1: Define a recursive function which accepts an integer as a parameter, say num.
Step 2: Intialize the value of i with “2”/
Step 3: Now, decide the base condition of the function. When the num value is equal to 0 or 1, the return is false.
Step 4: If the num value is equal to i then return true.
Step 5: If the num value is equal to i then return true.
Step 6: If the number is divisible by i then return false and increment i.
Step 7: Keep calling the function recursively until it reaches the base value.

C Program Implementation for Prime Number Using Recursion

Check the code below for the implementation of the prime number checker function using the recursion function.

C Program to Check for Prime Number Using Recursion

#include <stdio.h> #include <stdbool.h> // Recursive function to check for prime number bool is_Prime(int num) { static int i = 2; // Base Case if (num == 0 || num == 1) { return false; } // Recursive Case if (num == i) return true; // check if num is divisible by any number if (num % i == 0) { return false; } i++; // recursive function call. return is_Prime(num); } int main() { // test case 1 int num = 20; if (is_Prime(num)) { printf("%d is a Prime number\n", num); } else { printf("%d is not a Prime number \n", num); } } return 0; }

Output

C Program Algorithm to Check for Prime Number using Sieve of Eratosthenes

The Sieve of Eratosthenes is an efficient way of finding all prime numbers smaller than a given number n when n is smaller than 10 million or so. Given below is the algorithm to check for Prime numbers using the Sieve of Eratosthenes.

Step 1: Make an array of type boolean is_Prime[] and itntialize all its indexes with a value of 1 or true.
Step 2: Start with the first number, which will be 2.
Step 3: Iterate over each number from 2 to the square root of n.
Step 4: For each prime number, mark its multiples as non-prime in the array of is_Prime.
Step 5: The numbers in the array that are still true are prime numbers. Now return i.

C Program Implementation to Check for Prime Number using Sieve of Eratosthenes

Check the code below for implementation of Sieve of Eratosthenes in the table below.

C Program to Check for Prime Number using Sieve of Eratosthenes

#include <stdio.h> #include <stdbool.h> void sieve_of_eratosthenes(int n) { // Create a boolean array with all values marked as true bool is_prime[n+1]; for (int i = 0; i <= n; i++) { is_prime[i] = true; } // Start with the first prime number, which is 2. for (int p = 2; p * p <= n; p++) { // If is_prime[p] is not changed, then it is a prime if (is_prime[p] == true) { // Update all multiples of p to not prime for (int i = p * p; i <= n; i += p) { is_prime[i] = false; } } } // Print all prime numbers printf("Prime numbers up to %d are: ", n); for (int p = 2; p <= n; p++) { if (is_prime[p]) { printf("%d ", p); } } printf("\n"); } int main() { int n; printf("Enter a number to find all prime numbers up to: "); scanf("%d", &n); sieve_of_eratosthenes(n); return 0; }

Output

Learn C Programming Online: Resources

You can learn C programming online via:

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CodeChef
HackerRank
Coursera & edX
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C Program for Prime Numbers FAQs

How do I find prime numbers from 1 to 100 using the C program?

We can use the Sieve of Eratosthenes to print prime numbers in a given range from a given prime number to a given value n.

How do you check whether a given number is prime or not?

If a number is divisible by itself or by 1, then it is said to be a prime number. We can use c program to find if a number is prime or not.

What are different ways of checking whether a number is prime or not?

We can use loops, functions, recursions and other methods to check whether a given number is prime or not.

What is the sieve of Eratosthenes in C?

The sieve of Eratosthenes is an algorithm to find all prime numbers in a segment from the given prime number to n. Check out more about the algorithm in this article.

What is the Program of C?

What are the functions in C?

What are the four basic types of C?

What are the 4 Types of Functions in C?

What are the 4 principles of c?

What are the three methods of c programming?

How to define in C program?

How to write C program format?

Where Is C Mainly Used?

What are the three methods of C?

Using Functions

Using Pointers and Structures

Using File Handling

Importance of These Methods

Write a Program to Check Whether a Number Is Prime or Not in C

🧪 Sample Output

Write a C Program to Check Whether the Given Number Is Prime or Not

✅ Sample Output:

📌 Explanation:

Write an Algorithm to Find a Number Is Prime or Not in C

✅ Algorithm to Check Prime Number in C

💡 C programs for Practice Example in Words:

Flowchart or C Code

✅ Flowchart Steps (Text-Based Format)

🧠 C Code Based on This Flowchart

🧾 Sample Output

Write a C Program to Check Whether a Number Is Prime or Not

🧠 Logic to Check Prime Number

🔁 Algorithm (Step-by-Step)

📊 Flowchart (Text Version)

🧪 C Program Code

🧾 Sample Output

💡 Key Points

C Program Algorithm to Check Prime Numbers using a Loop

Implementation of C Program Using Loop for Checking Prime Number

C Program Algorithm to Check Prime Number Using Functions

C Program Implementation to Check Prime Number Using Function

C Program Algorithm to Check for Prime Using Recursion

C Program Implementation for Prime Number Using Recursion

C Program Algorithm to Check for Prime Number using Sieve of Eratosthenes

C Program Implementation to Check for Prime Number using Sieve of Eratosthenes

Learn C Programming Online: Resources

Learn Data Structure in C++ with PW Skills

C Program for Prime Numbers FAQs

How do I find prime numbers from 1 to 100 using the C program?

How do you check whether a given number is prime or not?

What are different ways of checking whether a number is prime or not?

What is the sieve of Eratosthenes in C?