Prime Number Program in C with Code

Prime Number Program in C with Code

• 

  Mr. Bambam Yadav / 1 days
• 0
• 4 min read

What is a Prime Number?

Let’s start simple: A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. So, numbers like 2, 3, 5, 7, 11, 13, etc., are primes.

Why is this important for programmers?

Some examples where APSP is essential:

• It strengthens your logical thinking.
• It introduces you to loops, conditionals, functions, and complexity analysis.
• It’s often used in cryptographic algorithms.
• It forms the basis for many advanced number theory concepts.

This is why, in any solid C Programming Course in Noida, this problem is introduced in the first few weeks.

A Simple Prime Number Program in C

Let’s look at the naive approach. This is what most beginners start with.

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

Explanation:

• Input a number from the user.
• Loop from 2 to n-1.
• If any number divides n evenly, it's not prime.

Downsides:

• Inefficient for large numbers.
• Time complexity is O(n).
• No support for modularity or function reuse.

Optimizing the Prime Check

Let’s now refine the logic. Instead of checking all numbers from 2 to n-1, we only need to go up to the square root of n.

Why?

If n = a * b, and both a and b are greater than √n, then a * b > n, which is a contradiction.

                           #include 
                            #include 
                            
                            int main() {
                                int n, i, flag = 0;
                                printf("Enter a positive integer: ");
                                scanf("%d", &n);
                            
                                if (n <= 1) { printf("%d is not a prime number.\n", n); return 0; } for (i="2;" i <="sqrt(n);" i++) if (n % 0) flag="1;" break; (flag="=" else pre>
                    

Time Complexity:

O(√n) — Much faster for large numbers.

Creating a Reusable Function

In your C Programming Course in Noida, you'll be encouraged to write modular code. So let’s put the logic into a function:

                           
                        int isPrime(int n) {
                        if (n <= 2="=" 1) return 0; if (n="=" 2) 1; % 0) for (int i="3;" <="sqrt(n);" +="2)" } int main() { num; printf("enter number: "); scanf("%d", &num); (isprime(num)) printf("%d is prime\n", num); else not pre>
                    

This version is concise, efficient, and reusable. You can now use isPrime() anywhere.

Batch Processing: Prime Numbers in a Range

This is where the challenge scales. Let’s say you want all prime numbers between 1 and 100:

                          #include 
                            #include 
                            
                            int isPrime(int n) {
                                if (n <= 2="=" 1) return 0; if (n="=" 2) 1; % 0) for (int i="3;" <="sqrt(n);" +="2)" } int main() { lower, upper; printf("enter range (lower and upper): "); scanf("%d %d", &lower, &upper); printf("prime numbers between %d are: \n", upper); i++) (isprime(i)) printf("%d ", i); printf("\n"); pre>
                    

You’ll often find such batch processing problems in online assessments or coding rounds.

Advanced Optimization: Sieve of Eratosthenes

For very large ranges (e.g., up to 10^6), use the sieve:

                          #include 
                            #define MAX 1000000
                            
                            int main() {
                                int prime[MAX] = {0};
                                for (int i = 2; i * i < MAX; i++) {
                                    if (prime[i] == 0) {
                                        for (int j = i * i; j < MAX; j += i)
                                            prime[j] = 1;
                                    }
                                }
                            
                                for (int i = 2; i < MAX; i++) {
                                    if (prime[i] == 0)
                                        printf("%d ", i);
                                }
                                return 0;
                            }


                        

This is a fast and efficient way to get all primes up to a high limit.

Why This Program is Taught in Noida-Based Courses

Noida, being a tech hub, sees a lot of students entering the IT workforce. Programming courses in this region often have:

• Hands-on labs where students write these programs.
• Assessments where they optimize and explain algorithms.
• Projectsthat use prime detection in encryption, hash generation, or random number validation.
• Expert mentorship focusing on code efficiency, readability, and structure.

Real-World Use Cases of Prime Number Algorithms

1. Cryptography: Algorithms like RSA use very large prime numbers.
2. Hashing: Prime number properties reduce hash collisions.
3. Computer Graphics: Some random number generation uses primes.
4. Data Security: Prime checks in encryption key generation.
5. Networking: Prime-based hashing improves packet distribution.
6. Simulation Systems: Use primes in probabilistic calculations.

Interview Readiness and Problem Solving

Most technical interviews don’t just want a solution—they want optimized, modular code. A good answer includes

• Base case handling (like n<= 1)< li>
• Time complexity awareness
• Logical reasoning
• Optional: Stress tests with large inputs

When you're taking a C Programming Course in Noida, these are the skills that are polished daily in labs and assignments.

FAQ Section

Q1: Why check only up to sqrt(n)?

Because any factor greater than sqrt(n) must be paired with a smaller factor, so no need to check beyond it.

Q2: Can I use arrays to store prime results?

Yes. You can even use a Sieve of Eratosthenes to precompute primes up to a range.

Q3: Is isPrime() efficient for huge numbers?

No. For very large values (e.g., >10^9), probabilistic or segmented sieve methods are better.

Q4: Do C courses in Noida include optimization topics?

Yes. Most courses teach both naive and optimized versions and encourage performance thinking early on.

Q5: How are primes used in job interviews?

They test logic, loops, and optimizations. Bonus if you show understanding of edge cases and complexity.

Q6: What is the difference between prime and composite numbers?

A prime number has exactly two factors: 1 and itself. A composite number has more than two factors.

Q7: Is 1 a prime number?

No. By definition, a prime number must have exactly two distinct positive divisors.

Conclusion

Writing a prime no program in Cis more than an academic exercise. In a structured C Programming Course in Noida, it represents the transition from syntax understanding to logical thinking. You’ve now seen how to:

• Write a basic version.
• Optimize for performance.
• Use functions and ranges.
• Understand real-world relevance.
• Apply it in interviews and large-scale projects.

From here, your next step could be sieve algorithms, cryptographic applications, or even diving into competitive programming. Prime numbers may be indivisible, but the lessons they teach are endless.