C PROGRAMMING PROGRAM • LOOPS & NUMBER PROBLEMS

Check Whether a Number Is Prime

Learn how to check whether a number is prime using a clear C program.

BeginnerPrime numberLoop boundDivisibility

PROBLEM UNDERSTANDING

Input and expected output

Sample input

Sample output

Prime number

COMPLETE C PROGRAM

Complete C implementation

#include <stdio.h>

int main(void)
{
    int number, isPrime = 1;

    printf("Enter an integer: ");
    scanf("%d", &number);
    if (number < 2) isPrime = 0;
    for (int divisor = 2; divisor <= number / divisor && isPrime; divisor++) {
        if (number % divisor == 0) isPrime = 0;
    }
    printf(isPrime ? "Prime number\n" : "Not a prime number\n");
    return 0;
}

CURRENT STEP

Select Start to walk through the important lines.

SELECTED LINE

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EXPECTED OUTPUT FOR THE SAMPLE

Prime number

0%Step 0 of 0

PROGRAM EXPLANATION

Algorithm and explanation

Read the required input values.
Test divisors only through the square root because factors occur in pairs.
Display the computed result.

Test divisors only through the square root because factors occur in pairs.

EFFICIENCY

Time and space complexity

Time complexity

O(√n)

Auxiliary space

O(1)

DEBUGGING CHECKLIST

Common mistakes

Check this

Use the correct format specifier for every variable.

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Initialize variables before using their values.

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Check braces, semicolons and input order carefully.

Try it yourself

Practice: Run the program with the sample input, predict its output, and then test one boundary case of your own.