C PROGRAMMING PROGRAM • FUNCTIONS & RECURSION

Check Prime Number Using a Function

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

BeginnerFunctionsParametersReturn values

PROBLEM UNDERSTANDING

Input and expected output

Sample input

Sample output

Prime number

COMPLETE C PROGRAM

Complete C implementation

#include <stdio.h>

int isPrime(int number)
{
    if (number < 2) return 0;
    for (int divisor = 2; divisor <= number / divisor; divisor++)
        if (number % divisor == 0) return 0;
    return 1;
}

int main(void)
{
    int number;

    printf("Enter an integer: ");
    scanf("%d", &number);
    printf(isPrime(number) ? "Prime number\n" : "Not a prime number\n");
    return 0;
}

CURRENT STEP

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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.
Return immediately when a divisor is found; otherwise return true after all candidates are tested.
Display the computed result.

Return immediately when a divisor is found; otherwise return true after all candidates are tested.

EFFICIENCY

Time and space complexity

Time complexity

O(√n)

Auxiliary space

O(1)

DEBUGGING CHECKLIST

Common mistakes

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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.