C PROGRAMMING PROGRAM • FUNCTIONS & RECURSION

Calculate Factorial Using a Function

Learn how to calculate factorial using a function using a clear C program.

BeginnerFunctionsParametersReturn values

PROBLEM UNDERSTANDING

Input and expected output

Sample input

Sample output

7! = 5040

COMPLETE C PROGRAM

Complete C implementation

#include <stdio.h>

unsigned long long factorial(int number)
{
    unsigned long long result = 1;
    for (int value = 2; value <= number; value++) result *= (unsigned long long) value;
    return result;
}

int main(void)
{
    int number;

    printf("Enter a non-negative integer up to 20: ");
    scanf("%d", &number);
    if (number < 0 || number > 20) return 1;
    printf("%d! = %llu\n", number, factorial(number));
    return 0;
}

CURRENT STEP

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EXPECTED OUTPUT FOR THE SAMPLE

7! = 5040

0%Step 0 of 0

PROGRAM EXPLANATION

Algorithm and explanation

Read the required input values.
Move the factorial loop into a reusable function that returns the computed value.
Display the computed result.

Move the factorial loop into a reusable function that returns the computed value.

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.