C PROGRAMMING PROGRAM • LOOPS & NUMBER PROBLEMS

Calculate Factorial Using a Loop

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

BeginnerFactorialfor loopAccumulator

PROBLEM UNDERSTANDING

Input and expected output

Sample input

Sample output

6! = 720

COMPLETE C PROGRAM

Complete C implementation

#include <stdio.h>

int main(void)
{
    int number;
    unsigned long long factorial = 1;

    printf("Enter a non-negative integer up to 20: ");
    scanf("%d", &number);
    if (number < 0 || number > 20) {
        printf("Input must be between 0 and 20.\n");
        return 0;
    }
    for (int value = 2; value <= number; value++)
        factorial *= (unsigned long long) value;
    printf("%d! = %llu\n", number, factorial);
    return 0;
}

CURRENT STEP

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SELECTED LINE

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EXPECTED OUTPUT FOR THE SAMPLE

6! = 720

0%Step 0 of 0

PROGRAM EXPLANATION

Algorithm and explanation

Read the required input values.
Multiply all integers from 2 through N while limiting N to avoid integer overflow.
Display the computed result.

Multiply all integers from 2 through N while limiting N to avoid integer overflow.

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.