`luc105.c`¶

Problem Statement

Write a function checkbits(x, p, n) which returns true if all 'n' bits starting from position 'p' are turned on.

Metadata¶

Property	Detail
Author	Amit Dutta amitdutta4255@gmail.com
Date	08 Feb 2026
License	MIT License (See the LICENSE file for details)
Difficulty	Beginner (index: 2 / 10)

Concepts¶

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Recursion

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Source Code¶

#include <stdio.h>
#include <stdlib.h>

int checkbits(unsigned int x, int p, int n);

int main()
{
    unsigned int x;
    int p, n;

    printf("Enter number (x): ");
    scanf("%u", &x);
    printf("Enter starting position (p) and count (n): ");
    scanf("%d %d", &p, &n);

    if (checkbits(x, p, n))
        printf("TRUE: %d bits starting at %d are ON.\n", n, p);
    else
        printf("FALSE: Not all specified bits are ON.\n");

    return 0;
}

int checkbits(unsigned int x, int p, int n)
{
    unsigned int mask;

    // Create a mask of n 1s. E.g., if n=3, mask=000...0111
    mask = (1 << n) - 1;

    // Shift mask to position p
    mask = mask << p;

    // Check if bits in x match the mask
    return (x & mask) == mask;
}

Explanation¶

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    You are explaining a C programming code to a beginner.

    STRICT RULES:

    - Only use the given code. Do NOT assume anything not present.

    - Do NOT add extra examples.

    - Keep explanation clear and short.

    - If something is unclear, say "Not clear from code".

    - Follow the exact format below. Do NOT change headings.

    FORMAT:

    [START]

    ## What it does

    (Explain the overall purpose in 1-2 sentences)

    ## Step-by-step

    (Explain how the code works in steps, simple language)

    ## Key Concepts

    (List concepts like loop, condition, function, etc.)

    ## Notes

    (Mention any limitations, errors, or assumptions)

    [END]

    CODE (luc105.c):

    #include <stdio.h>
    #include <stdlib.h>

    int checkbits(unsigned int x, int p, int n);

    int main()
    {
        unsigned int x;
        int p, n;

        printf("Enter number (x): ");
        scanf("%u", &x);
        printf("Enter starting position (p) and count (n): ");
        scanf("%d %d", &p, &n);

        if (checkbits(x, p, n))
            printf("TRUE: %d bits starting at %d are ON.\n", n, p);
        else
            printf("FALSE: Not all specified bits are ON.\n");

        return 0;
    }

    int checkbits(unsigned int x, int p, int n)
    {
        unsigned int mask;

        // Create a mask of n 1s. E.g., if n=3, mask=000...0111
        mask = (1 << n) - 1;

        // Shift mask to position p
        mask = mask << p;

        // Check if bits in x match the mask
        return (x & mask) == mask;
    }

luc105.c¶