Efficient way to multiply with 7

Given a number n, find the result alfter multiplying it with 7.

Example:

Input: n=8

Output: 56

Naive Approach: The basic way to solve this is to use a multiplication operator, or add the number 7 times, or use recursive function.

Efficient Approach: We can multiply a number by 7 using bitwise operator. 

• First left shift the number by 3 bits (you will get 8n) 
• Then subtract the original number from the shifted number 
• Return the difference (8n – n). 

Program:

CPP

# include<bits/stdc++.h>

using namespace std;
 //c++ implementation
long multiplyBySeven(long n)
{
    /* Note the inner bracket here. This is needed
       because precedence of '-' operator is higher
       than '<<' */
    return ((n<<3) - n);
}

/* Driver program to test above function */
int main()
{
    long n = 4;

    cout<<multiplyBySeven(n);

    return 0;
}

C

# include<stdio.h>

int multiplyBySeven(unsigned int n)
{
    /* Note the inner bracket here. This is needed
       because precedence of '-' operator is higher
       than '<<' */
    return ((n<<3) - n);
}

/* Driver program to test above function */
int main()
{
    unsigned int n = 4;
    printf("%u", multiplyBySeven(n));

    getchar();
    return 0;
}

Java

// Java program to multiply any
// positive number to 7

class GFG {

    static int multiplyBySeven(int n)
    {
        /* Note the inner bracket here.
        This is needed because precedence
        of '-' operator is higher
        than '<<' */
        return ((n << 3) - n);
    }

    // Driver code
    public static void main (String arg[])
    {
        int n = 4;

        System.out.println(multiplyBySeven(n));
    }
}

// This code is contributed by Anant Agarwal.

Python3

# Python program to multiply any
# positive number to 7

# Function to multiply any number with 7
def multiplyBySeven(n):

    # Note the inner bracket here.
    # This is needed because
    # precedence of '-' operator is
    # higher than '<<'
    return ((n << 3) - n)

# Driver code
n = 4
print(multiplyBySeven(n))

# This code is contributed by Danish Raza

C#

// C# program to multiply any
// positive number to 7
using System;

class GFG
{
    static int multiplyBySeven(int n)
    {
        /* Note the inner bracket here. This is needed
        because precedence of '-' operator is higher
        than '<<' */
        return ((n << 3) - n);
    }

    // Driver code
    public static void Main ()
    {
        int n = 4;
        Console.Write(multiplyBySeven(n));
    }
}

// This code is contributed by Sam007

PHP

<?php


function multiplyBySeven($n)
{

    // Note the inner bracket here.
    // This is needed because
    // precedence of '-' operator
    // is higherthan '<<'
    return (($n<<3) - $n);
}

    // Driver Code
    $n = 4;
    echo multiplyBySeven($n);

// This code is contributed by vt_m.
?>

Javascript

<script>

function multiplyBySeven(n)
{

    /* Note the inner bracket here. This is needed
      because precedence of '-' operator is higher
      than '<<' */

    return ((n << 3) - n);
}

// Driver program to test above function

     n = 4;
    document.write(multiplyBySeven(n));

  // This code is contributed by simranarora5sos
</script>

Output:

28

Time Complexity: O(1) 
Space Complexity: O(1)
Note: Works only for positive integers. 
Same concept can be used for fast multiplication by 9 or other numbers.