Efficient way to multiply with 7

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 numberfrom the shifted number and 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. 

Python

# Python program to multiply any  
# positive number to 7  
  
# Function to mutiply 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. 
?> 

Output:

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.

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My Personal Notes

CPP

C

Java

Python

C#

PHP

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