# Efficient way to multiply with 7

• Difficulty Level : Easy
• Last Updated : 13 Jun, 2022

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 `   `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<

## C

 `# include`   `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

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## Javascript

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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.

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