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Multiplying a variable with a constant without using multiplication operator
• Difficulty Level : Medium
• Last Updated : 05 Mar, 2021

As we know that every number can be represented as sum(or difference) of powers of 2, therefore what we can do is represent the constant as a sum of powers of 2.
For this purpose we can use the bitwise left shift operator. When a number is bitwise left shifted it is multiplied by 2 for every bit shift.
For example, suppose we want to multiply a variable say “a” by 10 then what we can do is

`a = a << 3 + a << 1;`

The expression a << 3 multiplies a by 8 ans expression a<<1 multiplies it by 2.
So basically what we have here is a = a*8 + a*2 = a*10
Similarly for multiplying with 7 what we can do is

```a = a<<3 - a;
or
a = a<<2 + a<<1 + a;```

Both these statements multiply a by 7.

## C++

 `#include``using` `namespace` `std;` `// Returns n * 7``int` `multiplyBySeven(``int` `n)``{``    ``// OR (n << 2) + (n << 1) + n``    ``return` `(n << 3) - n;``}` `// Returns n * 12``int` `multiplyByTwelve(``int` `n)``{``    ``return` `(n << 3) + (n << 2);``}` `int` `main()``{``    ``cout << multiplyBySeven(5) << endl;``    ``cout << multiplyByTwelve(5) << endl;``    ``return` `0;``}`

## Java

 `class` `GFG {``    ` `    ``// Returns n * 7``    ``static` `int` `multiplyBySeven(``int` `n)``    ``{``        ` `        ``// OR (n << 2) + (n << 1) + n``        ``return` `(n << ``3``) - n;``    ``}` `    ``// Returns n * 12``    ``static` `int` `multiplyByTwelve(``int` `n)``    ``{``        ``return` `(n << ``3``) + (n << ``2``);``    ``}``    ` `    ``// Driver code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``System.out.println(multiplyBySeven(``5``));``        ``System.out.println(multiplyByTwelve(``5``));``    ``}``}` `// This code is contributed by Anant Agarwal.`

## Python3

 `# Python3 program to Multiplying a``# variable with a constant` `# Returns n * 7``def` `multiplyBySeven(n):``    ` `    ``# OR (n << 2) + (n << 1) + n``    ``return` `(n << ``3``) ``-` `n` `# Returns n * 12``def` `multiplyByTwelve(n):``    ``return` `(n << ``3``) ``+` `(n << ``2``)``    ` `# Driver code``print``(multiplyBySeven(``5``))``print``(multiplyByTwelve(``5``))` `# This code is contributed by Anant Agarwal.`

## C#

 `// C# program to Multiplying a``// variable with a constant``using` `System;` `class` `GFG``{``    ``// Returns n * 7``    ``static` `int` `multiplyBySeven(``int` `n)``    ``{``        ``// OR (n << 2) + (n << 1) + n``        ``return` `(n << 3) - n;``    ``}``     ` `    ``// Returns n * 12``    ``static` `int` `multiplyByTwelve(``int` `n)``    ``{``        ``return` `(n << 3) + (n << 2);``    ``}``    ` `    ``// Driver code``    ``public` `static` `void` `Main()``    ``{``        ``Console.WriteLine(multiplyBySeven(5));``        ``Console.WriteLine(multiplyByTwelve(5));``    ``}``}` `// This code is contributed by Anant Agarwal.`

## PHP

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## Javascript

 ``

Output :

```35
60```

We just need to find the combination of powers of 2. Also, this comes really handy when we have a very large dataset and each one of them requires multiplication with the same constant as bitwise operators are faster as compared to mathematical operators.
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