Efficient way to multiply with 7

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本文介绍了一种使用位操作符高效实现数字乘以7的方法。通过将数字左移3位得到8倍值，然后从该值中减去原始数字，从而快速计算出结果。

reference:

http://www.geeksforgeeks.org/efficient-way-to-multiply-with-7/

Problem Definition:

Figure out a way to multiply with 7 efficiently.

Solution:

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).

Code:

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