Efficient way to multiply with 7

1.8

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:

# 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;
}

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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