Programming Technical Interview Questions
This site features a collection of common technical interview questions gathered by a group of programmers who have been through, and given, lots of technical interviews. There is an emphasis on C++ and game programming technical interviews, but most of the questions are relevant to any technical interview.
Lucky Number Seven
C/C++ Interview Questions, Optimization Questions

Write a fast routine for multiplying an unsigned integer by 7. Then write a fast routine for dividing an unsigned integer by 7.


4 Comments so far
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To Multiply

int MultBySeven(int iA)
{
int result = 0;
for(int i = 0; i

By Sundar on 07.13.07 4:23 am | Permalink

int MultBySeven(int iA)
{
int result = 0;
for(int i = 0; i

By Sundar on 07.13.07 4:23 am | Permalink

Fastest does not use loops!
7x = x + 2x + 4x
or
7x = x + x

By Rodneyk on 02.04.08 12:47 pm | Permalink

Fastest does not use loops!
This time with out the < inline
7x = x + 2x + 4x
or
7x = x + x<<1 + x<<2;

int MulBySeven(int x)
{
int x2 = x<<1;
int x4 = x2<<1;
return x + x2 + x4;
}

if you did not want to use the shift you get:
int MulBySeven(int x)
{
int x2 = x+x;
int x4 = x2+x2;
return x + x2 + x4;
}

This still is reduced the number of math operations to 4 (down from 7 for the loop case) and it comes with NO looping overhead…

By Rodneyk on 02.04.08 12:51 pm | Permalink

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