Describe a function (in C) that does the following
Input : an integer.
Output: if that integer is 4, the function returns 7. if that integer is 7, the function returns 4.

constraints:
the input is SURE to be either 4 or 7. no need to validate.

you are not allowed to use any condition/flow commands (if, switch, while, for, trinary operator...)

you are not allowed to use any external library or import anything (like math..)

you are not allowed to use %(modulu), * (multiplication)

Question

Describe a function (in C) that does the following
Input : an integer.
Output: if that integer is 4, the function returns 7. if that integer is 7, the function returns 4.

constraints:
the input is SURE to be either 4 or 7. no need to validate.

you are not allowed to use any condition/flow commands (if, switch, while, for, trinary operator...)

you are not allowed to use any external library or import anything (like math..)

you are not allowed to use %(modulu), * (multiplication)

Anonymous · Accepted Answer

The interviewer gave me the constrains each time i thought of a solution. so my first trivial solution was using an "if". than he said "now try without any condition" - so i came up with multiplication (return 38/num) - and than "try without multiplication as well..." and so on.

ultimatly there are (i think) 2 good solutions here:

1. return 11 -  num
2. return num XOR 3

Gregory · Answer

It's a simple bit manipulation question. The answer is "input xor 0x3". See below for details.

0111 xor 0011 = 0100
0100 xor 0011 = 0111

And it goes on in a loop.
xor is an operation that essentially checks if the input values are different, when applied to bits, it means "check if one of the bits is 0 AND the other  bit is 1".

xor table:
A B A xor B
0 0 0
0 1 1
1 0 1
1 1 0

tool4 · Answer

int func(int n)
{
    return 4 | (~(n & 3) & 3);
}

Anonymous · Answer

Input: x
Output: 11-x

Anonymous · Answer

XOR with 3

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