c - Divisiblity of 5 without using % and / operator

Question

how to check whether a number is divisible by 5 or not without using % and / operator. I want a quickest algorithm for this problem.

Answer 1

A good starting point is to look into how division can be accomplished with multiplication and bit-shifts. This question is one place to look.

In particular, you can follow the attached post to hit upon the following strategy. First, "divide by 5" using multiplication and bit-shifts:

 int32_t div5(int32_t dividend) {
     int64_t invDivisor = 0x33333333;
     return 1 + (int32_t) ((invDivisor * dividend) >> 32);
 }

Then, take the result and multiply by 5:

int result = div5(dividend) * 5;

Then, result == dividend if and only dividend is divisible by 5.

if(result == dividend) {
    // dividend is divisible by 5
}
else {
    // dividend is not divisible by 5
}