Using modulus in for loop

Question

I am trying to understand how to repeat loops using the mod operator.

If you have two strings, "abc" and "defgh", how can % be used to loop through abc, repeating it until the end of defgh is reached? Namely, what is the mod relationship of the length of abc and defgh?

I don't really understand this concept.

Solution 1

Simple.

std::string abc("abc");
std::string defgh("defgh");

for (size_t i = 0; i < defgh.length(); ++i)
{
    printf("%c", abc[i % abc.length()]);
}

Think about what the Modulus operator is doing, it discretely divides the left hand side by the right hand side, and spits back the integer remainder.

Example:

0 % 3 == 0
1 % 3 == 1
2 % 3 == 2
3 % 3 == 0
4 % 3 == 1

In our case, the left hand side represents the i'th position in "defgh", the right hand represents the length of "abc", and the result the looping index inside "abc".

Solution 2

The typical usage of mod is for generating values inside a fixed range. In this case, you want values that are between 0 and strlen("abc")-1 so that you don't access a position outside "abc".

The general concept you need to keep in mind is that x % N will always return a value between 0 and N-1. In this particular case, we also take advantage of the fact that if you increase x by 1 x % N also increases by 1. See it?

Another important property of modulus that we use here is the fact that it "rolls over". As you increase x by 1, x % N increases by 1. When it hits N-1, the next value will be 0, and so on.

Look at @Daniel's code. It's C++ but the concept is language-agnostic