Is using strlen() in the loop condition slower than just checking for the null character?

Question

I have read that use of strlen is more expensive than such testing like this:

We have a string x 100 characters long.

I think that<

Answer 1

I can suppose that in first variant you find strlen each iteration, while in second variant you don't do that.
Try this to check:

int a = strlen(x); 
for (int i = 0; i < a; i++) {...}