As far as I understand, when you write a for-loop similar to this one

for (int i = 0; i < SOME_NUM; i++) { 
  if (true)
    do_something();
  else
    do_something_else();
}

The time complexity of this operation is mostly affected by the if (true) statement because the for-loop iterations don't actually involve any comparisons of i to SOME_NUM, the compiler will just essentially run the code inside the for-loop SOME_NUM times. Please correct me if I am wrong.

However if this is correct, then how do the following nested for-loops behave?

for (int i = 0; i < SOME_NUM; i++) {
  for (int j = 0; j < i; j++) {
   do_something();
  } 
}

The j in the inner for-loop is now upper bound by i, a value that changes every time the loop restarts. How will the compiler compile this? Do these nested for-loops essentially behave like a for-loop with while-loop inside of it? If you're writing an algorithm that uses nested for-loops where the inner counting variable depends on the outer counting variable should you be concerned about what this will do to the complexity of your algorithm?