Moving a continue condition into the for loop produced something NOT logically equivalent?

Question

I'm setting up a hex grid. The column/row system I use puts 0, 0 as the top left corner, and increasing columns (x) moves to the right but also vertically upwards. So columns run straight up and down, but rows run to the right but slanted upwards a bit.

This code...

for (j = 0; j < bd*2-1; j += 1) // bd is "board dimension", in my case 9
{
  for (i = 0; i < bd*2-1 and i < j+bd; i += 1)
  {
    if (i <= j-bd)
      continue;

    scr_ini_hex(i, j, bxo + i*hs, byo + j*hh - i*vs); // bxo and byo are arbitrary board x/y origins where the first cell is drawn, hs, hh, and vs are the gaps between cells in horizontal and vertical directions.
  }
}

produced this:

which is exactly what I want, a large hexagon with 9 smaller hexagons on each side.

So I thought I would be sophisticated and move that continue condition into the for loop. I wrote this...

for (j = 0; j < bd*2-1; j += 1)
{
  for (i = 0; i < bd*2-1 and i < j+bd and i > j-bd; i += 1) // added "i > j-bd", basically migrated the condition into this inner loop
  {
    //if (i <= j-bd)
      //continue;

    scr_ini_hex(i, j, bxo + i*hs, byo + j*hh - i*vs);
  }
}

and it produced this:

What went wrong here? I thought they were logically equivalent (except negating the condition, because the continue code was like a do until, but the for condition is like a do while).

Answer 1

The difference is that continue only skips forward one step and the condition ends the whole loop.

To me it seems like the issue is, that you try to fit both the starting point and the end point into the loop-condition. When you should set the different starting point in the loop-initialization:

for (j = 0; j < bd*2-1; j += 1)
{
    for(i = max(0,j-bd);i < bd*2-1 and i < j+bd; i++)
    {
        scr_ini_hex(i, j, bxo + i*hs, byo + j*hh - i*vs);
    }
}

Answer 2

The condition part of the for-loop tells the loop when to stop looping. This is different from the continue statement that just skips the rest of the current step and moves on to the next.

Consider the following loop;

for(i = 0; i < 10; i += 1) {
    if (i % 2 == 0)
        continue;  // Skip even indices

    // Do stuff
}

If you were to extract the condition for the continue and place it as part of the for-loop you'd get

for(i = 0; i < 10 && !(i % 2 == 0); i += 1) {

    // Do stuff only on odd ones
}

But that means that the loop will exit as soon as it sees an even number, this is not what the continue statement does.