Either one is enough

Challenge 2 of 16

Two upright bars mean or, and that answer is true when either half is — one, the other, or both. It only comes out false when neither of them holds.

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Lesson Two Questions at Once

Last topic could not colour the middle of a row. Far enough along and not too far is two questions at once, and an if only holds one.

There are two ways to join a pair of questions into a single one.

if (i > 1 && i < 5) {

Two ampersands mean AND. The joined answer is true only when both halves are true. One half false and the whole thing is false, however good the other half was.

if (acrossPos < 100 || acrossPos > 300) {

Two upright bars mean OR. The joined answer is true when either half is — one, the other, or both. It is only false when neither of them holds.

These two are the only things in the whole taught language that are punctuation rather than words. There is a reason: the friendly names on this site are given to commands, and a joiner is not a command. The reference drawer has them written out in words if you need reminding which is which.

The way to picture them is to look at how much each one catches. On a grid, row == 0 || col == 0 catches a whole row and a whole column — an or is generous. Change nothing but the joiner and row == 0 && col == 0 catches one single square, the one where they cross. And is fussy.

Two questions can be about the same number, which is what makes a gate:

if (acrossPos > 100 && acrossPos < 300) {

Past the left post and before the right one. That is a band, a middle, an inside, and it takes no else if at all.

Brackets group a joined question so it can be joined again:

if ((row == 0 || row == 3) && (col == 0 || col == 3)) {

At one end of the rows, and at one end of the columns. Which is the four corners.

One caution. Both halves have to be whole questions. Writing acrossPos > 100 && < 300 does not work — the second half has nothing on its left to be about, and it has to say the name again.