Loops

Use for when you know how many times to repeat. for (int i = 1; i <= n; i++) has three parts separated by two semicolons: the value the counter starts at, a condition checked before each pass, and a step that runs after each pass. i++ adds one to the counter, and total += i adds i to the total. Watch the boundary: if you write < instead of <=, the last pass is lost without any warning.

Use while (n != 0) when you don't know the number of passes in advance: it keeps repeating as long as its condition is true, like dividing a number by 10 while there's something left in it. do { ... } while (n != 0); checks its condition after each pass instead of before, so it always runs at least once. And watch out for a loop that never changes what its condition checks: if you forget n /= 10, the loop runs forever, until the server stops it a few seconds later.

With a loop inside a loop you can draw shapes: the outer loop goes through the rows, the inner loop draws a single row, and the inner loop's limit comes from the outer loop's counter, as in for (int i = 0; i < row; i++). Give each loop a counter with a different name.