To illustrate a loop with a decrementing counter, we will rewrite the
triangle function so the counter decreases to zero.
This is the reverse of the earlier version of the function. In this case, to find out how many pebbles are needed to make a triangle with 3 rows, add the number of pebbles in the third row, 3, to the number in the preceding row, 2, and then add the total of those two rows to the row that precedes them, which is 1.
Likewise, to find the number of pebbles in a triangle with 7 rows, add the number of pebbles in the seventh row, 7, to the number in the preceding row, which is 6, and then add the total of those two rows to the row that precedes them, which is 5, and so on. As in the previous example, each addition only involves adding two numbers, the total of the rows already added up and the number of pebbles in the row that is being added to the total. This process of adding two numbers is repeated again and again until there are no more pebbles to add.
We know how many pebbles to start with: the number of pebbles in the last row is equal to the number of rows. If the triangle has seven rows, the number of pebbles in the last row is 7. Likewise, we know how many pebbles are in the preceding row: it is one less than the number in the row.