When is pre-decrement applied in a for loop? - c++

Question

I am trying to implement an algorithm that is specified using some C style pseudocode.
The bulk of the algorithm is a for loop:
for (i = 20;i > 0;i -= 2)
{
//...snip
}
It is claimed that the loop runs ten times:
The entire series of modifications is a series of 10 identical double-rounds.
I've ported the algorithm to my own language, and my results do not match the test vectors. So i'm trying to pick apart the minutia of the C-language in order to figure out what i could be doing wrong.
Perhaps the reason my code does not match the result of the test vectors is that it is supposed to only run nine times:
| i | i -= 2 | i > 0 | Perform iteration |
|----|--------|-------|-------------------|
| 20 | 18 | Yes | Yes - 1 |
| 18 | 16 | Yes | Yes - 2 |
| 16 | 14 | Yes | Yes - 3 |
| 14 | 12 | Yes | Yes - 4 |
| 12 | 10 | Yes | Yes - 5 |
| 10 | 8 | Yes | Yes - 6 |
| 8 | 6 | Yes | Yes - 7 |
| 6 | 4 | Yes | Yes - 8 |
| 4 | 2 | Yes | Yes - 9 |
| 2 | 0 | No | No |
If i were to read the if statement logically, it is only executed nine times.
There's still the confusion of why they say it runs ten rounds, but then use 20 as the initial value.
I tried duplicating the logic i see (execute it one less times than documented), but my results still don't match. Unfortunately i don't have access to any functional code to compare my results against a known good implementation. So i may be barking up the wrong tree.
Am i barking up the wrong tree?

Answer 1

The third part of the for condition is executed after the iteration.
The loop runs 10 times with i being 20, 18, 16, 14, 12, 10, 8, 6, 4, 2 for the body of each iteration.

Answer 2

A for loop goes "initialization, test, body, increment, test, body, increment, test, body, etc." The increment isn't applied before the first test.

Answer 3

As an aside, if you want to go from the last to the first element in a for loop but don't want the ugly:
for (let i = arr.length - 1; i >= 0; i--)
You can use:
for (let i = arr.length; i-- > 0;)
Example:
const arr = ['a', 'b', 'c'];
for (let i = arr.length; i-- > 0;) {
console.log(`arr[${i}] is: ${arr[i]}`);
}
Had you used pre-decrement here, the last iteration (which is the first element of the array) would have been missing:
const arr = ['a', 'b', 'c'];
for (let i = arr.length; --i > 0;) {
console.log(`arr[${i}] is: ${arr[i]}`);
}