Can someone explain this Insertion sort pseudocode?

Question

I got this pseudocode when studying "Schaum's Outline of Principles of Computer Science" chapter 2:

Insertion_Sort(num_list)

length <-- length of num_list

number_index <-- 2

while{number_index <= length} {
    newNum  <-- num_list[number_index]
    sorted_index <-- number_index - 1
    while newNum < num_list[sorted_index] AND sorted_index > 0 {
        num_list[sorted_index + 1]  <-- num_list[sorted_index]
        sorted_index            <-- sorted_index - 1
    }
    num_list[sorted_index + 1] = newNum
}
end

Won't the number_index be stuck at 2?

Shouldn't there be: number_index <-- number_index + 1 after the code num_list[sorted_index + 1] = newNum?

Is there an error, or it's just I didn't understand the pseudocode?

Can someone explain how this pseudocode works?

Note: I'm not a programmer (very new), I only studied "CODE" by Charles Petzold so far.

Answer 1

You are correct, the pseudo-code is incorrect as it is written because number_index is never incremented, so the code loops infinitely.

The code is syntactically incorrect in the second to last line with code, using an = sign to denote assignment instead of the <-- sign that it already established. The names are also not entirely accurate (newNum isn't actually a new number, just a stored number).

Here is what the code should look like using the given syntax.

Insertion_Sort(num_list)

length <-- length of num_list

number_index <-- 2

while{number_index <= length} {
    newNum  <-- num_list[number_index]
    sorted_index <-- number_index - 1
    while newNum < num_list[sorted_index] AND sorted_index + 1 > 0 {
        num_list[sorted_index + 1]  <-- num_list[sorted_index]
        sorted_index            <-- sorted_index - 1
    }
    num_list[sorted_index + 1] <-- newNum
    number_index <-- number_index + 1
}
end

Here's an animation showing what the code is supposed to do

And a link to the Wikipedia page on insertion sort (Which has the animation and the correct code).