Mastering Development

Is there a better way of copying elements of an array without knowing the length of the new array?

Say I have an array of 8 elements: old_array = [6, 3, 0, 12, 14, 2, 10, 5] and I start at the last element old_array[7]=5. I want to use the new element as the index of the next element I want to copy from old_array to new_array.
Result is new_array = [0,2,5].
Here is some pseudocode to do this:

old_array = [6, 3, 0, 12, 14, 2, 10, 5]
while i>0:

while i>0:

This method does work, however it does not seem efficient since it uses repeated code; one loop to get the length of the new array and another to copy the array.
My question is there a better way of doing this?

I am expecting some pseudocode that would do this better than my current code.
Note that the array can be arbritrary, except there will eventually be a 0 element and there is a way to get to it from the last element. Also we know the last index in the old array.
The preferred language is Python, but I am ok with other languages.

If this question has been answered before or can be improved kindly point me to it. Thank you.

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