Two easy and neat little algorithms to remove values (say ‘x’) from an array in-place (without allocating space for a new array).

The first idea is that I don’t need to sheft any element before the first ‘x’; I need to shift elements between the first ‘x’ and the second ‘x’ one place to the left, and so on.

def remove_element(value,array):
    shift = 0
    for index in xrange(len(array)):
        try:
            array[index] = array[index + shift]
            while array[index] == value:
                shift += 1
                array[index] = array[index + shift]
        except IndexError:
            array[index] = None

The second idea is to have two indeces, one for reading and one for writing, both starting at the begin of the array. I move them both forward for characters different than ‘x’; when char is ‘x’, only the reading index is moved forward.

def remove_element(value, array):
    reading_idx = writing_idx = 0
    while reading_idx < len(array):
        if array[reading_idx] != value:
            array[writing_idx] = array[reading_idx]
            writing_idx += 1
        reading_idx += 1
    while writing_idx < len(array):
        array[writing_idx] = None
        writing_idx += 1

(source here).