Safely remove items from an array table while iterating

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the general case of iterating over an array and removing random items from the middle while continuing to iterate

If you’re iterating front-to-back, when you remove element N, the next element in your iteration (N+1) gets shifted down into that position. If you increment your iteration variable (as ipairs does), you’ll skip that element. There are two ways we can deal with this.

Using this sample data:

    input = { 'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p' }
    remove = { f=true, g=true, j=true, n=true, o=true, p=true }

We can remove input elements during iteration by:

  1. Iterating from back to front.

    for i=#input,1,-1 do
    if remove[input[i]] then
        table.remove(input, i)
    end
end

  2. Controlling the loop variable manually, so we can skip incrementing it when removing an element:

    local i=1
while i <= #input do
    if remove[input[i]] then
        table.remove(input, i)
    else
        i = i + 1
    end
end

For non-array tables, you iterate using next or pairs (which is implemented in terms of next) and set items you want removed to nil.

Note that table.remove shifts all following elements every time it’s called, so performance is exponential for N removals. If you’re removing a lot of elements, you should shift the items yourself as in LHF or Mitch’s answer.

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