In-place way to apply a permutation to a list? (inverse of sorting-by-key)

Question

Here\'s a example of what I want to do

spam_list = [\"We\", \"are\", \"the\", \"knights\", \"who\", \"say\", \"Ni\"]
spam_order = [0,1,2,4,5,6,3]
spam_list.m         


        

Answer 1

If you actually don't care about the efficiency at all, and just want in-place semantics (which is a bit odd, because there are entire programming languages dedicated to avoiding in-place semantics), then you can do this:

def modifyList(toModify, newList):
    toModify[:] = newList

def permuteAndUpdate(toPermute, permutation):
    modifyList(toPermute, [toPermute[i] for i in permutation])

permuteAndUpdate(spam_list, spam_order)

print(spam_list)
# ['We', 'are', 'the', 'Ni', 'knights', 'who', 'say']

Credit goes to senderle for recognizing that this is what the OP may actually be after; he should feel free to copy this answer into his own. Should not accept this answer unless you really prefer it over his.