Appending to a multi-dimensional array in Python

Question

I have this code:

philips_trousers = []
for i in range(0, 5):
  philips_trousers.append(["T"] * 5)
  philips_trousers.append(["R"] * 5)
  philips_trousers.append(["O"] * 5)
  philips_trousers.append(["U"] * 5)
  philips_trousers.append(["S"] * 5)
  philips_trousers.append(["E"] * 5)
  philips_trousers.append(["R"] * 5)
  philips_trousers.append(["S"] * 5)
print philips_trousers

Which outputs the following:

[['T', 'T', 'T', 'T', 'T'], ['R', 'R', 'R', 'R', 'R'], ['O', 'O', 'O', 'O', 'O'], ['U', 'U', 'U', 'U', 'U'], ['S', 'S', 'S', 'S', 'S'], ['E', 'E', 'E', 'E', 'E'], ['R', 'R', 'R', 'R', 'R'], ['S', 'S', 'S', 'S', 'S'], ['T', 'T', 'T', 'T', 'T'], ['R', 'R', 'R', 'R', 'R'], ['O', 'O', 'O', 'O', 'O'], ['U', 'U', 'U', 'U', 'U'], ['S', 'S', 'S', 'S', 'S'], ['E', 'E', 'E', 'E', 'E'], ['R', 'R', 'R', 'R', 'R'], ['S', 'S', 'S', 'S', 'S'], ['T', 'T', 'T', 'T', 'T'], ['R', 'R', 'R', 'R', 'R'], ['O', 'O', 'O', 'O', 'O'], ['U', 'U', 'U', 'U', 'U'], ['S', 'S', 'S', 'S', 'S'], ['E', 'E', 'E', 'E', 'E'], ['R', 'R', 'R', 'R', 'R'], ['S', 'S', 'S', 'S', 'S'], ['T', 'T', 'T', 'T', 'T'], ['R', 'R', 'R', 'R', 'R'], ['O', 'O', 'O', 'O', 'O'], ['U', 'U', 'U', 'U', 'U'], ['S', 'S', 'S', 'S', 'S'], ['E', 'E', 'E', 'E', 'E'], ['R', 'R', 'R', 'R', 'R'], ['S', 'S', 'S', 'S', 'S'], ['T', 'T', 'T', 'T', 'T'], ['R', 'R', 'R', 'R', 'R'], ['O', 'O', 'O', 'O', 'O'], ['U', 'U', 'U', 'U', 'U'], ['S', 'S', 'S', 'S', 'S'], ['E', 'E', 'E', 'E', 'E'], ['R', 'R', 'R', 'R', 'R'], ['S', 'S', 'S', 'S', 'S']]

So far so good you're probably thinking, but I'm wondering something -- why does Python fill philips_trousers in quite the way it does? That is, why does the append() function create new child arrays with the form ['T', 'T', 'T', 'T', 'T'], ['R', 'R', 'R', 'R', 'R'] etc, rather than: ['T', 'R', 'O', 'U', 'S', 'E', 'R', 'S']?

Why would you expect ['T', 'R', 'O', 'U', 'S', 'E', 'R', 'S']? — user2357112
– user2357112, Commented Apr 6, 2016 at 22:01
Hhhhhmmm, it's difficult to put into words, but I think I expected it to create five child arrays and populate them with ["T"], then append ["R"] to those arrays, and so on and so forth. — Peter Carter
– Peter Carter, Commented Apr 6, 2016 at 22:03
You're calling append on the philips_trousers list, not on its sublists. You're not going to append to the list's sublists. — user2357112
– user2357112, Commented Apr 6, 2016 at 22:10
So... I think the sticking point for me is this: where in the code does it end one child array (say ['R', 'R', 'R', 'R', 'R']) and begin another (say ['O', 'O', 'O', 'O', 'O'])? That's the bit which doesn't make sense to me. What instruction am I passing to the compiler that causes it to end one child array and begin another? — Peter Carter
– Peter Carter, Commented Apr 6, 2016 at 22:29
["T"] * 5 is one list. ["R"] * 5 is another. Nothing here would stick them together. — user2357112
– user2357112, Commented Apr 6, 2016 at 22:51

zondo · Accepted Answer · 2016-04-07 16:34:42Z

1

["T"] + ["T"] is the same thing as ["T", "T"]. Multiplication is just addition several times. Therefore, ["T"] * 5 is the same thing as ["T", "T", "T", "T", "T"]¹. What you see in your end list is several of such lists. If you want a bunch of ["T", "R", "O", ...], you can do this:

philips_trousers = []

for _ in range(5):
    philips_trousers.append(list("TROUSERS"))

print philips_trousers

which is pretty much the shorter version of Remuze's answer.

_{¹Don't use list multiplication with mutable objects because each item in the list will be the same object. That means that modifying one will modify them all.}

edited Apr 7, 2016 at 16:34

answered Apr 7, 2016 at 0:06

zondo

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Trev Davies · Accepted Answer · 2016-04-07 08:40:49Z

0

["T"] * 5 first gets converted into [['T'],['T'],['T'],['T'],['T']], which is then appended on to the empty array. The same then happens for each of the other letters.

To get the functionality you are looking for, you should try this:

philipsTrousers = []
for i in range(0, 5):
  philipsTrousers.append(["T","R","O","U","S","E","R","S"])
print philipsTrousers

edited Apr 7, 2016 at 8:40

answered Apr 6, 2016 at 15:22

Trev Davies

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1 Comment

jDo Over a year ago

Well, no... [T] * 5 isn't valid (no quotes) and ["T"] * 5 creates a single list containing 5 "T"s; not 5 lists containing a single "T" in a list of lists. To get [['T'],['T'],['T'],['T'],['T']] you'll need [["T"]] * 5

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