Troubles with matrix in python

Question

Yesterday, I was trying to solve a problem with python, and I encountered something really odd:

# create matrix
for i in range(N):
    tmp.append(0)

for i in range(N):
    marker.append(tmp)

# create sum of 2 first cards
for i in range(N) :
    for j in range(N):
      if i != j and marker[i][j] == 0:
          comCard.append(deck[i]+deck[j])
          taken[deck[i]+deck[j]] = [i,j]
          marker[i][j] = 1
          marker[j][i] = 1

The idea is that I want to calculate all the possible sums of each pair of cards in the deck (these cards need to be different), so I think that with a marker, I can avoid calculating the same 2 cards again. for example: deck[1]+deck[2] and deck[2]+deck[1]. But these lines didn't work as they were supposed to do:

marker[i][j] = 1
marker[j][i] = 1

all your rows point to the same list; the one you have initialize with `tmp`. possible duplicate of This https://stackoverflow.com/questions/5957341/python-list-confusion . — hiro protagonist, Jan 08 '17 at 07:53

score 2 · Answer 1 · answered Jan 08 '17 at 08:07

I can recommend another way using standard python modules:

# suppose this is a deck - list of cards (I don't know how to mark them :)
>>> deck = ['Ax', 'Dy', '8p', '6a']

>>> from itertools import combinations
# here's a list of all possible combinations of 2 different cards
>>> list(combinations(deck, 2)))
[('Ax', 'Dy'), ('Ax', '8p'), ('Ax', '6a'), ('Dy', '8p'), ('Dy', '6a'), ('8p', '6a')]

You may work with this list: check some combinations and so on.

I recommend to pay attention to library itertools - it's really awesome for such type computations!

score 0 · Answer 2 · edited Sep 26 '17 at 22:07

You're using the same instance of tmp, only shallow-copied. That's why it doesn't work. You need a new copy of each line to add in your matrix.

This can be done with:

marker.append(list(tmp))

Also, you may benefit from using True and False instead of 0 and 1 someday. So the initialisation of your matrix could rahter look like this:

tmp = list()
marker = list()
for i in range(N):
    tmp.append(False)
for j in range(N):
    marker.append(list(tmp))

This way, when you try marker[i][j] = True, only the index (i, j) will be affected.

That being said, Eugene Lisitsky's answer gives you a far more adapted tool for this kind of matter (permutation listing).

Troubles with matrix in python

2 Answers2