Question

I am creating a list of lists using this code:

zeroArray = [0]*Np
zeroMatrix = []
for i in range(Np):
    zeroMatrix.append(zeroArray[:])

Is there a more efficient way to do this? I’m hoping for something along the lines of zeroArray = [0]*Np; zeroMat = zeroArray*Np but can’t find anything similar.

Answer 1

You could do this:

zeroMatrix = [[0] * Np for i in range(Np)]

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Update: Well if we’re going to make it into a race, I’ve found something faster (on my computer) than Omnifarious’ method. This doesn’t beat numpy of course; but this is all academic anyway right? I mean we’re talking about microseconds here.

I think this works because it avoids append and avoids preallocating zeroMatrix.

zeroArray = [0] * Np
zeroMatrix = [zeroArray[:] for i in range(Np)]

My test results:

$ python -m timeit -s "Np = 80" "zeroMatrix = [[0] * Np for i in range(Np)]"
1000 loops, best of 3: 200 usec per loop
$ python -m timeit -s "Np = 80" "zeroArray = [0] * Np" "zeroMatrix = [None] * Np" "for i in range(Np):" "    zeroMatrix[i] = zeroArray[:]"
10000 loops, best of 3: 171 usec per loop
$ python -m timeit -s "Np = 80" "zeroArray = [0] * Np" "zeroMatrix = [zeroArray[:] for i in range(Np)]"
10000 loops, best of 3: 165 usec per loop