What does square brackets on their own represent?

Question

Hi I'm just working with adapting some python code and making sure I understand everything within it as I've never really worked with Python before. What does the [0] alone mean in the code mean? (qtable is 2 dimensional array, holding states(s) and actions(a), actions is a list, current_s is an integer)

self.q_table[current_s] = ([0] * len(self.actions))

list with only one element. multiplying by len provides a vector of zeroes — Jean-François Fabre, Mar 31 '20 at 10:44
or in other words list of 0s, brackets in python are very important, slicing, accessing dict keys, indices, list shortcuts, accessing individual chars(strings) etc. — Vitaliy Terziev, Mar 31 '20 at 10:52
To find out simple things like this for your self, I find that: `print([], type([]))` is quite revealing, output: `[] ` — quamrana, Mar 31 '20 at 10:52

Jean-François Fabre · Accepted Answer · 2020-03-31T11:02:21.007

3

[0] is a list with one element: zero.

Then the program uses * operator (list multiplication in this case) to replicate the left operand list len(self.actions) times.

This creates a list of zeroes of len(self.actions) length.

With immutable types, that is the preferred initialization. A slower alternative (which must be used for mutable types) would be:

[0 for _ in len(self.actions)]

edited Mar 31 '20 at 11:02

answered Mar 31 '20 at 10:47

Jean-François Fabre

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Perfect thankyou, I thought it was just the value 0 and couldn't understand why it was being multiplied by that. Makes a lot of sense with the rest of the code now :) – TheButterWorm Mar 31 '20 at 10:52

What does square brackets on their own represent?

1 Answers1