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I'm looking at the solution for how to create a hash table in python and came across this function:

def __init__(self, size):
    self.size = size
    self.table = [[] for _ in range(self.size)]

The third line of code creating the multi-dimensional array is confusing me. Mainly this part:

for _ in range(self.size)

What exactly is this doing? And why is it needed instead of just doing something like this:

[[] self.size]

Which would create a 2-dimensional array with the size of self.size correct? Any sort of visual aids would really help too.

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  • 1
    [[] self.size] creates an array whose first element is an empty array, and second element is a number. Commented Mar 27, 2017 at 21:49
  • 1
    Did you mean [] * self.size? Commented Mar 27, 2017 at 21:49
  • for the underscore part, see : stackoverflow.com/questions/1739514/… Commented Mar 27, 2017 at 21:50
  • 1
    [[] self.size] is a syntax error. [[], self.size] would be a list containing an empty list and whatever self.size is. Commented Mar 27, 2017 at 21:53
  • Okay I was mistaken thinking that [[] self.size] was declaring an array of self.size size with each index containing an empty array. Thanks for clearing that up. Commented Mar 27, 2017 at 21:55

2 Answers 2

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2 
>>> good_table = [[] for _ in range(5)]
>>> good_table
[[], [], [], [], []]
>>> good_table[0].append(3)
>>> good_table
[[3], [], [], [], []]
>>> suggested_table = [[] 5]
  File "<input>", line 1
    suggested_table = [[] 5]
                          ^
SyntaxError: invalid syntax
>>> suggested_table = [[] * 5]
>>> suggested_table
[[]]
>>> [] * 5
[]
>>> bad_table = [[]] * 5
>>> bad_table
[[], [], [], [], []]
>>> bad_table[0].append(3)
>>> bad_table
[[3], [3], [3], [3], [3]]

For the last part, see List of lists changes reflected across sublists unexpectedly.

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1 Comment

I would love to know why people are downvoting this. What information do I need to add? What hasn't already been thoroughly covered elsewhere?
1

The line

self.table = [[] for _ in range(self.size)]

creates an array of buckets for holding the contents of the hash table.

The variable name _ is used to indicate that the variable doesn't matter and its contents are, essentially, thrown away. This is useful for unused variables with a short scope, like this one.

You suggest initializing things like this:

self.table = [[]]*self.size

But this is a bad idea because you actually get self.size copies of the same list! That is:

a=[[]]*4
>>> [[], [], [], []]
a[0].append(3)
>>> [[3], [3], [3], [3]]
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4 Comments

Of the same list.
@juanpa.arrivillaga: Quite right. I've fixed the mistake.
Yeah, sorry for coming off as pedantic, but it's a pet peeve of mine. Python lists are different than arrays, especially arrays in languages like C++ and Java. They are heterogeneous, resizable array-lists with constant-time append/extend operations.
@juanpa.arrivillaga: It was fuzzy wording on my part, and I appreciate the reminder that these are inherently different data structures.

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