(About Radix Sort) I'm in trouble understanding the code from my professor

Question

I cannot understand what this line means:

queue = [list() for _ in range(k)]

This is the full code:

for i in range(1, maxlen+1):     
    queue = [list() for _ in range(k)]   
    for word in words:

        word += (maxlen - len(word)) * ' '

        if ord(word[-i]) >= 97:
            queue[ord(word[-i]) - (97)].append(word)
        else:
            queue[0].append(word)


    words = sum(queue, [])

for i in range(len(words)):
    words[i] = words[i].replace(" ", "")

print(words)

This is not the full code because we don’t know what are `k`, `maxlen`, `words`. — bfontaine, Dec 28 '18 at 09:29

score 1 · Answer 1 · answered Dec 28 '18 at 09:30

1

That simply just creates a list the number of the variable k times of a empty list, so see example:

a=3
print([list() for i in range(a)])

Output:

[[], [], []]

answered Dec 28 '18 at 09:30

U13-Forward

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score 0 · Accepted Answer · answered Dec 28 '18 at 09:33

0

queue = [list() for _ in range(k)]

it is creating a list of empty lists which is called queue. Queue is a list containing k empty lists

answered Dec 28 '18 at 09:33

Vaibhav Sahu

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score 0 · Answer 3 · answered Dec 28 '18 at 09:36

0

_ is simply a variable such as i or j. You are using an underscore to show that it is a throwaway varaible which you do not use again.

See also What is the purpose of the single underscore "_" variable in Python?

answered Dec 28 '18 at 09:36

tfv

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score 0 · Answer 4 · answered Dec 28 '18 at 09:37

This is python’s list comprehension, which is shortened code for

queue = []
for _ in range(k):
    queue.append(list())

underscore _ is a python variable meant to represent the last accessed variable and typically is used as a throwaway variable that the script code does not wish to reference

(About Radix Sort) I'm in trouble understanding the code from my professor

4 Answers4