Python LPTHW: What is this loop statement doing in his function

Question

I'm following the book "Learn Python The Hard Way" And I don't understand what is going on in this function in Exercise 39.

def new(num_buckets=256): #So the function is made and its named "new" and the argument inside is """Initializes a Map with the given number of buckets.""" aMap = [] #then it creates an empty list and names it aMap for i in range(0, num_buckets): #This loop is created named i and it is ranging from 0-255. aMap.append([]) #then it adds a list inside our aMap variable. return aMap

I don't know what the "for i in range(0,num_bucketss) is doing here. What is it doing the append command?

Add a line print(i) and think how aMap changes each time. Also, think about the variable name num_buckets... — cphlewis
– cphlewis, Commented Apr 13, 2015 at 21:19
I've tried adding a print command. It gives me this though: "<function new at 0x01DB6870>" — Lia
– Lia, Commented Apr 13, 2015 at 21:23
Well, then you're definitely doing something wrong -- did you use the exact print statement I suggested? Being able to make looping explicit is helpful while learning. — cphlewis
– cphlewis, Commented Apr 13, 2015 at 22:51

Community · Accepted Answer · 2017-05-23 11:57:03Z

0

It's adding as many empty lists in aMap as num_buckets specifies. So, if num_buckets = 5, aMap will be a list of 5 empty lists.

As for why it is doing that, we'll need more context.

EDIT:

Seeing the context, it's doing this to create a number of empty buckets for the hashmap. You can read more about how a hashmap uses buckets here: What is meant by number of buckets in the HashMap?

edited May 23, 2017 at 11:57

CommunityBot

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answered Apr 13, 2015 at 21:11

Ayush

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5 Comments

Lia Over a year ago

It's used to create our own dictionary module:learnpythonthehardway.org/book/ex39.html . so if it was num_buckets = 3. It's basically making 3 lists inside a list? Like [ [],[],[] ] ?

Ayush Over a year ago

@Lia: Correct, it would make [[],[],[]]. Seeing the context, it's doing this because a dictionary operates using a number of buckets. See edit for more details.

Lia Over a year ago

Ok I have one more question. How is the "for i in range(0, num_buckets): applied to aMap.append([])? I thought since the "for loop" is named "i" that it has to be applied somewhere in aMap.append() to have it create 255 lists, but I'm thinking aMap.append([]) has no relationship to the loop

Ayush Over a year ago

i is the loop counter here. There is no requirement that you have to use the counter in the loop. For example: for i in range(1, 100): print "Hello World!"

Lia Over a year ago

ahh ok I get it. Thanks for the help. This has been bothering me all day and I couldnt understand why. The book didn't go as in depth with loops as I would have wanted. Your Hello World example was very useful.

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