Python list iteration

Question

So i have a list of heights:

heights = [1, 2, 3, 5, 7, 8, 8, 13]

And im using this function to store each height integer value and its index in the list in a class i called Node.

def initializeNodes(heights):
    ans = []
    for height in heights:
        ans.append(Node(heights.index(height), height))
    return ans

But my problem is, because their are two 8's in the list, its giving them both the same first 8 position of 5 in the list:

How can i go around this? Thanks!

Martijn Pieters · Accepted Answer · 2013-07-10 14:55:23Z

6

Use enumerate() to generate an index:

def initializeNodes(heights):
    ans = []
    for i, height in enumerate(heights):
        ans.append(Node(i, height))
    return ans

You can collapse the four lines into 1 using a list comprehension:

def initializeNodes(heights):
    return [Node(i, height) for i, height in enumerate(heights)]

answered Jul 10, 2013 at 14:55

Martijn Pieters

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1 Comment

Sean Over a year ago

This is perfect and surprisingly a lot simpler than i thought! lol haven't used python in a while and forgot how to use enumerate, silly me

Ashwini Chaudhary · Accepted Answer · 2013-07-10 15:05:04Z

1

The problem with list.index is that it'll only return the index of first occurrence of the item.

>>> heights = [1, 2, 2, 3, 5, 5, 7, 8, 8, 13]
>>> heights.index(2)
1
>>> heights.index(5)
4
>>> heights.index(8)
7

help on list.index:

L.index(value, [start, [stop]]) -> integer -- return first index of value.

You can do provide a different start value to list.index than 0, to get the index of repeated items:

>>> heights.index(5,heights.index(5)+1) #returns the index of second 5
5

But that is very cumbersome, a better solution as @MartijnPieters already mentioned is enumerate

answered Jul 10, 2013 at 15:05

Ashwini Chaudhary

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Comments

mbdavis · Accepted Answer · 2013-07-10 15:13:19Z

0

The problem is that you're generating the index from the values, why not do it the other way around?

heights = [1, 2, 3, 5, 7, 8, 8, 13]

def initializeNodes(heights):
    ans = []
    for index in range(len(heights)):
        ans.append(Node(index, heights[index]))
    return ans

This creates a list from 0 to the length of heights, and then will append the index then the height at this index.

answered Jul 10, 2013 at 15:13

mbdavis

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1 Comment

mbdavis Over a year ago

The enumerate way suggested by @Martijn Pieters is better though, mind.

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