python list with fixed number of elements

Question

I want to create a fixed size list of size 6 of tuples in Python. Note that in the code below i am always re-initializing the values in the outer for loop in order to reset the previously created list which was already added to globalList. Here is a snippet:

for i,j in dictCaseId.iteritems():
    listItems=[None]*6
    for x,y in j:
        if x=='cond':
            tuppo = y
            listItems.insert(0,tuppo)
        if x=='act':
            tuppo = y
            listItems.insert(1,tuppo)
        if x=='correc':
            tuppo = y
            listItems.insert(2,tuppo)
            ...
            ...
    globalList.append(listItems)

But when I try to run the above (snippet only shown above) it increases the list size. I mean, stuff gets added but I also see the list contains more number of elements. I dont want my list size to increase and my list is a list of 6 tuples.

For example:

Initially: [None,None,None,None,None,None]
What I desire: [Mark,None,Jon,None,None,None]
What I get: [Mark,None,Jon,None,None,None,None,None]

Ashwini Chaudhary · Accepted Answer · 2013-05-07T09:22:07.897

4

Instead of inserting you should assign those values. list.insert inserts a new element at the index passed to it, so length of list increases by 1 after each insert operation. On the other hand assignment modifies the value at a particular index, so length remains constant.

for i,j in dictCaseId.iteritems():
    listItems=[None]*6
    for x,y in j:
        if x=='cond':
            tuppo = y
            listItems[0]=tuppo
        if x=='act':
            tuppo = y
            listItems[1]=tuppo
        if x=='correc':
            tuppo = y
            listItems[2]=tuppo

Example:

>>> lis = [None]*6
>>> lis[1] = "foo"   #change the 2nd element
>>> lis[4] = "bar"   #change the fifth element
>>> lis
[None, 'foo', None, None, 'bar', None]

Update:

>>> lis = [[] for _ in xrange(6)] # don't use  [[]]*6
>>> lis[1].append("foo")
>>> lis[4].append("bar")
>>> lis[1].append("py")
>>> lis
[[], ['foo', 'py'], [], [], ['bar'], []]

edited May 07 '13 at 09:22

answered May 07 '13 at 09:10

Ashwini Chaudhary

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Thanks Ashwini. That works. However my list is a list of tuples. What I mean is it has 6 elements and each element can be a tuple. But when I insert like above it will rewrite the previously inserted element. So how can I insert a tuple. For example I need [(foo,bar),None,bar,None,None,None] – Yavar May 07 '13 at 09:16
@Yavar you may have to use a list of lists then instead of lists of None, see my updated answer. – Ashwini Chaudhary May 07 '13 at 09:19
1

Thanks buddy! That's perfect. I am a C++ coder new to Python, learning on the job. That really helped me!! Accepted with a +1 :) – Yavar May 07 '13 at 09:22
@Yavar You're welcome. Make sure you don't use something like this : `[[]]*6`. Reason : http://stackoverflow.com/questions/13058458/python-list-index – Ashwini Chaudhary May 07 '13 at 09:24

score 3 · Answer 2 · edited May 23 '17 at 12:03

Ashwini fixes your main issue, however I will make a suggestion here.

Because (from what you have shown us) you are simply assigning an element to a specific index based on a condition, it would be better to do something like this:

for i, j in dictCaseId.iteritems():
    listItems = [None] * 6
    lst = ['cond', 'act', 'correc', ...]
    for x, y in j:
        idx = lst.index(x)
        listItems[idx] = y
    globalList.append(listItems)

Or with a list of lists:

for i, j in dictCaseId.iteritems():
    listItems = [[] for _ in xrange(6)]
    lst = ['cond', 'act', 'correc', ...]
    for x, y in j:
        idx = lst.index(x)
        listItems[idx].append(y)
    globalList.append(listItems)

This allows each of the conditions to be dealt with in one go, and it condenses your code significantly.

Thanks Volatility, that does help, indeed a lot!! – Yavar May 07 '13 at 09:18 — Yavar, May 07 '13 at 09:18

score 1 · Answer 3 · answered May 07 '13 at 09:10

1

Instead of listItems.insert(0, tuppo), do listItems[0] = tuppo etc.

answered May 07 '13 at 09:10

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score 1 · Answer 4 · answered May 07 '13 at 09:11

thats becauce you insert the names indestead of changing the value. do like this indstead

for i,j in dictCaseId.iteritems():
    listItems=[None]*6
    for x,y in j:
        if x=='cond':
            tuppo = y
            listItems[0] = tuppo
        if x=='act':
            tuppo = y
            listItems[1] = tuppo
        if x=='correc':
            tuppo = y
            listItems[2] = tuppo                ...
            ...
    globalList.append(listItems)

python list with fixed number of elements

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