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# which lists python

Posted on 2009-05-03
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Hi,

Very short question. How do you search (without using for loops) in which position of an array certain condition holds.

I.e., list = [3,4,5,2,4,3,5,6,3,2,1,0]
which(list < 2) = [10, 11] cause in positions 10 and 11 the elements of the list are less than 2.
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Question by:dfernan
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LVL 41

Expert Comment

ID: 24292059
Don't use "list" as a variable name.

L = [3,4,5,2,4,3,5,6,3,2,1,0]
L.index( 5 )

Tells the location of the list variable that contains the specified value.

Is this what you want?
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LVL 41

Accepted Solution

HonorGod earned 125 total points
ID: 24292081
List comprehensions kind of work, so

>>> L = [3,4,5,2,4,3,5,6,3,2,1,0]
>>> [ i for i in L if i < 2 ]
[1, 0]

But that gives us the values, not the index.  Combining them

>>> [ L.index( i ) for i in L if i < 2 ]
[10, 11]

appears to work.  Is that what you want?
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Author Comment

ID: 24292082
No. I want to check for a condition. L.which(>=5) ... index in the list where the values are bigger or equal to 5. In your case, [2,6,7] cause in those positions L is bigger or equal to 5.
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Author Comment

ID: 24292086
sorry now i read ur second answer! yes, that's what i want. thanks!
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LVL 41

Expert Comment

ID: 24292101
Great.  Glad to be of some help.

Thanks for the grade & points.

Good luck & have a great day.
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