Question about lists

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November 8th, 2012, 08:42 PM

Nick Wyden

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Question about lists

Hi, what could I do if I wanted to find out how many times a certain value appeared in a list. For example how could I find out how many times "Z" appeared in list 1

thanks

November 8th, 2012, 10:16 PM

Lux Perpetua

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Code:

>>> from collections import defaultdict
>>> d = defaultdict(int)
>>> a = ['a', 'b', 'c', 'a', 'b', 'd']
>>> for x in a:
...     d[x] += 1
...     
>>> d['a']
2
>>> d['b']
2
>>> d['c']
1
>>> d['d']
1

This counts the occurrences of every value in the list, assuming each value is hashable. If you only care about one value, I think you're better off manually comparing each list item to the value and incrementing a counter when equality occurs.

November 8th, 2012, 10:31 PM

b49P23TIvg

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I'd use the interpreter. Browsing through the output of

>>> help(list())

I'd find a count method. And then I'd learn more detail:

>>> help(list().count)

And maybe I'd look in the iterables or list section of the module library reference at docs.python.org .

And I'd practice with a few cases back in the interpreter to make sure the funny variations I can think of work in reasonable ways.

And then I'd be ready to use the list.count method.

Comments on this post

Lux Perpetua agrees: This is certainly simpler than my methods.

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November 9th, 2012, 06:55 PM

Nick Wyden

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alright thanks

November 9th, 2012, 08:36 PM

KING-OLE

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Here's an example of what b49P23TIvg was referring to:

Code:

>>> a=['A','B','A','C']
>>> a.count('A')
2
>>> a.count('B')
1
>>> a.count('E')
0
>>> for i in range (0,a.count('A')):
...   e = a.index('A',i)
...   print ("element ",e," is the letter ",a[e])
...
element  0  is the letter  A
element  2  is the letter  A
>>>

Question about lists

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