counting the number of values in a python list

Question

 x = [20, 6, 99, 3, 6, 2, 1,11,41, 31, 99, 6, 7, 8, 99, 10, 99 ,6] 

    y =[]
    for j in x:
        y[j]+=1

I want to count the occurrence of the list values in x. I get an error when I try to do that using the above code. what is wrong In my code?

I am new to programming so kindly explain it in simple terms. my desired output is a list that contains the number of occurences of the values in x.

What is `y[0]` supposed to mean when `y` has no elements in it? — Ignacio Vazquez-Abrams, Mar 12 '15 at 05:54
**Duplicate of the many questions on `collections.Counter` See [unutbu's answer to How to count the frequency of the elements in a list?](http://stackoverflow.com/a/2162045/202229)** — smci, Mar 12 '15 at 06:02
@IgnacioVazquez-Abrams ok ok, that's the reason I got an error. Ok now I understood. a simple concept. — panni sekar, Mar 12 '15 at 06:03

score 3 · Accepted Answer · answered Mar 12 '15 at 05:58

You should use a dictionary instead of a list, and also be careful - you can't add +1 if the value doesn't exist in the dictionary, which is why you should use .get(j, 0) (zero will be returned as a default value in case it's the first occurrence):

x = [20, 6, 99, 3, 6, 2, 1,11,41, 31, 99, 6, 7, 8, 99, 10, 99 ,6] 

y ={}
for j in x:
    y[j] = y.get(j,0) + 1
print y # {1: 1, 2: 1, 3: 1, 6: 4, 7: 1, 8: 1, 41: 1, 10: 1, 11: 1, 99: 4, 20: 1, 31: 1}

Tejas Pendse · Answer 2 · 2015-03-12T06:11:22.720

0

In your code y is a list. So y[j] for j=6, for example gets you the 6th element of y. Instead, what you want to do is initialize y as a dict, like y={}.

Your code would become:

y = {}
for j in x:
    y[j] = y.get(j,0) + 1

An even better way for Python 2.6+ would be to use a Counter

edited Mar 12 '15 at 06:11

answered Mar 12 '15 at 05:59

Tejas Pendse

551
6
19

score 0 · Answer 3 · answered Mar 12 '15 at 06:03

You could use collection.counter function.

>>> from collections import Counter
>>> x = [20, 6, 99, 3, 6, 2, 1,11,41, 31, 99, 6, 7, 8, 99, 10, 99 ,6]
>>> Counter(x)
Counter({99: 4, 6: 4, 1: 1, 2: 1, 7: 1, 8: 1, 41: 1, 10: 1, 11: 1, 3: 1, 20: 1, 31: 1})
>>> dict(Counter(x))
{1: 1, 2: 1, 99: 4, 6: 4, 7: 1, 8: 1, 41: 1, 10: 1, 11: 1, 3: 1, 20: 1, 31: 1}

score 0 · Answer 4 · answered Mar 12 '15 at 06:06

You can't just access elements of an array if the computer doesn't know the size of the array. Imagine the computer thought your array y had length 10, and set aside a small block of memory for it, and used the rest of its memory for other stuff, like computer games. Now imagine one of the numbers in x was 10000000000. Then the computer would have to modify a cell far, far away from the start of the array, which would be bad, because the computer was already using that memory for other stuff, and you didn't warn it in advance.

In lower level languages, like C, you do this more explicitly by first declaring an array (which sets aside memory for it), then assigning values to its elements.

In your example, your array y actually has length 0, so the computer understandably balks when you try to access elements of it.

score 0 · Answer 5 · answered Mar 12 '15 at 06:08

There is also Counter in collections.

from collections import Counter

x = [20, 6, 99, 3, 6, 2, 1, 11, 41, 31, 99, 6, 7, 8, 99, 10, 99, 6] 
cnt = Counter(x)

Example views of the output:

dict(cnt)
Out[113]: {1: 1, 2: 1, 3: 1, 6: 4, 7: 1, 8: 1, 10: 1, 11: 1, 20: 1, 31: 1, 41: 1, 99: 4}

list(cnt)
Out[114]: [1, 2, 3, 6, 7, 8, 41, 10, 11, 99, 20, 31]

list(cnt.viewitems())
Out[115]: 
[(1, 1),
 (2, 1),
 (3, 1),
 (6, 4),
 (7, 1),
 (8, 1),
 (41, 1),
 (10, 1),
 (11, 1),
 (99, 4),
 (20, 1),
 (31, 1)]

counting the number of values in a python list

5 Answers5