Adding 1 to elements of an empty list using another list to count up (maybe with += ?)

Question

I'm very new to python and programming, so please keep answers simple if you can.

I'm trying to create a script that will add 1 to elements in an empty list based on an unordered other list. My inputs are shown below as well as my desired output. Since 5 appears twice in the 'data' list, element 5 in 'input' should count up to 2.

Here's the code I tried, with the error it gives:

input = [0, 0, 0, 0, 0]
data = [5, 1, 2, 5]

input[data] += 1

#desired output: [1, 1, 0, 0, 2]

The error this gives:

TypeError: list indices must be integers, not list

I'm confused, because I have a list of integers, but I guess that doesn't count as type 'int'. I searched to try to convert a list to integers, but all the solutions I found result in still having type 'list'. I'm thinking += might not be the right operator to use in this instance, but I'm not sure what else to use.

Edit: Oops, I forgot about lists starting at 0. Thanks for pointing that out, and thanks for all the great answers! Hard to choose just one.

Answer 1

You will have to loop through the data list, since you cannot pass a list to the index of a list (which is what the error is telling you).

Something like:

for ind in data:
    input[ind]+=1

Note also that Python lists are 0-indexed, which means the first element is called using:

input[0]

I notice you have 5's in your data list - there is no 5th element in your input - the indices are 0,1,2,3,4 so you'd need to modify your code (or input list) for it to run without an error.

Answer 2

Python basically is telling you, you can't use a list as an index to another list . When you try to index the input list with the data list, Python raises an error because the index object is not valid(Also, while I'm at it, input is a reserved keyword in Python and should not be used as variable name . But for sake of clarity, I'll keep using it).

This code

input[data] += 1

is equivalent to this

input[[5, 1, 2, 5]] += 1

Which of course makes no sense, and Python complains.

Now if I understand you right, you want to increment the element in input which corresponds to the element in data , each time an occurrences of the element is found. If this is the case, a dictionary would be a much better choice.

Each key in the dictionary would the a number from data , and the value of each key is zero. Each time an occurrence of a key is found, the corresponding value will be incremented. eg.

input = {5: 0, 2:0, 1:0}
data = [5, 1, 2, 5]

for element in data:
    inpt[element] += 1

However, a much better idea would be to use the collections.Counter() class:

from collections import Counter

data = [5, 1, 2, 5]
input = Counter(data)

Answer 3

What you can do to speed things up is to use a collections.Counter to count the number of times it appeared in data.

input = [0, 0, 0, 0, 0]
data = [5, 1, 2, 5]


from collections import Counter
counted = Counter(data)
for i, b in counted.items():
    input[i-1] = b

And as a side note, input or any other pre-defined names isn't good variable names since they could mess things up in the future.

A easier way to create the input is to do this in case you don't know how many zeroes you need.

input = [0 for _ in range(max(a))] # place this after you defined counted

And I believe you don't even need to do this, just use the Counter instance can surely get the job done. Counter({5: 2, 1: 1, 2: 1}) It behaves almost the same way as a dict.

Answer 4

list is accessed by its index suppose list1 = [3,4,1,2,5] to access list element i need to provide index of that element. index start with 0 to n. suppose i want to access second element of list

print list1[1]

#out: 4

to iterate through list element

for i in list1:
   print list1[i]

in your code you are accessing list index by another list.

Answer 5

welcome to python!

Your problems arise from a misunderstanding of how indexing works. There are two problems. The first has to do with data types, and is neatly encapsulated in your error message: TypeError: list indices must be integers, not list . Let's break this down a bit:

'Indexing' is what happens when you use square brackets after a list to access a particular element of that list. ie if a is a list, a[N] will return the Nth element of a, where N is an integer. You can only access one element at a time when indexing. Python doesn't know what to do if you give it a list, and your error message is basically telling you "You've asked me to index a list with another list, and I don't know how to do that. I only understand integers!".

Because you can only index one value at time, the best way of doing what you're trying to do is with a loop:

for i in data:
    input[i] += 1

How is this different from what you've done? Using a loop goes through each element of data individually, temporarily calls it i , and uses that to index input, and then the += 1 does what you were originally trying to do.

HOWEVER , if you run the loop, you'll get a second error message, because of a different problem. This error will say IndexError: list index out of range , which tells you that you're requesting to access a part of the list that doesn't exist. But why? input has 5 items in it, and you're only asking it for indices between 1 and 5! This error arises because Python's indexing starts at zero, not one. Valid indices of the input list are therefore [0, 1, 2, 3, 4] , not [1, 2, 3, 4, 5] . Because your data list contains the number 5, at some point during the loop i = 5 , and you're asking to access the sixth element of index ( index[5] ), which does not exist.

You can get past this either by changing your data list so that it contains valid indices, or adjusting your loop so that:

for i in data:
    input[i - 1] += 1

Hope that helps!