Loop in a list (Python v3) beginner

Question

I am stuck in making a loop that will eliminate the values(from the alist) that are below average. Thanks for the help.

a=input("Enter a list of values separated by a coma : ")
alist=eval(a)
print("the list is : ",alist)

average = sum(alist)/len(alist)
print("the average is : ",average)

for i in alist:
    if alist[i]<average:
        alist.remove[i]

Answer 1

You are almost there. Instead of removing elements, select elements you want to retain instead:

alist = [a for a in alist if a>=average]

Your mistake here is that for i in alist: is iterating over list elements themselves, not indexes, so alist[i] is throwing an error (or returning nonsense).

Answer 2

For the "loop" you can use a filter and a lambda function.

above_average = list(filter(lambda x: x >= average, alist))

For the rest of your code, I suggest you clean it up to something which is safer (use of eval is very bad)

import ast
user_string = raw_input('input a list of numbers separated by a commas: ')
alist = list(ast.literal_eval(user_string)))

---

So, in all, I would write your code as something like this:

import ast
user_string = raw_input('input a list of numbers separated by a commas: ')
numbers = list(ast.literal_eval(user_string)))
average = sum(numbers)/len(numbers)
print('The numbers: {}'.format(numbers))
print('The average: {}'.format(average))
above_average = list(filter(lambda x: x >= average, numbers))
# now do what you want with the above_average numbers.

Answer 3

You are mixing two ways of iterating a list: By index, and by element. In your loop, i is not the index, but the element of the list itself, thus alist[i] won't work.

If you use the for x in somelist loop, then x is the element itself, not the index of the element. For iterating over the indices, you can use for i in range(len(somelist)), or you could use for i, x in enumerate(somelist) to loop over tuples of index and element.

Also note that removing elements from a list or other kinds of collections while you are looping them generally is a bad idea. Better create a copy of the list.

for x in list(alist):     # creates a copy of alist
    if x < average:       # remember: x is the element itselt
        alist.remove(x)   # remove element x from list

But the way you do it (with eval of a comma-separated string of numbers), alist is a tuple, not a list, and thus has no remove method at all. Thus you either have to convert it to a list before (alist = list(eval(a)), or use one of the approaches given in the other answers, creating a new list using list comprehension or filter and retaining the "good" elements.

Answer 4

Other answers tell you how to do it. I'll tell you why it doesn't work:

You iterate over the list and, at the same time, modify it.

This leads to items being missed during the iteration.

Why?

Internally, the iteration works via an index to the list. So it is the same as doing

idx = 0
while True:
    try:
        i = alist[idx]
    except IndexError: 
        break
    idx += 1
    if alist[i] < average:
        alist.remove(i)

What happens if you are at the element #3, go to the next one and then remove #3? Right, the indexes of the remaining ones move down and you are pointing to the one which formerly was #5. The old #4 is skipped at this test.

(BTW, I don't know if you noticed, I have replaced your [] behind .remove with ().)

Answer 5

As a general principle for asking StackOverflow questions like this, you should always include example input and output -- show what happens, and what you expect to happen.

In this case, I believe there are two three problems with your code:

Edit: Third, but possibly most importantly, look at glglgl's answer. If you implement the two fixes I describe below, you'll still have one problem: your code won't necessarily remove all the items you want to remove, because it'll skip over some items.

First, you say alist[i], which grabs the element of alist at index i. But saying for i in alist makes i be successive elements in the list already. Example:

mylist = [1, 2, 4]
for i in mylist:
    print(i)

Would give you the output:

1
2
4

If you instead said this (which is like what you wrote)

mylist = [1, 2, 4]
for i in mylist:
    print(mylist[i])

It wouldn't work as you'd expect, because you'd get the element at index 1, the element at index 2, and then try to get the element at index 4, but that wouldn't exist. You'll get something like this:

2
4
IndexError: list index out of range

Second, your syntax for removing an element is wrong. You should use alist.remove(i) instead of alist.remove[i]. You want to call a function, so you use parentheses. The square brackets are for indexing and slicing.