Problems with a simple loop

Question

I want to make a loop, that sums up the first two items of a list, and stores the result in a new list. After that it should take the first three items of the first list and sum up these and store the result in the same list aswell.

What I try to do:

x = [1, 2, 3, 4]
y = []

for i in x:
     s = i + (i+1)
     y.append(s)

This does not work at all. Because of my bad understanding its hard to get help from google. I hope some might understand my problem.

y = [sum(x[:i]) for i in (2, 3)] or y = [sum(x[:2]), sum(x[:3])] — Steven Rumbalski
– Steven Rumbalski, Commented Apr 13, 2020 at 15:38
In your loop, i is not an index, it's the value from the list. — Mark
– Mark, Commented Apr 13, 2020 at 15:38
When you are doing i +(i +1) it's not sums the first tow elements you can do it with enumerat of the list and by the index get the second element — Leo Arad
– Leo Arad, Commented Apr 13, 2020 at 15:38
First of all thanks to your fast responses. @StevenRumbalski I tried both of your versions. Can i somehow change the code that i do not have to tipe in every Index of the List? I need this for Lists that are much bigger than the text version. — stnrl
– stnrl, Commented Apr 13, 2020 at 15:52

user13301358 · Accepted Answer · 2020-04-13 15:50:28Z

2

This will work for you

x = [1, 2, 3, 4]
y = []

for i in range(2,len(x)+1):
    y.append(sum(x[:i]))
print(y)

answered Apr 13, 2020 at 15:50

user13301358

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Comments

Darien Schettler · Accepted Answer · 2020-04-13 15:49:26Z

0

Looks like you want a list comprehension that sums incrementally larger and larger lists as you iterate through

This will work.

>>> x = [1,2,3,4]
>>> x
[1,2,3,4]

# Note we put the condition `if i != 0` so that we ignore the first entry in the list
#
# This comprehension essentially is taking the indices for the given list
# and utilizing the indices to generate a subset list of indices (`range(i+1)`)
# and by using these subset indices we can generate a subset of values which
# we can then sum as you requested.
>>> y = [sum([x[idx] for idx in range(i+1)]) for i in range(len(x)) if i !=0]
>>> y
[3, 6, 10]

Let me know if you have any questions!

answered Apr 13, 2020 at 15:49

Darien Schettler

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Comments

user10599980 · Accepted Answer · 2020-04-13 15:50:44Z

0

You don't really need a loop but if you want:

x = [1, 2, 3, 4]
new_list = []

for count in range(len(x) - 2):
  if count <2:
    new_list.append( sum(x[:count+2]) )

print(new_list)

# OR without loop 

x = [1, 2, 3, 4]
new_list = []

sums_of_the_first_two = sum(x[:2])
new_list.append(sums_of_the_first_two)
sums_of_the_first_three = sum(x[:3])
new_list.append(sums_of_the_first_three)

print(new_list)

Edit: The 2nd part in the comment of Steven Rumbalski is the best solution i think

answered Apr 13, 2020 at 15:50

user10599980

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Looks like you want a list comprehension that sums incrementally larger and larger lists as you iterate through

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Looks like you want a list comprehension that sums incrementally larger and larger lists as you iterate through

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