How do I iterate through lists with nested lists in Python / why doesn't my code work?

Question

Can someone explain why Python won't let me use i in this manner?

unit1 = [["cats allowed", True], ["bedrooms", 0], ["Balcony", False]]

userPref = []
for i in unit1:
   userPref = userPref.append(unit1[i][1])
   print(unit1[i][1])

I get this error message:

TypeError: list indices must be integers or slices, not list

If I want to iterate through the second item in each nested list, how would I go about doing that?

(FYI: the for loop in nested in an if statement. I omitted that for simplicity.)

did you debug it ? print(i) in the loop you'll understand, you iterate over values, not indices — azro
– azro, Commented Aug 6, 2022 at 16:56
Does this answer your question? TypeError: list indices must be integers or slices, not list — Joooeey
– Joooeey, Commented Aug 6, 2022 at 17:01
you didn't use append correctly, you can see the correct format in my answer — Masoud Gheisari
– Masoud Gheisari, Commented Aug 6, 2022 at 17:32

Masoud Gheisari · Accepted Answer · 2022-08-06 17:17:39Z

2

Some options you have to iterate over a list:

1)

for item in unit1:
   userPref.append(item[1])
   print(item[1])

which item[1] is the second parameter of nested list

2)

for i in range(len(unit1)):
    userPref.append(unit1[i][1])
    print(unit1[i][1])

or if you need item and index together:

for i,item in enumerate(unit1):
    userPref.append(item[1])
    print(item[1])

answered Aug 6, 2022 at 17:17

Masoud Gheisari

1,5271 gold badge16 silver badges23 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

John Gordon · Accepted Answer · 2022-08-06 17:01:19Z

1

for i in unit1:

When you iterate over a list in this way, i becomes each value in the list, not the list index.

So on the first iteration, i is the sub-list ["cats allowed", True].

If you want to iterate over the indexes of a list, use range():

for i in range(len(unit1)):

answered Aug 6, 2022 at 17:01

John Gordon

33.8k9 gold badges48 silver badges72 bronze badges

Comments

josix · Accepted Answer · 2022-08-06 17:10:17Z

0

Python’s for statement iterates over the items of any sequence (a list or a string), in the order that they appear in the sequence.

You can found that differ in python official tutorial, the iteration will traverse all the value in the sequence rather index of that, which is quite different from other languages.

answered Aug 6, 2022 at 17:10

josix

13 bronze badges

Collectives™ on Stack Overflow

How do I iterate through lists with nested lists in Python / why doesn't my code work?

3 Answers 3

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related