How to take Nested List as input in Python

Question

I have to take input from the user in the following format and make a nested list from it. The first line is the number of rows.

3  
Sourav Das 24 M  
Titan Das 23 M  
Gagan Das 22 F

The nested list should be like :

parentlist = [  
['Sourav', 'Das', '24', 'M']  
['Titan', 'Das', '23', 'M']  
['Gagan', 'Das', '22', 'M']  
]

I have written the following code :

k = int(raw_input())
parentlist = [[]]
for i in range(0, k):
    str1 = raw_input()
    parentlist[i] = str1.split()

But it gives some index out of bound exception after entering the 2nd row (as shown below). What's wrong in the code for which it is giving this exception ?

3
Sourav Das 24 M
Titan Das 23 M
Traceback (most recent call last):
  File "nested.py", line 5, in <module>
    parentlist[i] = str1.split()
IndexError: list assignment index out of range

(I am new to Python. So point out any other mistakes too if you find any in my code.)

I actually need to take input as shown in my question and then sort it according to the "age" like this : stackoverflow.com/questions/409370/… But I am having problem in taking the input from the user. — titan7585
– titan7585, Commented Nov 7, 2014 at 15:08

Jakube · Accepted Answer · 2020-08-17 15:49:07Z

9

When your reading the second line, you attempt to store the splitted line into parentlist[1]. But your parentlist has only one element (parentlist[0]).

The solution is to append the list.

k = int(raw_input())
parentlist = []
for i in range(0, k):
    str1 = raw_input()
    parentlist.append(str1.split())

edited Aug 17, 2020 at 15:49

answered Nov 7, 2014 at 15:08

Jakube

3,6024 gold badges26 silver badges44 bronze badges

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

1 Comment

titan7585 Over a year ago

That worked. Thanks. Being new to Python, I wasn't able to trace how this was going out of bound.

Vishnu Upadhyay · Accepted Answer · 2014-11-07 15:19:23Z

2

You should simply use list comprehension as code will be shorter and faster.

k = int(raw_input())
l = []
print [raw_input().split() for i in range(0, k)]

answered Nov 7, 2014 at 15:19

Vishnu Upadhyay

5,0611 gold badge17 silver badges24 bronze badges

Comments

Georgy · Accepted Answer · 2019-01-22 16:30:07Z

2

In Python 3:

l= [[input(), float(input())] for _ in range(int(input()))]
print l

Input:

5
Harry
37.21
Berry
37.21
Tina
37.2
Akriti
41
Harsh
39

Output:

[[Harry,37.21],[Berry,37.21],[Tina,37.2],[Akriti,41],[Harsh,39]]

edited Jan 22, 2019 at 16:30

Georgy

14k7 gold badges69 silver badges80 bronze badges

answered Jan 22, 2019 at 16:01

coderaky

215 bronze badges

1 Comment

Georgy Over a year ago

print l is not Python 3

Daniel Hepper · Accepted Answer · 2014-11-07 15:13:39Z

1

Your parentlist is a list with one element. On the second iteration for your for loop, you try to access the second element of parentlist, that causes the IndexError. Lists in Python work different from e.g. arrays in JavaScript or PHP.

What you actually want to do is create an empty list and then append the result of str1.split() to it.

k = int(raw_input())
parentlist = []
for i in range(0, k):
    str1 = raw_input()
    parentlist.append(str1.split())

answered Nov 7, 2014 at 15:13

Daniel Hepper

30.1k10 gold badges78 silver badges79 bronze badges

2 Comments

gboffi Over a year ago

@? This answer is correct and exhaustive, why the down vote? +1

titan7585 Over a year ago

I was wondering the same. Someone has downvoted the question and some of the answers.

gboffi · Accepted Answer · 2014-11-07 15:30:02Z

1

You were quite close

k = int(raw_input())
parentlist = []
for i in range(k):
    str1 = raw_input()
    parentlist.append(str1.split())

initialize parentlist to an empty list, it is important because later we want to extend it
- read an input line
- split the input line -> a list
- tell the parentlist to append the new list to what it has already got

If you want to go in a bad direction, you can do, similar to your code

parentlist = [[]]*k
for i in range(k):
    parentlist[i] = raw_input().split()

Exercise, find what the syntax [[]]*k stands for...

edited Nov 7, 2014 at 15:30

answered Nov 7, 2014 at 15:09

gboffi

25.4k10 gold badges62 silver badges98 bronze badges

5 Comments

titan7585 Over a year ago

I tried it in this way, but it still gave the same error.

gboffi Over a year ago

I edited my answer, s/[[]*3]/[[]]*3/, as in Mr. Kozlov's answer.

gboffi Over a year ago

A bad direction because if you replace a list item, as in our discussion, everything is fine, but if you modify a list item, strange things are going to happen... Try the following in the interpreter l = [[]]*5; l[0]=1 ; l[1].append(5) You've already seen what is append, can you tell me what you get by printing l? Well, now try print l and come back with an explanation of what happened...

titan7585 Over a year ago

It is appending 5 to the unused l[2], l[3] and l[4] also along with the l[1]. So what I understood is that we can use [[]] * k format only when we are sure of how many spaces we are going to need.

gboffi Over a year ago

Yes, mutating all elements is danger #1, danger #2 well, you found it yourself, you must know how many slots you need and in many cases (eg processing a file) you don't know it in advance, so imho it's better to memorize the idiom that is safer and more general, isn't it?

Sergii Kozlov · Accepted Answer · 2014-11-07 15:20:32Z

0

Size of your parentlist [[]] is 1, where on 0 position empty list. Your code would be work if you put into parentlist k lists:

parentlist = [[]] * k

Or use append instead, but with append additional look up of method name is necessary.

edited Nov 7, 2014 at 15:20

answered Nov 7, 2014 at 15:13

Sergii Kozlov

8488 silver badges12 bronze badges

Comments

Somanshu Choudhary · Accepted Answer · 2020-05-03 07:55:46Z

0

l=[]
for _ in range(int(input())):
    name = input()
    score = float(input())
    l.append([name,score])
print(l)

answered May 3, 2020 at 7:55

Somanshu Choudhary

11 bronze badge

Comments

Sergey Shubin · Accepted Answer · 2020-07-21 10:04:10Z

0

scorelist =[] 
n = int(input())
for i in range(n):
    name.append(str(input()))
    score.append(float(input()))

for i in range(n):    
    scorelist.append([name[i],score[i]])

print(scorelist)

edited Jul 21, 2020 at 10:04

Sergey Shubin

3,2684 gold badges27 silver badges40 bronze badges

answered Jul 21, 2020 at 8:39

nikhitha m

1

1 Comment

greyfade Over a year ago

It would be helpful to expand on this answer with some explanations.

Puja Kumari · Accepted Answer · 2020-09-18 04:59:21Z

0

k = int(input())
parentlist = [None]*k
for i in range(0, k):
    str1 = input()
    parentlist[i] = str1.split()
print(parentlist)

answered Sep 18, 2020 at 4:59

Puja Kumari

1

1 Comment

BDL Over a year ago

Although this code might solve the problem, a good answer should also explain what the code does and how it helps.

SNEHEL · Accepted Answer · 2021-09-23 16:56:30Z

0

You can also use something like this :-


    k = int(input())
    parentlist = []
    for i in range(k):
        parentlist.append([j for j in input().split()])
    
    print(parentlist)

answered Sep 23, 2021 at 16:56

SNEHEL

1

1 Comment

Simas Joneliunas Over a year ago

Code alone makes for a pretty low quality answer. Could you explain us what your code snippet does?

Collectives™ on Stack Overflow

How to take Nested List as input in Python

10 Answers 10

1 Comment

Comments

1 Comment

2 Comments

5 Comments

Comments

Comments

1 Comment

1 Comment

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

10 Answers 10

1 Comment

Comments

1 Comment

2 Comments

5 Comments

Comments

Comments

1 Comment

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related