CODEWITHSUNDEEP
pythonlistpython-3.x
Electric
Electric

Reputation: 537

Filter list based on the element in nested list at specific index

I have a list of list containing:

[['4.2','3.4','G'],['2.4','1.2','H'],['8.7','5.4','G']]

and i want to obtain the value from the list of list by referring to the alphabet in the third section of every list inside the list of list.

example, I want python to print the element represented by letter 'G' for every item in the list of list.

output = [4.2,3.4]
         [8.7,5.4]

Here's what I've tried:

L = [['4.2','3.4','G'],['2.4','1.2','H'],['8.7','5.4','G']]
newList = []

for line in L:
    if line[0][2] == 'G'
        newList.append([float(i) for i in line[0:2]])
print(newList)

my error would be on line 5 as I'm not sure if i am able to do it this way. Regards.

Upvotes: 3

Views: 3034

Answers (7)

nitzpo
nitzpo

Reputation: 517

I would suggest using a collections.defaultdict, as a multi-value dictionary:

from collections import defaultdict
d = defaultdict(list)
for x in L:
    d[x[2]].append(x[:2])

Now you can use d['G'] to get what you wanted, but also d['H'] to get the result for 'H'!

Edit: Source append multiple values for one key in Python dictionary

Upvotes: 2

Moinuddin Quadri
Moinuddin Quadri

Reputation: 48077

You may create a function to return sub-lists based on the element using an list comprehension expression along with the usage of map as:

def get_element_by_alpha(alpha, data_list):
    #        v map returns generator object in Python 3.x,hence type-cast to `list`
    return [list(map(float, s[:2])) for s in data_list if s[2]==alpha]
    #                 ^ type-cast the number string to `float` type

Sample Runs:

>>> my_list = [['4.2','3.4','G'],['2.4','1.2','H'],['8.7','5.4','G']]

>>> get_element_by_alpha('G', my_list)
[[4.2, 3.4], [8.7, 5.4]]

>>> get_element_by_alpha('H', my_list)
[[2.4, 1.2]]

>>> get_element_by_alpha('A', my_list)  # 'A' not in the list
[]

Upvotes: 0

Fuji Komalan
Fuji Komalan

Reputation: 2047

@Electric your edited code.

L = [['4.2','3.4','G'],['2.4','1.2','H'],['8.7','5.4','G']]
newList = []

# line = ['4.2','3.4','G']
for line in L:
    if line[2] == 'G': # ':' was missing.
        newList.append(line[:2]) # line[:2] => ['4.2','3.4']
print(newList)

Upvotes: 0

Martin
Martin

Reputation: 34

A list comprehension will do this:

newList = [[float(j) for j in i[:-1]] for i in L if i[2]=='G']

Upvotes: 0

SuperNova
SuperNova

Reputation: 27466

you can use a dictionary by iterating the list of lists.

lst = [['4.2','3.4','G'],['2.4','1.2','H'],['8.7','5.4','G']]
dict1 = {}

for l in lst:
    if l[2] in dict1.keys():
        dict1[l[2]].append(l[0:2])
    else:
        dict1[l[2]] = [l[0:2]]
        print l[0:2]

print dict1['G']

Upvotes: 0

JkShaw
JkShaw

Reputation: 1947

There are 2 issues in your code,

1. line = ['4.2', '3.4', 'G'] for 1st iteration
hence to check for 'G', look out for line[2] == 'G' instead of line[0][3] == 'G'
2. use 'G' instead off 'house'.

>>> for line in L:
...   if line[2] == 'G':
...     newList.append([float(i) for i in line[0:2]])
... 
>>> newList
[[4.2, 3.4], [8.7, 5.4]]

Upvotes: 1

RomanPerekhrest
RomanPerekhrest

Reputation: 92854

Simple list comprehension:

L = [['4.2','3.4','G'],['2.4','1.2','H'],['8.7','5.4','G']]
newList = [l[0:2] for l in L if l[2] == 'G']

print(newList)

The output:

[['4.2', '3.4'], ['8.7', '5.4']]

Upvotes: 3

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