CODEWITHSUNDEEP
pythonpython-3.x
user12286313
user12286313

Reputation:

Python iterating a list based a condition

This is my current snippet:

l = ['i', 'b', 'c', 'a', '3', 'a', '7', 'a', '9', 'k', 'a', '66', 'k', 'd']

new_list = []
a = {}
for i in range(len(l)):
    if 'a' == l[i]:
        a['plus_line'] = l[i+1]
        new_list.append(a)


print(new_list)

I am expecting output like this below:

[{'plus_line': '3'}, {'plus_line': '7'}, {'plus_line': '9'}, {'plus_line': '66'}]

But I am getting:

[{'plus_line': '66'}, {'plus_line': '66'}, {'plus_line': '66'}, {'plus_line': '66'}]

I am trying to achieve when a will be in the list, then the next item of the a will be the value of the dict.

Upvotes: 1

Views: 64

Answers (4)

Vicrobot
Vicrobot

Reputation: 3988

See this:

>>> d = {}
>>> d['hi there'] = 23
>>> d['hi there'] = 24
>>> d
{'hi there': 24}

You're overwriting the key 'plus_line' of dictionary a.

Items you're appending in list new_list are referencing to dictionary a. Thus the problem.

Possible solution for that can be:

>>> new_list = []
>>> for i in range(len(l)):
...     if 'a' == l[i] and i < len(l) - 2:
...             new_list.append({'plus_line':l[i+1]})
... 
>>> new_list
[{'plus_line': '3'}, {'plus_line': '7'}, {'plus_line': '9'}, {'plus_line': '66'}]

Upvotes: 0

RightmireM
RightmireM

Reputation: 2492

You simply needed to reset your a = {} at the start of each loop (not outside the loop). By initializing it outside the loop, you created one dictionary named a, which has only one key, plus line which keeps getting assigned a new value. 

l = ['i', 'b', 'c', 'a', '3', 'a', '7', 'a', '9', 'k', 'a', '66', 'k', 'd']

new_list = []
for i in range(len(l)):
    a = {}
    if 'a' == l[i]:
        a['plus_line'] = l[i+1]
        new_list.append(a)


print(new_list)

OUTPUT:

[{'plus_line': '3'}, {'plus_line': '7'}, {'plus_line': '9'}, {'plus_line': '66'}]

Upvotes: 0

Dan
Dan

Reputation: 1587

itzhaki explains the problem with your code, here's another possible way of doing it:

l = ['i', 'b', 'c', 'a', '3', 'a', '7', 'a', '9', 'k', 'a', '66', 'k', 'd']

result = [{'plus_line':l[n+1]} for n, i in enumerate(l) if i == 'a']

print(result)

Output:

[{'plus_line': '3'},
 {'plus_line': '7'},
 {'plus_line': '9'},
 {'plus_line': '66'}]

Upvotes: 2

itzhaki
itzhaki

Reputation: 840

Your problem is that you're mutating the dict named a. Instead you could do something like:

l = ['i', 'b', 'c', 'a', '3', 'a', '7', 'a', '9', 'k', 'a', '66', 'k', 'd']

new_list = []
for i in range(len(l)):
    if 'a' == l[i]:
        new_list.append({'plus_line': l[i+1]})

print(new_list)

This will create a new dict instead of mutating an existing one.

Upvotes: 4

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