CODEWITHSUNDEEP
pythonnested-lists
Janmajay
Janmajay

Reputation: 51

Appending a list within a for loop?

There are 2 given lists, suppose:

list1 = ['node1','node2','node3','node4']
list2 = ['goal1','goal2','goal3']

I require a list which returns:

result = [['node1','node2','node3','node4','goal1'],
       ['node1','node2','node3','node4','goal2'],
       ['node1','node2','node3','node4','goal3']]

Here is what I have:

result = []
for i in range (len(list2)):
    list1.append(list2[i])
    result.append(list1)
    list1.pop()

The problem is, result is not being appended with the desired value. It prints,

[['node1', 'node2', 'node3', 'node4'],
 ['node1', 'node2', 'node3', 'node4'],
 ['node1', 'node2', 'node3', 'node4']]

after the for loop is completed.

What am I doing wrong?

Upvotes: 2

Views: 96

Answers (3)

Padraic Cunningham
Padraic Cunningham

Reputation: 180512

You can add list1 each goal to the end to list1 using a list comp:

list1 = ['node1', 'node2', 'node3', 'node4']
list2 = ['goal1', 'goal2', 'goal3']
print([list1 + [gl] for gl in list2])

Output:

[['node1', 'node2', 'node3', 'node4', 'goal1'], 
['node1', 'node2', 'node3', 'node4', 'goal2'], 
['node1', 'node2', 'node3', 'node4', 'goal3']]

Upvotes: 7

glibdud
glibdud

Reputation: 7880

What your loop is doing is repeatedly appending the same list object (list1) to result. You can verify that by doing something like list1.append(True) after running the loop and checking result again:

[['node1', 'node2', 'node3', 'node4', True], 
 ['node1', 'node2', 'node3', 'node4', True], 
 ['node1', 'node2', 'node3', 'node4', True]]

What you want to do is instead make a copy of list1 to append each time, such as:

for i in range (len(list2)):
    list1.append(list2[i])
    result.append(list(list1))
    list1.pop()

Although I would probably instead make use of list concatenation, which implicitly makes a new list:

for item in list2:
    result.append(list1 + [item])

Upvotes: 4

Blair
Blair

Reputation: 6693

You can fix this using extend:

>>> for i in range(len(list2)):
    tmp = []
    tmp.extend(list1)
    tmp.append(list2[i])
    all_paths.append(tmp)


>>> all_paths
[['node1', 'node2', 'node3', 'node4', 'goal1'], ['node1', 'node2', 'node3', 'node4', 'goal2'], ['node1', 'node2', 'node3', 'node4', 'goal3']]

Upvotes: 0

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