Replacing an element from one list with all elements in another list

Question

I'm having trouble splicing one list into another at a given index. The idea is as such:

"For each word in the primary list, if that word is X, replace it with all the words in the secondary list."

Here's an example. It inserts the secondary list into the primary list at the correct index:

primary_list = ['the', 'black', 'cat', 'jumped']
secondary_list = ['big', 'grey']

for i, word in enumerate(primary_list):
    if word == "black":
        primary_list[i] = secondary_list

primary_list 
>> ['the', ['big', 'grey'], 'cat', 'jumped']

The ideal output is:

['the', 'big', 'grey', 'cat', 'jumped']

I have also tried list comprehension:

primary_list[i] = [item for item in secondary_list]

Not only is it redundant, but it also just does the same as above.

There are some other answers here that used list slicing, but I wasn't sure how to use them in my syntax. Thoughts?

Answer 1

Here's an option using an intermediate result list:

>>> primary_list = ['the', 'black', 'cat', 'jumped']
>>> secondary_list = ['big', 'grey']
>>> result = []
>>> for word in primary_list:
...   if word == "black":
...     result += secondary_list
...   else:
...     result.append(word)
... 
>>> result
['the', 'big', 'grey', 'cat', 'jumped']

Answer 2

The problem in your code was because of two reasons in the following line:

1. trying to assign a list to a specific element in primary_list .
2. not using secondary_list.copy() .

If you had multiple occurrences of 'black' , then this would copy the secondary list there. Any one of which if changed, it would later on change the others as they would share the same memory location.

primary_list[i] = secondary_list

Here is the solution.

final_list = list()
for word in primary_list:
    if word=='black':
        final_list += secondary_list.copy()
    else:
        final_list.append(word)

final_list

Output :

['the', 'big', 'grey', 'cat', 'jumped']

Answer 3

You should create another list and add primer list elements and secondary list elements.

primary_list = ['the', 'black', 'cat', 'jumped']
secondary_list = ['big', 'grey']
result_list=[]
for i, word in enumerate(primary_list):
    if word == "black":
        for secod_val in secondary_list:
            result_list.append(second_val)
    else:
        result_list.append(word)

Answer 4

A simple approach using slicing.

>>> a = [1, 2, 3, 7, 8]
>>> b = [4, 5, 6]
>>> a[3:3] = b
>>> a
[1, 2, 3, 4, 5, 6, 7, 8]

Answer 5

primary_list = ['the', 'black', 'cat', 'jumped']
secondary_list = ['big', 'grey']

for i, word in enumerate(primary_list):
    if word == "black":
        primary_list[i:len(secondary_list)] = secondary_list

print(primary_list)

['the', 'big', 'grey', 'cat', 'jumped']

Answer 6

Another option using strings:

primary_list = ['the', 'black', 'cat', 'jumped']
secondary_list = ['big', 'grey']
for i in range(len(primary_list)):
    if primary_list[i] == 'black':
        primary_list[i] = secondary_list

final_list = repr(primary_list).replace('[', '').replace(']', '').replace("'", '').split(', ')
print(final_list)

Output:

['the', 'big', 'grey', 'cat', 'jumped']