Assume that I have a list:
l = ['foo', 'bar', 'baz']
How to append pre to bar to have:
l = ['foo', 'prebar', 'baz']
?
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4 Answers
Lists are mutable; strings are not. Therefore, assuming you know the index of the string you want to change:
l = ['foo', 'bar', 'baz']
i = 1
l[i] = 'pre' + l[i]
This gives l = ['foo', 'prebar', 'baz'].
Using a list comprehension constructs a whole new list in memory, which is wasteful of resources (though immaterial for small lists). This method modifies the existing list in place.
1 Comment
kindall
Using a slice assignment with a generator expression eliminates the objection of creating a new list, but I agree that directly modifying the desired item is the best approach when possible. If you don't know what item you want to change, that's what
list.index() is for!You can use + operand to concatenate strings together.So you can use a list comprehension to loop over your list and change the item that is equal with 'bar' :
>>> l1=['pre'+i if i=='bar' else i for i in l1]
>>> l1
['foo', 'prebar', 'baz']
1 Comment
supermario
Thanks Kasra. BTW, love your motto! :)
Alternatively to the other answer you can also alter the list directly:
l[1] = l[1] + "bar"
Comments
l=['pre'+i if i=='bar' else i for i in l]
l
['foo', 'prebar', 'baz']
result
['foo', 'prebar', 'baz']
