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jackcrouch
jackcrouch

Reputation: 33

Python: Why do these lines of code display different values?

I am relatively new to coding and python and I am trying to wrap my head round some concepts. One I am struggling on is the split() function and why these two pieces of code produce two different outputs

y = ["hello\n", "world\n", "python\n"]
x = [line.strip() for line in y]
print(x)

and

y = ["hello\n", "world\n", "python\n"]
for line in y:
    x = [line.strip()]
print(x)

The first piece of code produces an output of 

['hello', 'world', 'python']

Whilst the second produces

['python']

Does anyone know why this is, as to me they should do the same thing by both producing an output of 

['python']

thanks

Upvotes: 3

Views: 69

Answers (5)

kederrac
kederrac

Reputation: 17322

in your first case:

y = ["hello\n", "world\n", "python\n"]
x = [line.strip() for line in y]
print(x)

you are using a list comprehension to define your variable x by iterating over all the elements from y and keeping them into a new list, this line is equivalent with:

x = []
for line in y:
    x.append(line.strip)

in your second case:

y = ["hello\n", "world\n", "python\n"]
for line in y:
    x = [line.strip()]
print(x)

you are assigning to the same variable x a different value on each iteration of the for loop this means that your variable x will end to have the last value from your list y (with str.strip applied)

another nice way to achieve your desired output:

list(map(str.strip, y))

Upvotes: 1

abhiarora
abhiarora

Reputation: 10430

Does anyone know why this is, as to me they should do the same thing by both producing an output of ['python'].

Your first code uses list comprehension which is used to create a sequence. It provides a concise way to create lists or sequence of elements.

Your second code is basically using a for loop just to get the last element of list y. for loop is used if you want to iterate through a sequence. However, you are also overwriting the variable x in each of your iteration and hence, when you come out of for loop, it will be pointing to the last element (i.e., ['python']). Try this:

for line in y:
    x = [line.strip()]
    print(x)

If you just want the last element of your list y and strip() it, you can also do that via:

x = [ y[-1].strip() ]

OR

If you want the list, then use your first code.

MORE INFORMATION

If you can change your second code to:

x = [ ]
y = ["hello\n", "world\n", "python\n"]
for line in y:
    x.append(line.strip())
print(x)

Then both will print same output (i.e., ['hello', 'world', 'python']).

Upvotes: 2

Code-Apprentice
Code-Apprentice

Reputation: 83527

x = [line.strip()] creates a new list with a single element each time you iterate through the loop. You can more easily see what is happening by moving the print() inside the loop:

y = ["hello\n", "world\n", "python\n"]
for line in y:
    x = [line.strip()]
    print(x)

Upvotes: 1

Óscar López
Óscar López

Reputation: 236004

The first version of your code is using a list comprehension, where you accumulate the results of processing each element in an output list that contains all the elements.

Whereas in the second version, you process one element at a time inside the loop, saving the result in a single-element list and overwriting the previous value, so when you exit the loop the variable will contain the last element that was processed.

Upvotes: 2

Pete
Pete

Reputation: 569

In the second one you overwrite x for each loop iteration, and you only see x's value from the last iteration.

Upvotes: 3

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