Python 列表 - 在新列表中复制列表值

Question

This is what I am trying:

I have a text file with few lines of input in the format - parameter value

This text file is read line by line and parameters & values are taken into as list elements.

a = []
for line in fileinput.input():
    a.append((line.strip()).split(' '))
print a

Output:

[['parameter1': 'value1'], ['parameter2': 'value2'], ['parameter3': 'value3']]

I am now trying to move these values - value1, value2, value3, & so on - in to a new list.

I can't figure this out. copy function will copy the full list. Do I use loops to iterate through these values and append to new list? Is it possible to add these values directly to new list while reading from the text file..?

Answer 1

You can do that with a list comprehension:

new_a = [sub[1] for sub in a]

You could also do this:

import operator

new_a = list(map(operator.itemgetter(1), a))

An equivalent of that is to use a lambda function:

new_a = list(map(lambda x: x[1], a))

As mentioned in a comment, you could also do this:

new_a = [value for parameter, value in a]

That last will throw an error if any of the lists in a is not of length two. That could be a good thing or a bad thing depending on what you want.

You could also just use your for loop only add just the second element:

a = []
for line in fileinput.input():
    a.append(line.split(' ')[1])
print a

I must confess, I didn't even notice that until I saw flaschbier's answer.

Answer 2

b.append(line.split(' ')[1])

will append only the value from each line to another list b you have prepared before the loop.

The output of the program from the question will be

[['parameter1', 'value1'], ['parameter2', 'value2'], ['parameter3', 'value3']]

btw...