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Nov 3rd, 2009

File to dictionary

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If I have a file that has a content of this kind:
a,b
c,d
e,f
g,h

I need to make a list of the letters in the first column, i.e., [a, c, e, g]. I wrote some code which prints these letters but I don't know how to put them into a list. Here is the code:

Python Syntax (Toggle Plain Text)
def file_to_dict(f):
    for line in f:
        columns = line.split(",")
        print columns[0]
def file_to_dict(f):
    for line in f:
        columns = line.split(",")
        print columns[0]

Your help is appreciated.

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pyprog

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Nov 3rd, 2009

Re: File to dictionary

Hint:

Python Syntax (Toggle Plain Text)
>>> l = []
>>> l += [ "a" ]
>>> l
['a']
>>>
>>> l = []
>>> l += [ "a" ]
>>> l
['a']
>>>

masterofpuppets

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Nov 3rd, 2009

Re: File to dictionary

Click to Expand / Collapse Quote originally posted by masterofpuppets ...

Hint:
Python Syntax (Toggle Plain Text)
>>> l = []
>>> l += [ "a" ]
>>> l
['a']
>>>
>>> l = [] >>> l += [ "a" ] >>> l ['a'] >>>

I did the following

Python Syntax (Toggle Plain Text)
def file_to_dict(f):
    list1 = []
    for line in f:
        columns = line.split(",")
        letters = columns[1]
    list1 += [letters]
    print list1
def file_to_dict(f):
    list1 = []
    for line in f:
        columns = line.split(",")
        letters = columns[1]
    list1 += [letters]
    print list1

but the output is just ['g']. It always prints only the last letter. How to print them all in a list?

pyprog

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Nov 3rd, 2009

Re: File to dictionary

hi again,
the problem is that you have to add the letter in the list while you are in the for loop

Python Syntax (Toggle Plain Text)
for line in f:
    columns = line.split(",")
    letters = columns[ 0 ]
    list1 += [ letters ]
for line in f:
    columns = line.split(",")
    letters = columns[ 0 ]
    list1 += [ letters ]

also I am not sure why you put columns[ 1 ], I thought you wanted the first element, which is columns[ 0 ]

masterofpuppets

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Nov 3rd, 2009

Re: File to dictionary

Click to Expand / Collapse Quote originally posted by masterofpuppets ...

hi again,
the problem is that you have to add the letter in the list while you are in the for loop
Python Syntax (Toggle Plain Text)
for line in f:
    columns = line.split(",")
    letters = columns[ 0 ]
    list1 += [ letters ]
for line in f: columns = line.split(",") letters = columns[ 0 ] list1 += [ letters ]
also I am not sure why you put columns[ 1 ], I thought you wanted the first element, which is columns[ 0 ]

yeah sorry...never mind the [1]...my mistake...thank you for your help...much appreciated...

pyprog

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Nov 3rd, 2009

Re: File to dictionary

The more common approach would be to use append() ...

python Syntax (Toggle Plain Text)
for line in f:
    columns = line.split(",")
    letters = columns[ 0 ]
    list1.append(letters)
for line in f:
    columns = line.split(",")
    letters = columns[ 0 ]
    list1.append(letters)

I think it is less mistake prone. I also want to point out, that should you want to read the second column (the end of each line), you will find a newline character attached.

Since you title is File to dictionary this may become important.

Last edited by vegaseat; Nov 3rd, 2009 at 5:00 pm. Reason: newline

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DaniWeb's Hypocrite

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Nov 3rd, 2009

Re: File to dictionary

To make it short with python list comprehensions.
Not that this make code better or more readable in many cases.
But think this line is not so bad to understand.

Python Syntax (Toggle Plain Text)
>>> f = [line[0] for line in open('ab.txt')]
>>> f
['a', 'c', 'e', 'g']
>>>
>>> f = [line[0] for line in open('ab.txt')]
>>> f
['a', 'c', 'e', 'g']
>>>

Read every characters.
So colum 2 will be line[2]

Last edited by snippsat; Nov 3rd, 2009 at 7:04 pm.

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