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Nov 7th, 2009

Working with files

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I have a file like this:
a,z,1
b,y
c,x,1
d,w,1
e,v
f,u
What I need to do is to create a dictionary that has the characters in the first column as keys and characters in the third column as values. The rest should be ignored, i.e. {a:1, c:1, d:1}. This is what I have so far:

Python Syntax (Toggle Plain Text)
def create_dict(f):
    f = open("something.txt")
    d = {}
    for line in f:
        columns = line.split(",")
        letters = columns[0]
        numbers = columns[2]
def create_dict(f):
    f = open("something.txt")
    d = {}
    for line in f:
        columns = line.split(",")
        letters = columns[0]
        numbers = columns[2]

I am confused about next steps. Please, help.

Last edited by pyprog; Nov 7th, 2009 at 2:28 am.

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pyprog

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since Oct 2009

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Nov 7th, 2009

Re: Working with files

The only problem is the txt file must have 3 rows or the index becomes out of range.
Edit: You can use a Try Except block to only update the dict if there is a value in the 3ed row.

Python Syntax (Toggle Plain Text)
def create_dict():
    f = open("something.txt")
    d = {}
    for line in f:
        try:
            columns = line.split(",")
            letters = columns[0]
            numbers = columns[2]
            data = {letters:numbers}
            d.update(data)
        except(IndexError):
            pass
 
        print d
 
create_dict()
def create_dict():
    f = open("something.txt")
    d = {}
    for line in f:
        try:
            columns = line.split(",")
            letters = columns[0]
            numbers = columns[2]
            data = {letters:numbers}
            d.update(data)
        except(IndexError):
            pass

        print d

create_dict()

Last edited by ShadyTyrant; Nov 7th, 2009 at 3:04 am. Reason: May have found solution

ShadyTyrant

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Junior Poster

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since Nov 2009

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Nov 7th, 2009

Re: Working with files

I usually whip up a little test program. Here you use the length of your columns list to avoid problems ...

Python Syntax (Toggle Plain Text)
# possible test data
data = """\
a,z,1
b,y
c,x,1
d,w,1
e,v
f,u"""
 
fname = "something.txt"
 
# write test data file ...
fout = open(fname, "w")
fout.write(data)
fout.close()
 
 
def create_dict(fname):
    fin = open(fname, "r")
    d = {}
    for line in fin:
        columns = line.split(",")
        if len(columns) > 2 :
            letter = columns[0]
            number = columns[2].rstrip()
            d[letter] = number
    return d
 
print( create_dict(fname) )  # {'a': '1', 'c': '1', 'd': '1'}
# possible test data
data = """\
a,z,1
b,y
c,x,1
d,w,1
e,v
f,u"""

fname = "something.txt"

# write test data file ...
fout = open(fname, "w")
fout.write(data)
fout.close()


def create_dict(fname):
    fin = open(fname, "r")
    d = {}
    for line in fin:
        columns = line.split(",")
        if len(columns) > 2 :
            letter = columns[0]
            number = columns[2].rstrip()
            d[letter] = number
    return d

print( create_dict(fname) )  # {'a': '1', 'c': '1', 'd': '1'}

vegaseat

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DaniWeb's Hypocrite

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5,792 posts
since Oct 2004

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Nov 9th, 2009

Don't get too noisy, try this much shorter hack...

Resort to regular expressions. Begin with:

Python Syntax (Toggle Plain Text)
import re
s = file('something.txt', 'rt').read()
import re
s = file('something.txt', 'rt').read()

Then, if it's OK for the dictionary values to be strings, use:

Python Syntax (Toggle Plain Text)
d = dict( re.findall(r'^(\S),\S,(\d)$', s, re.M) )
d = dict( re.findall(r'^(\S),\S,(\d)$', s, re.M) )

Otherwise use:

Python Syntax (Toggle Plain Text)
d = dict( (key, int(val)) for (key, val) in re.findall(r'^(\S),\S,(\d)$', s, re.M) )
d = dict( (key, int(val)) for (key, val) in re.findall(r'^(\S),\S,(\d)$', s, re.M) )

In both cases, the key is

Python Syntax (Toggle Plain Text)
re.findall(r'^(\S),\S,(\d)$', s, re.M)
re.findall(r'^(\S),\S,(\d)$', s, re.M)

It will extract only the lines that match the expected pattern (non-blank,non-blank,digit), and return a list of tuples containing the 1st and 3rd item. All that's left to do then is to convert these tuples into a dictionary of the desired format (string => string or string => int).

Depending on what your expected pattern is, you may substitute '\W' or '[A-Za-z]' for '\S'. Consult the re module docs for more information.

Last edited by pythopian; Nov 9th, 2009 at 12:17 pm.

pythopian

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