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I am both new to this forum & programming & Python. I am trying to develop my first program, however I keep coming up against a brick wall regarding one particular issue. I am hopng that some kind sole can put me out of my misery & show me how to do what I want to do correctly. I'm sure it's simple if you know what you are doing, but at the moment I am stupid & do not have a clue what I'm doing :-)

Example:

I need to work with 2 files, A & B

File A contains the text:

This is a test

While file B contains the text:

h
t
s
i
a

I need to create a program that will grab 1 character at a time from file A and then search through file B looking for an identical character. Once the program has found a match, I want it to print the row number where it found the match, then move on grab another character from file A & repeat this process until EOF.

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    Hi, welcome to StackOverflow! You've chosen an interesting challenge (congratulations!) and you've described it well. However, you didn't say at all where you're stuck - reading a file, iterating over it, taking substrings, comparing them... Could you show what you've written so far? Then it's easier for us to give you specific advice. Commented Jun 16, 2013 at 13:57

3 Answers 3

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OK, let's take this one step at a time. First, I would read file B into a structure that's well suited for fast lookup since we're going to be doing that quite often:

chars = {}
with open("B") as lookupfile:
    for number,line in enumerate(lookupfile):
        chars[line.strip()] = number

Now we have a dictionary chars that contains letters as keys and their row number as values:

>>> chars
{'t': 1, 'a': 4, 'i': 3, 'h': 0, 's': 2}

Now we can iterate over the first file. The standard Python iterator for files consumes one line per iteration, not one character, so it's probably best to simply read the entire file into a string and then iterate over that (because for strings, iteration is character-by-character):

with open("A") as textfile:
    text = textfile.read()

Now we iterate over the string and print matching values:

for char in text:
    if char in chars:
        print("Character {0} found in row {1}".format(char, chars[char]))

If you don't like accessing the dictionary twice, you can also use

for char in text:
    found = chars.get(char):    # returns None if char isn't a key in chars
    if found:
        print("Character {0} found in row {1}".format(char, found))

or, using exceptions:

for char in text:
    try:
        print("Character {0} found in row {1}".format(char, chars[char]))
    except KeyError:
        pass
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1 Comment

Hi Tim, really appreciate your help on this. Tried out your code & it works perfectly. I just need to modify now to suit my own little program. I'll also spend some time studying your code, so that I can learn from it for the future. Thanks again! Clinton.
0 
import os
fA = open('C:\\Desktop\\fileA.txt', 'r')
fB = open('C:\\Desktop\\fileB.txt', 'r')

fileb_content = []
for line in fB:
    fileb_content.append(fB.read().split('\n'))

rA = fA.readline().split('\n')[0]

for c in list(rA):
        if(c.strip()):
            if(c.lower() in fileb_content[0]):
                print(fileb_content[0].index(c.lower()))

here i test that character is not empty.

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1 Comment

Hi antitrust, thanks for your input & time. All sorted now after trying the code from another person who kindly replied. Clinton.
0

Read the file A first and store it's content in a variable(using file.read).

with open('A.txt') as f:

    data = f.read()  # now data is: "This is a test"
    # now data is string that dontains all data of the file A.
    # But as searching a character in a string is an O(N) operation
    # so we must convert this string to a better data-structure.
    # As we need the item as well as their first index so we
    # should create a dict here, with character as the key and
    # it's first appearance(first index) as it's value. 
    # Dicts provide O(1) lookup.

    dic = {}
    for ind, char in enumerate(data):
        # store the character in dict only if it is an alphabet
        # also check if it's already present is dict or not.
        if char.isalpha() and char not in dic:
            dic[char] = ind
    #dic is {'a': 8, 'e': 11, 'i': 2, 'h': 1, 's': 3, 'T': 0, 't': 10}

Now open the file B and iterate over it using a for-loop, a for-loop over an file iterator returns one line at a time.(memory efficient approach).

with open('B.txt') as f:
    for char in f:            #iterate one line at a time 
        char = char.strip()   #str.strip strips off whitespaces like '\n'
        if char in dic:
           print dic[char]     # if character is found in dict then
                              # print it's value, i.e index
...             
1
10
3
2
8
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2 Comments

Hi Ashwini,thanks for all your help. I have tried your option, also the option below by Tim. Unfortunately when I tried out your code there were some errors & nothing really happen. so rather than try & work out what was wrong I tried Tim's code which worked perfectly. Thanks for your help though! I do really appreciate it :-)
@user2490853 what errors exactly? I fixed a typo now it should work fine.

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