How to read a string one letter at a time in python

Question

I need to convert a string inputed by a user into morse code. The way our professor wants us to do this is to read from a morseCode.txt file, seperate the letters from the morseCode into two lists, then convert each letter to morse code (inserting a new line when there is a space).

I have the beginning. What it does is reads the morseCode.txt file and seperates the letters into a list [A, B, ... Z] and the codes into a list ['– – . . – –\n', '. – . – . –\n'...].

We haven't learned "sets" yet, so I can't use that. How would I then take the string that they inputed, go through letter by letter, and convert it to morse code? I'm a bit caught up. Here's what I have right now (not much at all...)

EDIT: completed the program!

# open morseCode.txt file to read
morseCodeFile = open('morseCode.txt', 'r') # format is <letter>:<morse code translation><\n>   
# create an empty list for letters
letterList = []    
# create an empty list for morse codes
codeList = []
# read the first line of the morseCode.txt
line = morseCodeFile.readline()    
# while the line is not empty
while line != '':        
    # strip the \n from the end of each line
    line = line.rstrip()        
    # append the first character of the line to the letterList        
    letterList.append(line[0])           
    # append the 3rd to last character of the line to the codeList
    codeList.append(line[2:])        
    # read the next line
    line = morseCodeFile.readline()        
# close the file    
morseCodeFile.close()


try:
    # get user input
    print("Enter a string to convert to morse code or press <enter> to quit")    
    userInput = input("")  
    # while the user inputs something, continue   
    while userInput:
        # strip the spaces from their input
        userInput = userInput.replace(' ', '')
        # convert to uppercase
        userInput = userInput.upper()

        # set string accumulator
        accumulateLetters = ''
        # go through each letter of the word
        for x in userInput:            
            # get the index of the letterList using x
            index = letterList.index(x)
            # get the morse code value from the codeList using the index found above
            value = codeList[index]
            # accumulate the letter found above
            accumulateLetters += value
        # print the letters    
        print(accumulateLetters)
        # input to try again or <enter> to quit
        print("Try again or press <enter> to quit")
        userInput = input("")

except ValueError:
    print("Error in input. Only alphanumeric characters, a comma, and period allowed")
    main()   

Answer 1

Why not just iterate through the string?

a_string="abcd"
for letter in a_string:
    print letter

returns

a
b
c
d

So, in pseudo-ish code, I would do this:

user_string = raw_input()
list_of_output = []
for letter in user_string:
   list_of_output.append(morse_code_ify(letter))

output_string = "".join(list_of_output)

Note: the morse_code_ify function is pseudo-code.

You definitely want to make a list of the characters you want to output rather than just concatenating on them on the end of some string. As stated above, it's O(n^2): bad. Just append them onto a list, and then use "".join(the_list).

As a side note: why are you removing the spaces? Why not just have morse_code_ify(" ") return a "\n"?