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"?

Answer 2

A couple of things for ya:

The loading would be "better" like this:

with file('morsecodes.txt', 'rt') as f:
   for line in f:
      line = line.strip()
      if len(line) > 0:
         # do your stuff to parse the file

That way you don't need to close, and you don't need to manually load each line, etc., etc.

for letter in userInput:
   if ValidateLetter(letter):  # you need to define this
      code = GetMorseCode(letter)  # from my other answer
      # do whatever you want

Answer 3

# Retain a map of the Morse code
conversion = {}

# Read map from file, add it to the datastructure
morseCodeFile = file('morseCode.txt')
for line in moreCodeFile:
    conversion[line[0]] = line[2:]
morseCodeFile.close()

# Ask for input from the user
s = raw_input("Please enter string to translate")
# Go over each character, and print it the translation.
# Defensive programming: do something sane if the user 
# inputs non-Morse compatible strings.    
for c in s:
    print conversion.get(c, "No translation for "+c)

Answer 4

Use 'index'.

def GetMorseCode(letter):
   index = letterList.index(letter)
   code = codeList[index]
   return code

Of course, you'll want to validate your input letter (convert its case as necessary, make sure it's in the list in the first place by checking that index != -1), but that should get you down the path.

Answer 5

I can't leave this question in this state with that final code in the question hanging over me...

dan: here's a much neater and shorter version of your code. It would be a good idea to look at how this is done and code more this way in future. I realise you probably have no further need of this code, but learning how you should do it is a good idea. Some things to note:

• There are only two comments - and even the second is not really necessary for someone familiar with Python, they'll realise NL is being stripped. Only write comments where it adds value.

• The with statement (recommended in another answer) removes the bother of closing the file through the context handler.

• Use a dictionary instead of two lists.

• A generator comprehension ((x for y in z)) is used to do the translation in one line.

• Wrap as little code as you can in a try/except block to reduce the probability of catching an exception you didn't mean to.

• Use the input() argument rather than print()ing first - Use '\n' to get the new line you want.

• Don't write code across multiple lines or with intermediate variables like this just for the sake of it:

  a = a.b()
  a = a.c()
  b = a.x()
  c = b.y()
  

  Instead, write these constructs like this, chaining the calls as is perfectly valid:

  a = a.b().c()
  c = a.x().y()
  

code = {}
with open('morseCode.txt', 'r') as morse_code_file:
    # line format is <letter>:<morse code translation>
    for line in morse_code_file:
        line = line.rstrip()  # Remove NL
        code[line[0]] = line[2:]

user_input = input("Enter a string to convert to morse code or press <enter> to quit\n")
while user_input:
    try:
        print(''.join(code[x] for x in user_input.replace(' ', '').upper()))
    except KeyError:
        print("Error in input. Only alphanumeric characters, a comma, and period allowed")

    user_input = input("Try again or press <enter> to quit\n")

Answer 6

# Open the file
f = open('morseCode.txt', 'r')

# Read the morse code data into "letters" [(lowercased letter, morse code), ...]
letters = []
for Line in f:
    if not Line.strip(): break
    letter, code = Line.strip().split() # Assuming the format is <letter><whitespace><morse code><newline>
    letters.append((letter.lower(), code))
f.close()

# Get the input from the user
# (Don't use input() - it calls eval(raw_input())!)
i = raw_input("Enter a string to be converted to morse code or press <enter> to quit ") 

# Convert the codes to morse code
out = []
for c in i:
    found = False
    for letter, code in letters:
        if letter == c.lower():
            found = True
            out.append(code)
            break

    if not found: 
        raise Exception('invalid character: %s' % c)

# Print the output
print ' '.join(out)

Answer 7

For the actual processing I'd keep a string of finished product, and loop through each letter in the string they have entered. I'd call a function to convert a letter to morse code, then add it to the string of existing morse code.

finishedProduct = []
userInput = input("Enter text")
for letter in userInput:
    finishedProduct.append( letterToMorseCode(letter) )
theString = ''.join(finishedProduct)
print(theString)

You could either check for space in the loop, or in the function that is called.

Answer 8

Create a lookup table first:

morse = [None] * (ord('z') - ord('a') + 1)
for line in moreCodeFile:
    morse[ord(line[0].lower()) - ord('a')] = line[2:]

Then convert using the table:

for ch in userInput:
    print morse[ord(ch.lower()) - ord('a')]