title: String index not working? - Python
body: <p>I'm Making a sort of cipher that requires a string to be read backwards and i get an indexing error even though the index I reference is well within range:</p>
  
  <pre><code>M = str(input("Input Message: "))
  M = M.upper()
  L = len(M)
  A = ["A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z"]
  
  def DECRYPT():
      global L
      global M
      global A
  
      if L%2 != 0:
          POS = False
      else:
          POS = True
  
      i = L-1
      NM = ""
      while 1:
          if M[i] != " ":
              INDEX = A.index(M[i])
              if POS == True:
                  INDEX += (i + 1)
              else:
                  INDEX -= (i)
              INDEX %= 26        
              NM = NM+A[INDEX]
              i += 1
              if POS == True:
                  POS = False
              else:
                  POS = True
      print("\n"+NM)
  
  def ENCRYPT():
      global L
      global A
      global M
  
      POS = True
      M = M[::-1]
      i = 0
      NM = ""
  
      while 1:
          if i == L:
              break
          if M[i] != " ":
              INDEX = A.index(M[i])
              if POS == True:
                  INDEX += (i + 1)
              else:
                  INDEX -= (i + 1)
              INDEX %= 26        
              NM = NM+A[INDEX]
          i += 1
          if POS == True:
              POS = False
          else:
              POS = True
      print("\n"+NM)
  
  while 1:
      C = int(input("\nWhat do you want to do:\n1) Encrypt Something\n2)Decrypt Something\n\n"))
      if C == 1:
              ENCRYPT()
      if C == 2:
              DECRYPT()
  </code></pre>
  
  <p>where i is a placeholder value. I run it and get this:</p>
  
  <pre><code>Input Message: ABC
  
  What do you want to do:
  1) Encrypt Something
  2)Decrypt Something
  
  2
  Traceback (most recent call last):
    File "C:\Users\Danny\Google Drive\SHIFT.py", line 67, in &lt;module&gt;
      DECRYPT()
    File "C:\Users\Danny\Google Drive\SHIFT.py", line 19, in DECRYPT
      if M[i] != " ":
  IndexError: string index out of range
  </code></pre>
  
  <p>I have tried changing value of i to no avail.</p>