Python Text Processing

• a string is like a list, each character is indexed
• text_variable[index] to access a character
• strings know their length, use len(string_name)
• can iterate through a string by index
• Examples of functions you can call on strings using x = 'this is a Test '
  • 'must'know:
    • split--> x.split(' ') ['this', 'is', 'a', 'test']
    • upper--> x.upper() 'THIS IS A TEST '
    • lower--> x.lower() 'this is a test
    • replace--> x.replace('is', 'lol') 'thlol lol a Test '
    • find--> x.find('is') 2
    • strip--> x.strip() 'this is a Test
  • 'good' to know:
    • startswith--> x.startswith('th') True
    • endswith--> x.endswith('end') False
    • title--> x.title() 'This Is A Test '
    • isalpha--> x.isalpha() False
    • isdigit--> '521'.isdigit() True
    • isspace--> ' '.isspace() True (works with spaces, tabs, or newlines)
• strings are not good at being modified, so usually you create a new string to work with them
  • example:

    • raw_string = 'My phone number is 6508675309. Please call!'
      print(just_number(raw_string)
      
      def just_number(str):
          only_number = ''        # use to build new string rather than trying to delete from existing string
          for ch in str:
              if ch.isdigit():
                  only_number = only_number + ch
      return only_number    

      prints 6508675309
• characters are just a giant enumeration (An enumeration is a complete, ordered listing of all the items in a collection. The term is commonly used in mathematics and computer science to refer to a listing of all of the elements of a set.)
  • big look up table
    • ASCII
    • ASCII2
    • Unicode (bigger ASCII)
  • 'A' -> 'Z' are sequential
  • 'a' -> 'b' are sequential
  • '0' -> '9' are sequential
  • ord(ch) gives us the number associated with the character
  • functions which take strings (same example x as above):
    • len--> len(x) 15
    • ord--> ord('A') 65
    • hash--> hash(x) 2466759895439727657
    • < --> 'abc' < 'zabc' True
    • == --> x == 'this is a Test True
    • in--> 'his' in x True
• Python strings are immutable
  • once a string has been created you cannot set characters
  • to change a string:
    • create a new string holding the new value you want to give it via concatenation
      • see earlier example with the function that only returned the numbers
    • reassigning the string variable (that's allowed)
      • example:

        x = 'abc'
        
        x[1] = 'z'        # TypeError: 'str' object does not support item assignment
        
        x = 'azc'         # can reassign the string

    • often build up new string through concatenation
      • example:

        def main():
          s1 = 'CS106'
          s2 = 'A'
          s3 = 'I got an ' + s2 + ' in ' + s1 +s2
          print(s3)

        prints I got an A in CS106A
  • important consequence: if you pass a string to a function, you are guaranteed your string won't be changed
• many string algorithms use the 'loop and construct' method
  • 3 examples that give the same result:

    • def reverse_string(str):
          result = ''
          for i in range(len(str)):
              result = str[i] + result
          return result

    • def reverse_string_v2(str):
          result = ''
          for ch in str:
              result = ch + result
          return result

    • def reverse_string_v3(str):
          '''
          This uses the slice operator in a special way. With no start, no end,
          and a delta of -1, slice reverses.
          '''
          return strt[::-1]