Lesson 10 Notes: String Things

• The Things in Strings
  • Strings are actually made up of smaller units, each individual character.
  • Each character has an address number within the string, with the first character being number zero (0).
  • Each individual character, or set of characters, in a string can be accessed using their address number.
• Bits of String
  • Use [ ] to access characters in a string.
  • Example:
    word = "computer"
letter = word[0]
• More Bits of String
  • Use [ # :#] to get set of letters
    part = word[0:3]
  • To pick from beginning to a set point:
    part = [:4]
  • To pick from set point to end:
    part = [3:]
  • To pick starting from the end:
    part = [-1]
• How Long is the String
  • Find out how many characters are in a string by using the len() function.
  • The len() function requires a string as an argument.
  • Example
    word = "computer"
length = len(word)
• Walking Through a String
  • May want to do a test on a string one letter at a time. Two ways to do this: while or for loop.
  • while loop:
    index = 0
while index < len(word):
letter = word[index]
print letter
index=index +1

• For Loop
  • Shorter way to write that same "while" loop is with a "for" loop.
  • Example:
    word = "computer"
for letter in word:
print letter

  • Note that we made up the variable "letter" when we created the for loop.
• Useful "for" loop
  • Sample Program:
    # Vowel counting program
word = raw_input("Enter a word")
vowels = "aeiouy"
count = 0
for letter in word:
    if letter in vowels:
       count = count+1
    else:
       pass
print "Number of vowels =", count
• Comparing Strings
  • Strings can be used in comparison tests.
  • Example:
    if word < "cebu":
print word

  • According to Python, all uppercase letters come before lowercase letters when comparing. (so Z > a).
  • Good idea to convert to same case before comparing.
• String Module
  • The string module contains a number of useful functions including:
    • lower()
    • upper()
    • find()
    • count()
  • See documentation for more functions and uses.
• Review
  • What code is used to access parts of a string?
  • What function finds the number of characters in a string?
  • How is a "for" loop written?
  • What are the guidelines for comparing strings?