CS150 - Fall 2011 - Class 6

  • Problem set 5 problem 1

  • admin
       - Section A: sorry for going over
       - Do the Lab prep for Friday! It's particularly important for this lab.
       - CS lunch talk tomorrow 12:20 in MBH 104: Identification of Individual Spotted Salamanders
          - extra credit if you attend and submit a 1 paragraph summary

  • A quick review of strings
       - we can concatenate strings
          >>> "this is " + "multiple " + "strings"
          'this is multiple strings'
       - if we want to merge a string and a number, we need to convert the number to a string
          >>> year = 2011
          >>> "The current year is " + str(year)
          'The current year is 2011'
       - we can index particular characters in the string using []'s. Indexing starts at 0
          >>> test = "a string"
          >>> test[0]
          'a'
          >>> test[2]
          's'
          >>> test[-2]
          'n'
       - we can get a substring (or part) of a string using slicing, giving a range of indices
          >>> test[2:4]
          'st'
          >>> test[1:6]
          ' stri'
       - We commonly want to iterate over the characters in a string. We can do this using range over the length of the string

          for i in range(len(test)):
             print test[i] # do something with test[i]

       - Strings are also objects
          - objects represent data
          - but they also have "methods" that we can call on them
             - to call a method, we use the '.' notation

             >>> "this is a string".upper()
             'THIS IS A STRING'
             >>> "this is a string".capitalize()
             'This is a string'
             >>> "this is a string".find("a")
             8

  • What other methods might we want for strings?
       - upper / lower
       - find
       - replace
       - startswith
       - isalnum
       - isalpha
       - isdigit
       - islower
       - isupper
       - Many more, look at the documentation: http://docs.python.org/library/string.html

  • A few examples to try
       >>> test = " A string"
       >>> test[4]
       't'
       >>> test[-5]
       't'
       >>> test[-10]
       Traceback (most recent call last):
        File "<string>", line 1, in <fragment>
       IndexError: string index out of range
       >>> test[0:2]
       ' A'
       >>> test[2:7]
       ' a stri'
       >>> test[7:]
       'ng'
       >>> test.lower()
       " a string"
       >>> test.upper()
       " A STRING"
       >>> test.find("i")
       6
       >>> test.find("ring")
       5
       >>> test.find("A string")
       1
       >>> test.find("banana");
       -1
       >>> " This is a string".find("i", 4)
       6
       >>> test.replace("s", "S")
       " A String"
       >>> test.replace("stri", "ris");
       " A rising"
       >>> "This is a string".replace("i", "")
       "Ths s a strng"
       >>> test.upper()[3:]
       "STRING"
       >>> test.upper().replace("i", "n").lower().find("n")
       >>> test[0:3] + "longer " + test[3:]
       " A longer string"
       >>> test.startswith(" A ")
       True
       >>> test.startswith(" a")
       False
       >>> test.endswith("n");
       False
       >>> test.endswith("string");
       True
       >>> test.lower()[6:8].startswith("i")
       True

  • strings are immutable!
       - test.replace("i", "a") does NOT change test
       - test.upper() does NOT changes test to upper case, but returns a new string that is the capitalize version
    •