CMU 15-112: Fundamentals of Programming and Computer Science
Class Notes: Strings

  1. String Literals
  2. Some String Constants
  3. Some String Operators
  4. Looping over Strings
  5. Example: isPalindrome
  6. Strings are Immutable
  7. Strings and Aliases
  8. Some String-related Built-In Functions
  9. Some String Methods
  10. String Formatting (optional)
  11. String Formatting with f Strings
  12. Basic File IO

  1. String Literals
    1. Four kinds of quotes
      # Quotes enclose characters to tell Python "this is a string!" # single-quoted or double-quoted strings are the most common print('single-quotes') print("double-quotes") # triple-quoted strings are less common (though see next section for a typical use) print('''triple single-quotes''') print("""triple double-quotes""") # The reason we have multiple kinds of quotes is partly so we can have strings like: print('The professor said "No laptops in class!" I miss my laptop.')
      # See what happens if we only use one kind of quote. # It causes a syntax error! We don't even know how to color this. print("The professor said "No laptops in class!" I miss my laptop.")

    2. Newlines in strings
      # A character preceded by a backslash, like \n, is an 'escape sequence'. # Even though it looks like two characters, Python treats it as one special character. # Note that these two print statements do the same thing! print("abc\ndef") # \n is a single newline character. print("""abc def""") print("""\ You can use a backslash at the end of a line in a string to exclude the newline after it. This should almost never be used, but one good use of it is in this example, at the start of a multi-line string, so the whole string can be entered with the same indentation (none, that is). """)

    3. More Escape Sequences
      print("Double-quote: \"") print("Backslash: \\") print("Newline (in brackets): [\n]") print("Tab (in brackets): [\t]") print("These items are tab-delimited, 3-per-line:") print("abc\tdef\tg\nhi\tj\\\tk\n---")

      An escape sequence produces a single character:
      s = "a\\b\"c\td" print("s =", s) print("len(s) =", len(s))

    4. repr() vs. print()
      Sometimes it can be difficult to debug strings! Two strings that look identical when printed may not actually be the same. Have you ever had trouble distinguishing a tab from several spaces in a word processor? The repr() function is sort of like a 'formal' version of print(). Where print() is meant to produce output intended for the user, repr() shows us a representation of the data contained in the string. This can be really useful for debugging! Looking at an example will help you understand the difference:
      print("These look the same when we print them!") s1="abc\tdef" s2="abc def" print("print s1: ",s1) print("print s2: ",s2) print("\nThey aren't really though...") print("s1==s2?", s1==s2) print("\nLet's try repr instead") print("repr s1: ",repr(s1)) print("repr s2: ",repr(s2)) print("\nHere's a sneaky one") s1="abcdef" s2="abcdef \t" print("print s1: ",s1) print("print s2: ",s2) print("s1==s2?", s1==s2) print("repr s1: ",repr(s1)) print("repr s2: ",repr(s2)) print("repr() lets you see the spaces^^^")

    5. String Literals as Multi-line Comments
      """ Python does not have multiline comments, but you can do something similar by using a top-level multiline string, such as this. Technically, this is not a comment, and Python will evaluate this string, but then ignore it and garbage collect it! """ print("wow!")

  2. Some String Constants
    import string print(string.ascii_letters) # abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ print(string.ascii_lowercase) # abcdefghijklmnopqrstuvwxyz print("-----------") print(string.ascii_uppercase) # ABCDEFGHIJKLMNOPQRSTUVWXYZ print(string.digits) # 0123456789 print("-----------") print(string.punctuation) # '!"#$%&\'()*+,-./:;<=>?@[\\]^_`{|}~' print(string.printable) # digits + letters + punctuation + whitespace print("-----------") print(string.whitespace) # space + tab + linefeed + return + ...

  3. Some String Operators
    1. String + and *
      print("abc" + "def") # What do you think this should do? print("abc" * 3) # How many characters do you think this prints? print("abc" + 3) # ...will this give us an error? (Yes)

    2. The in operator
      # The "in" operator is really really useful! print("ring" in "strings") print("wow" in "amazing!") print("Yes" in "yes!") print("" in "No way!")

    3. String indexing and slicing
      1. Indexing a single character
        # Indexing lets us find a character at a specific location (the index) s = "abcdefgh" print(s) print(s[0]) print(s[1]) print(s[2]) print("-----------") print("Length of ",s,"is",len(s)) print("-----------") print(s[len(s)-1]) print(s[len(s)]) # crashes (string index out of range)

      2. Negative indexes
        s = "abcdefgh" print(s) print(s[-1]) print(s[-2])

      3. Slicing a range of characters
        # Slicing is like indexing, but it lets us get more than 1 character. # is this like range(a,b)? s = "abcdefgh" print(s) print(s[0:3]) print(s[1:3]) print("-----------") print(s[2:3]) print(s[3:3])

      4. Slicing with default parameters
        s = "abcdefgh" print(s) print(s[3:]) print(s[:3]) print(s[:])

      5. Slicing with a step parameter
        print("This is not as common, but perfectly ok.") s = "abcdefgh" print(s) print(s[1:7:2]) print(s[1:7:3])

      6. Reversing a string
        s = "abcdefgh" print("This works, but is confusing:") print(s[::-1]) print("This also works, but is still confusing:") print("".join(reversed(s))) print("Best way: write your own reverseString() function:") def reverseString(s): return s[::-1] print(reverseString(s)) # crystal clear!

  4. Looping over Strings
    1. "for" loop with indexes
      s = "abcd" for i in range(len(s)): print(i, s[i])

    2. "for" loop without indexes
      s = "abcd" for c in s: print(c)

    3. "for" loop with split
      # By itself, names.split(",") produces something called a list. # Until we learn about lists (soon!), do not store the result of # split() and do not index into that result. Just loop over the # result, as shown here: names = "fred,wilma,betty,barney" for name in names.split(","): print(name)

    4. "for" loop with splitlines
      # splitlines() also makes a list, so only loop over its results, # just like split(): # quotes from quotes = """\ Dijkstra: Simplicity is prerequisite for reliability. Knuth: If you optimize everything, you will always be unhappy. Dijkstra: Perfecting oneself is as much unlearning as it is learning. Knuth: Beware of bugs in the above code; I have only proved it correct, not tried it. Dijkstra: Computer science is no more about computers than astronomy is about telescopes. """ for line in quotes.splitlines(): if (line.startswith("Knuth")): print(line)

  5. Example: isPalindrome
    A string is a palindrome if it is exactly the same forwards and backwards.
    # There are many ways to write isPalindrome(s) # Here are several. Which way is best? def reverseString(s): return s[::-1] def isPalindrome1(s): return (s == reverseString(s)) def isPalindrome2(s): for i in range(len(s)): if (s[i] != s[len(s)-1-i]): return False return True def isPalindrome3(s): for i in range(len(s)): if (s[i] != s[-1-i]): return False return True def isPalindrome4(s): while (len(s) > 1): if (s[0] != s[-1]): return False s = s[1:-1] return True print(isPalindrome1("abcba"), isPalindrome1("abca")) print(isPalindrome2("abcba"), isPalindrome2("abca")) print(isPalindrome3("abcba"), isPalindrome3("abca")) print(isPalindrome4("abcba"), isPalindrome4("abca"))

  6. Strings are Immutable
    1. You cannot change strings! They are immutable.
      s = "abcde" s[2] = "z" # Error! Cannot assign into s[i]

    2. Instead, you must create a new string.
      s = "abcde" s = s[:2] + "z" + s[3:] print(s)

  7. Strings and Aliases
    Strings are immutable, so aliases do not change!
    s = 'abc' # s references the string 'abc' t = s # t is an alias to the one-and-only string 'abc' s += 'def' print(s) print(t)

  8. Some String-related Built-In Functions
    1. str() and len()
      name = input("Enter your name: ") print("Hi, " + name + ". Your name has " + str(len(name)) + " letters!")

    2. chr() and ord()
      print(ord("A")) # 65 print(chr(65)) # "A" print(chr(ord("A")+1)) # ?

    3. eval()
      # eval() works but you should not use it! s = "(3**2 + 4**2)**0.5" print(eval(s)) # why not? Well... s = "reformatMyHardDrive()" print(eval(s)) # no such function! But what if there was?

  9. Some String Methods
    Methods are a special type of function that we call "on" a value, like a string. You can tell it's a method because the syntax is in the form of value.function(), like s.islower() in the code below.
    1. Character types: isalnum(), isalpha(), isdigit(), islower(), isspace(), isupper()
      # Run this code to see a table of isX() behaviors def p(test): print("True " if test else "False ", end="") def printRow(s): print(" " + s + " ", end="") p(s.isalnum()) p(s.isalpha()) p(s.isdigit()) p(s.islower()) p(s.isspace()) p(s.isupper()) print() def printTable(): print(" s isalnum isalpha isdigit islower isspace isupper") for s in "ABCD,ABcd,abcd,ab12,1234, ,AB?!".split(","): printRow(s) printTable()

    2. String edits: lower(), upper(), replace(), strip()
      print("This is nice. Yes!".lower()) print("So is this? Sure!!".upper()) print(" Strip removes leading and trailing whitespace only ".strip()) print("This is nice. Really nice.".replace("nice", "sweet")) print("This is nice. Really nice.".replace("nice", "sweet", 1)) # count = 1 print("----------------") s = "This is so so fun!" t = s.replace("so ", "") print(t) print(s) # note that s is unmodified (strings are immutable!)

    3. Substring search: count(), startswith(), endswith(), find(), index()
      print("This is a history test".count("is")) # 3 print("This IS a history test".count("is")) # 2 print("-------") print("Dogs and cats!".startswith("Do")) # True print("Dogs and cats!".startswith("Don't")) # False print("-------") print("Dogs and cats!".endswith("!")) # True print("Dogs and cats!".endswith("rats!")) # False print("-------") print("Dogs and cats!".find("and")) # 5 print("Dogs and cats!".find("or")) # -1 print("-------") print("Dogs and cats!".index("and")) # 5 print("Dogs and cats!".index("or")) # crash!

  10. String Formatting (optional)
    See these optional notes on string formatting using the % operator and using the format() method.

  11. String Formatting with f Strings
    # We saw this example back in week1! # It shows a nice relatively new way to format strings: x = 42 y = 99 # Place variable names in {squiggly braces} to print their values, like so: print(f'Did you know that {x} + {y} is {x+y}?')

  12. Basic File IO
    # Note: As this requires read-write access to your hard drive, # this will not run in the browser in Brython. def readFile(path): with open(path, "rt") as f: return def writeFile(path, contents): with open(path, "wt") as f: f.write(contents) contentsToWrite = "This is a test!\nIt is only a test!" writeFile("foo.txt", contentsToWrite) contentsRead = readFile("foo.txt") assert(contentsRead == contentsToWrite) print("Open the file foo.txt and verify its contents.")