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lab44
strings as arrays
so-far variables revisited

Recall how we used loops, along with a “so-far” variable, to build up (say) multiple copies of a string, back in lab33—Accumulating an answer and lab34—loop practice: accumulating a string.

# stringTimes: int, string -> string
# Return multiple copies of `txtToRepeat`.
# Example:  stringTimes( 3, "Koala" )  ⇒ "KoalaKoalaKoala"
#
def stringTimes( numCopies, txtToRepeat ):
    stringSoFar = ""  # one-time setup
    for currentCopyNumber in range(numCopies):
        stringSoFar = stringSoFar + txtToTRepeat
    return stringSoFar

Arrays: Strings are arrays of characters

We know that substring can pull out sections of a string. We can also access individual characters by just referring to them by index, using square-brackets ¹:

  pet = "wallaby"
  pet[1]
  pet[5]
  pet[6]
  pet[7]    # ERROR!

Accessing individual items by their index is powerful, when combined with arrays:

for i in range(8):
  printNow("I got me a " + pet + "; it has a " + pet[i] + "!")

This gives us a way to process each character of a string. For example:

# substring : string, int, int → string
#   Given some text, a start-index, and a stop-index,
#   return the characters of `txt` from `start` up until (but not including) `stop`.
#   Indices are 0-based.
#   Examples:   substring( "radford", 3, 6) = "for"
#               substring( "radford", 0, 3) = "rad"
#
def substring(txt, start, stop):
    if (0 <= start  and  start <= stop  and  stop <= len(txt)):
        resultSoFar = ""
        for i in range(stop-start):
            resultSoFar = resultSoFar + txt[start+i]
        return resultSoFar
    else:
        raise IndexError("substring: bounds out of range: " + str(start) + "," + str(stop) + " for '" + txt + "'")

As a class, we'll write:

# countVowels: string -> int
# Return the number of vowels in a string.
#




def test():
    print( "Actual: " + str( countVowels( "a" ) ) )
    print( "Expect: " + str(1) )
    print( "Actual: " + str( countVowels( "b" ) ) )
    print( "Expect: " + str(0) )
    print( "Actual: " + str( countVowels( "" ) ) )
    print( "Expect: " + str(0) )
    print( "Actual: " + str( countVowels( "aa" ) ) )
    print( "Expect: " + str(2) )
    print( "Actual: " + str( countVowels( "ba" ) ) )
    print( "Expect: " + str(1) )
    print( "Actual: " + str( countVowels( "hello" ) ) )
    print( "Expect: " + str(2) )
    print( "Actual: " + str( countVowels( "abcde" ) ) )
    print( "Expect: " + str(2) )
    print( "Actual: " + str( countVowels( "zzzzz" ) ) )
    print( "Expect: " + str(0) )
    print( "Actual: " + str( countVowels( "zaeiouyAEIOUYzzzz" ) ) )
    print( "Expect: " + str(12) )

solution

YOUR TASK (part of next lab's problems): Write a function countUpperCase, which returns the number of upper-case letters in a given string.

hint: As you write this, and you reach something which you wish was a built-in function, use the technique of wishful-thinking, and make up a helper-function to do what you want. Then you can come back later and write that, perhaps even getting some help.

¹ In fact, substring itself uses the square-bracket notation to do its work! ↩

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©2014, Ian Barland, Radford University
Last modified 2014.May.05 (Mon) Please mail any suggestions
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lab44 strings as arraysso-far variables revisited

Arrays: Strings are arrays of characters

lab44
strings as arrays
so-far variables revisited