ITEC 109 2014spring ibarland

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lab34
loop practice
accumulating a string

Last time we saw a loop where we modified a variable each time through:

def stringTimes( numCopies, txtToRepeat ):
  stringSoFar = ""  # one-time setup
  for currentCopyNumber in range(numCopies):
    stringSoFar = stringSoFar + txtToTRepeat
  return stringSoFar

We say that currentCopyNumber is the “index variable”.

Separately, we also noted that the variable currentCopyNumber in the above code was a bit odd: we never actually used the value of that variable! We did discuss what happens in the following variation though:

def stringTimes_v2( numCopies, txtToRepeat ):
  stringSoFar = ""  # one-time setup
  for currentCopyNumber in range(numCopies):
    stringSoFar = stringSoFar + txtToTRepeat + str(currentCopyNumber)
  return stringSoFar

YOUR TASKS:

1. Write a function stutter, which builds a string which has every character of the input repeated: For example, stutter("cats") = "ccaattss". See more tests below.

   Hints that apply to any loop problem: To solve the problem, think of a specific input (like “"cats"”), and consider these questions:

   • Can you write out a series of (repeated) statements that do the task for this particular input? Do it using a so-far variable.

     (This is what we did in yesterday's lab.)

   • How many times will we want to repeat the loop, in general?
   • What type is our our so-far variable?
   • What do we set our so-far varible to initially, before we've even gone through the loop once?
   • Suppose we're about halfway through the problem (in this case, say twice — so our index-variable might be 2).
     What is our so-far variable holding as we start the loop? (In this case: "ccaa".)
     What do we want our so-far varible to hold when we finish the loop? (In this case: "ccaatt".)
     What's a general way of getting from the current value to the desired value, using the input and the index-variable?

   Recall the code for substring:

   # 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): return txt[start:stop]

2. Write a function stutter2, which builds a string which has every pair of characters of the input repeated: For example, stutter2("cats") = "cacatsts". See more tests below.

   Hint: see Hint from above, which applies to all loop-problems.

############# Test cases; place your code above these lines ##############

# test : None -> None
# Run tests for `stutter` and `stutter2`, printing actual and expected results.
#
def test():
  testStutter()
  testStutter2()
  
  
# testStutter : None -> None
# Run tests for `stutter`, printing actual and expected results.
#
def testStutter():
  print( "Testing `stutter`:" )
  print( "Actual: " + stutter("") )
  print( "Expect: " + "" )
  print( "" )
  print( "Actual: " + stutter("a") )
  print( "Expect: " + "aa" )
  print( "" )
  print( "Actual: " + stutter("TODO: REPLACE THIS WITH A SHORT INPUT (but longer than the previous test)") )
  print( "Expect: " + "TODO: REPLACE THIS WITH THE EXPECTED OUTPUT" )
  print( "" )
  print( "Actual: " + stutter("TODO: REPLACE THIS WITH A MEDIUM-LENGTH INPUT") )
  print( "Expect: " + "TODO: REPLACE THIS WITH THE EXPECTED OUTPUT" )
  print( "" )
  print( "Actual: " + stutter("aaa") )
  print( "Expect: " + "aaaaaa" )
  print( "" )

# testStutter2 : None -> None
# Run tests for `stutter2`, printing actual and expected results.
#
def testStutter2():
  print( "Testing `stutter2`:" )
  print( "Actual: " + stutter2("") )
  print( "Expect: " + "" )
  print( "" )
  print( "Actual: " + stutter2("ab") )
  print( "Expect: " + "abab" )
  print( "" )
  print( "Actual: " + stutter("TODO: REPLACE THIS WITH A SHORT INPUT (but longer than the previous test)") )
  print( "Expect: " + "TODO: REPLACE THIS WITH THE EXPECTED OUTPUT" )
  print( "" )
  print( "Actual: " + stutter("TODO: REPLACE THIS WITH AN INPUT OF *ODD* LENGTH") )
  print( "Expect: " + "TODO: REPLACE THIS WITH THE EXPECTED OUTPUT" )
  print( "" )
  print( "Actual: " + stutter2("a") )
  print( "Expect: " + "" )
  print( "" )
  print( "Actual: " + stutter2("aaaa") )
  print( "Expect: " + "aaaaaaaa" )
  print( "" )
  print( "Actual: " + stutter2("aaa") )
  print( "Expect: " + "aaaa" )
  print( "" )

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©2014, Ian Barland, Radford University Last modified 2014.Apr.07 (Mon)

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