BFOIT Introduction to Programming Words/Sentences and     Operations on Them

Introduction

1. learn a bunch of operators that you can use to manipulate words and sentences, e.g.,
   • get the first character of a word or the first word of a sentence,
   • get the number of characters in a word or the number of words in a sentence,
   • how to join two words to form a single word, and more...
2. write a program that plays Hangman

Use the following TG applet to try these examples out.  Do they print what you expected?

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TurtleGraphics Applet

first, butfirst, and empty? Operators

New jLogo Procedures
Name	Input(s)	Description	Example
BUTFIRST BF	wordOrSentence	If the input is a word, all characters except its first are output. If the input is a sentence, all words except the first are output.	BUTFIRST :W
EMPTYP EMPTY?	wordOrSentence	Outputs true if its input is a word that has no characters in it, otherwise false. Outputs true if its input is a sentence that has no words in it, otherwise false.	EMPTY? :S
FIRST	wordOrSentence	If the input is a word, its first character is output. If the input is a sentence, its first word is output.	FIRST :W

Go back up to the TG applet and play with each of these operators.  Use println to display the outputs produced by these operators on a variety of words and sentences.

Write a procedure, an operator, called rotate.  It should take the first letter of a word off and append it onto the end of the word.  Here are some examples of it in action:

    ? to rotate :wd
    >   ...
    >   end
    ? println rotate "word
    ordw
    ? println rotate "ab
    ba

Iteration Through Words/Sentences

Let's write a procedure, a command, named printPiecePerLine which has one input which can be a word or sentence. It iterates through all of the pieces of its input, displaying each on a separate line.  Here's the skeleton:

    to printPiecePerLine :thing
      ...
      end

•     printPiecePerLine "ABCD
      

  produces the following in the terminal window:

      A
      B
      C
      D
      

•     printPiecePerLine [A four word sentence]
      

  produces the following in the terminal window:

      A
      four
      word
      sentence
      

1. write code to print a piece of the input,
2. write the recursive invocation, changing the input such that its new value is moving toward ending the iteration, and
3. write a stop rule that catches the end condition.

If you got it to work, congratulations - skip ahead to the first exercise.

A Hint/Coaching

Working with words and sentences is a little different than iterating in order to do something a number of times.  What you tend to want to do with words and sentences is to do something to their pieces.  The most straight-forward way to iterate through all of the pieces of a word or sentence is:

    to iterateThroughPieces :thing
      if empty? :thing [stop]
      ;do something/whatever to the first piece of :thing
      iterateThroughPieces butfirst :thing
      end

The recursive invocation modifys the input, removing the first piece, moving it to, or one step closer to, an empty word or sentence.

Type this procedure into TG or the TG applet and then trace it.  Watch how the input changes.  Invoke it with a word for its input.  Invoke it with a sentence as its input.

Now, complete printPiecePerLine.

Memorize the iterateThroughPieces template so that you have another example of recursion in your toolbox of techniques for writing programs.

Exercise: Reverse Iteration Through Word/Sentence Elements

Here are two more primitive operators that take a word or sentence input, complements of first and butfirst:

New jLogo Procedures
Name	Input(s)	Description	Example
BUTLAST BL	wordOrSentence	If the input is a word, all characters except its last are output. If the input is a sentence, all words except the last one are output.	BUTLAST :W
LAST	wordOrSentence	If the input is a word, its last character is output. If the input is a sentence, its last word is output.	LAST :SENT

Write a procedure that prints the pieces of its input in reverse order, e.g.

•     printReversedPieces "ABCD
      

  produces the following in the terminal window:

      D
      C
      B
      A
      

•     printReversedPieces [A four word sentence]
      

  produces the following in the terminal window:

      sentence
      word
      four
      A
      

Finally...

If you would like to see a detailed walk-through of writing recursive procedures that manipulate words, here is Brian Harvey's chapter "Introduction to Recursion," from his book Computer Science Logo Style.

Iteration Through Words/Sentences With Counting

In this example, we write a procedure, a command, named printIndexedPiece which displays the specified piece of a sentence or word. 

    to printIndexedPiece :index :thing
      ...
      end

The new first input, named index is the number of the desired piece, i.e., 1 for the first piece, 2 for the second piece, etc...  Here are a couple of examples of what it should do.

•     printIndexedPiece 2 "ABCD
      

  produces the following in the terminal window:

      B
      

•     printIndexedPiece 4 [A four word sentence]
      

  produces the following in the terminal window:

      sentence
      

If you got it to work, congratulations - skip ahead to the next exercises.

A Hint/Coaching

If the specified index is 1, then all you need to do is print the first piece of thing.

    if equal? :index 1 [ println first :thing ]

Exercise: From Command to Operator

Write a procedure, an operator, named indexedPiece which outputs the specified piece of a sentence or word.  Like, the previous procedure (printIndexedPiece), its first input is the number of the desired piece.  But, instead of printing this piece, it should be the result of invoking your procedure - it should be your procedure's output. 

    to indexedPiece :index :thing
      ...
      end

•     println indexedPiece 3 "QWERTY
      

  produces the following in the terminal window:

      E
      

•     println indexedPiece 1 [Two Words]
      

  produces the following in the terminal window:

      Two
      

Exercise: Acronym Operator

Here's another interesting operator to write: acronym.  It outputs the first characters of all the words in its input - a sentence. 

    to acronym :fullName
      ...
      end

•     println acronym [Artificial Intelligence]
      

  produces the following in the terminal window:

      AI
      

•     println acronym [Digital Versatile Disk]
      

  produces the following in the terminal window:

      DVD
      

Once you get this working, improve it...  Acronyms do not contain noise words, e.g., "of," "the," "for," etc...  jLogo has a primitive procedure that will help you to find and ignore these in the input sentences. 

New jLogo Procedure
Name	Input(s)	Description	Example
MEMBERP MEMBER?	char or word wordOrSentence	Outputs true if its first input is in its second input, otherwise it outputs false.	MEMBER? "e :W

Use this operator to extend your version of acronym to ignore noise words.

Hangman

Click the mouse on [Start] to begin play.  Use the keyboard to enter guess characters.

A Detailed Look at my Version of the Program

Here's the structure of my program.

Figure 14.1

Now, this is a simplified version of the program - a subset of the applet I included above.  Notice that there is no "Start" and "Restart" button.  I also left out the xxxHelper procedures.  Both drawGuessWord and replGuessChars procedures are fronts for corresponding xxxHelper procedures that iterate through words, one character at a time.  By now you should know that I always favor getting something to work first, then enhance it.

To reduce the time it will take you to write your hangman program, I'm going to provide you with part of my version of the program.  I'm doing this because I want you to concentrate on the new/interesting parts of the program - not drawing the scaffold, or man.  You did this sort of stuff 10 or so lessons ago. 

Some of my hangman program - as a starting point. .

So, now you have 80% of the program.  But, the remaining 20% that you need to add is the interesting - NEW - stuff.

The Operation: replGuessChars

    to replGuessChars :ch
      ...
      end

1. make "secretWord "logo
   make "guessWord "____
   replGuessChars "o
   println :guessWord
       

   produces the following in the terminal window:

   _o_o
       

2. replGuessChars "g
   println :guessWord
       

   produces the following in the terminal window:

   _ogo
       

3. replGuessChars "l
   println :guessWord
       

   produces the following in the terminal window:

   logo
       

Flashback to keypressed and ASCII Character Numbers

Well, back in the first lesson, I talked about about how all symbolic information is represented inside the computer as numbers.  Go back and checkout the Numeric and Symbolic Information chart.  It has a few ASCII characters in it.  You will need to convert these numbers for the characters a to z into single character words.  You do this with the char operator.  It takes a single input that is a number and outputs a single character word for the corresponding ASCII symbol.:  The numerical value for 'a' is 97, 'b' is 98, 'c' is 99, 'd' is 100, etc...  Try out the operator!

    println char 97

New jLogo Procedures Used In This Lesson
Name	Input(s)	Description	Example
BUTFIRST BF	wordOrSentence	If the input is a word, all characters except its first are output. If the input is a sentence, all words except the first are output.	BUTFIRST :W
BUTLAST BL	wordOrSentence	If the input is a word, all characters except its last are output. If the input is a sentence, all words except the last one are output.	BUTLAST :S
CHAR	number	outputs a single character word which is the ASCII character for the given number.	CHAR 65
COUNT	wordOrSentence	If the input is a word, the number of characters in it is output. If the input is a sentence, the number of words in it is output.	COUNT :W
EMPTYP EMPTY?	wordOrSentence	Outputs true if its input is a word that has no characters in it, otherwise false. Outputs true if its input is a sentence that has no words in it, otherwise false.	EMPTY? :S
FIRST	wordOrSentence	If the input is a word, its first character is output. If the input is a sentence, its first word is output.	FIRST :W
ITEM	number wordOrSentence	Outputs the number-th element of its second input. If its second input is a word, it outputs a word consisting on the number-th character. If its second input is a sentence, it outputs the number-th word of it.	ITEM 2 :S
LAST	wordOrSentence	If the input is a word, its last character is output. If the input is a sentence, its last word is output.	LAST :W
MEMBERP MEMBER?	char or word wordOrSentence	Outputs true if its first input is in its second input, otherwise it outputs false.	MEMBER? "e :W
SE SENTENCE	wordOrSentence wordOrSentence	Outputs a sentence consisting of its first input concatenated with its second input.	SENTENCE "word :sent
WORD	word  word	Outputs a word consisting of its first input concatenated with its second input.	WORD "pre :wd