CSE 505 -- Smalltalk Mini-Exercises -- Answer Key

1. Write a Smalltalk expression to return true if x is either equal to 0 or if 10 divided by x equals y.

x=0 or: [10/x=y]

2. Let a be an array of some sort. Write a loop that prints every second element on an output stream strm.

     | a strm |
     strm := WriteStream with: (String new: 100).
     a := Array new: 10.
     a at: 1 put: 'fee fi fo fum'.
     a at: 2 put: 3.14159.
     a at: 3 put: 8.
     "THE FOLLOWING LINE IS THE ONLY NEW PART"
     1 to: a size by: 2 do: [:i | (a at: i) printOn: strm].
     strm contents
   

3. Add a repeatUntil control structure to Smalltalk. Hint: this is trivial -- do it by defining a repeatUntil: method for BlockContext.

   Add this method to BlockContext:

   repeatUntil: doneTest
     self value.
     doneTest whileFalse: self.
   

   Or if you wanted to define it primitively, without using while, the following will work. (It is less efficient, but maybe more interesting.)

   repeatUntil: doneTest
     self value.
     doneTest value ifFalse: [self repeatUntil: doneTest]
   

4. Consider the following Smalltalk class definition.

   Object subclass: #Octopus
      instanceVariableNames: 'myblock'
      classVariableNames: ''
      poolDictionaries: ''
   
   setValue: x
      myblock := [x].
   
   setBlock: y
      myblock := y.
   
   getValue
     ^ myblock value
   

   What is the result of evaluating the following code? (The value in each case will be the value of the last o getValue expression.)

         | o a |
      a := 3.
      o := Octopus new.
      o setValue: a.
      a := a+1.
      o getValue
   

   Answer: 3

   And what about this code?

         | o a b |
      a := 3.
      b := [a].
      o := Octopus new.
      o setBlock: b.
      a := a+1.
      o getValue
   

   Answer: 4