Introduction

Students who have programmed in other languages will find this tutorial fairly easy.

For the exercises in this lesson. Start the LearningWorks program provided by the OU (Open University) M206 course. Use either LearningBooks LB 12, 13 or 14.
Or use an alternative Smalltalk program to LearnWorks.

• Simple expression(s).
• Cascading expression.
• Expression series.

Question 1. What punctuation is used to separate the expressions in an expression series?
Go to Answer 1.

In order to make computers do more than execute such very simple programs you require methods that will allow you to:

1. Repeat sections of code, a number of times. Such code is often referred to as a loop.
2. Branch. Branching refers to code where some decision making takes place. This is done by coding a Boolean Conditional Expression. This results in only certain sections (branches), of code being executed depending on the result of the Boolean condition, i.e. a condition that evaluates to true or false
3. Or a decision making loop.. I.e. Methods that provide a combination of Repeating and branching, they repeat sections of code depending on the result of a conditional block.

Top of Page

Conditional Expressions

Conditional Expressions sometimes called Boolean Conditional Expressions return a boolean answer of either true or false.
You can think of conditional expression as asking a question. examples of these expressions could be:-

(object1 = object2)

(object3 < object4)

Note that the parenthesis surrounding a conditional expression is optional. The course convention is to use parenthesis.The use of parenthesis along with indentation makes code easier to read. This in turn means that it is easier to debug if it is not working.

Top of Page

Comparison Operators

'a' == 'a'
2 = 2

'a' = 'a'
 
2 = 2

'a'  ~= 'a'
2 ~= 2

'z' < 'Z'
3 < 4

'z' > 'Z'
3 > 4

'a' <= 'z'
2 <= 2

'a' >= 'z'
2 >= 2

('a' >= 'z') not
 
(2 >= 2) not

Messages that answer true or false

Top of Page

Blocks

• The start of a block is denoted by the opening bracket [
• The end of a block is denoted by the closing bracket ]
• A block can contain any number of valid executable expressions or any number of comments.
• Each expression must end with a period except for the last expression in the block, when the period is optional.
• The whileTrue: or whileFalse: messages require the use of a block both in front of, and after the message selector.
• A block could be reguarded as a method without a name.
• Blocks belong to the class BlockClosure.
• One or more of the expressions may be a further construct that includes [aBlock] that is nested within the block.
• There are many other methods, besides those discussed in this lesson that require blocks.
• The blocks used in this lesson are required, but any expression, or expression series, can be turned into a block by enclosing it in square brackets. The message value is often used with this type of block. e.g.

  ['abcd' asUppercase] value

• When typing in the start [bracket, type 3 spaces then the end ]bracket. This ensures that you do not forget it.
• It may help if you also insert any required periods at this stage. Or at least think about where they will go.
• Then insert your code between the 2 square brackets. This makes the insertion of nested blocks much easier.
• Use indentation if your block contains an expression series.
• Use further indentation for any nested blocks.

Top of Page

Use of Periods

• Periods are used to separate expressions, in a series of expressions.
• Expressions examples
  • Receiver message.
  • Receiver keywordMessage: argument.
  • Receiver keywordMessage: argument keywordMessage: argument.
• Blocks can often contain expression series.
• A period is not used between a receiver and the message sent to that receiver. Note:- Boolean conditions or Boolean condition blocks are receivers.
• ifTrue:ifFalse is an example of a keyword message selector with more than one argument. A period is not used to separate keywords with more than one argument, although a period may be used within either, or both of the 2 blocks, if more than 1 expression is required.
• The wrong positioning of periods will lead to an error message. You may like to experiment with wrong positions to build up your error message list.

Top of Page

Decision making

• Conditional logic allows a selected portion of code to be executed, depending on the value of a boolean condition. This control structure is known as branching.
• A Boolean condition can be any expression that results in a value of true or false
• The keyword message selectors that can be used are:-

        ifTrue:    ifFalse:   ifTrue:ifFalse:   ifFalse:ifTrue:   whileTrue:   whileFalse:

ifTrue: and ifFalse:

(aBooleanCondition)
    ifTrue: [ Execute this block of code only if the condition is true ] .

( 4 = 4 ) 
    ifTrue: [ ^true ] .

( aBooleanCondition ) 
ifFalse: [ Execute this block of code only if the condition is false ] .

( 4 < 4 ) 
    ifFalse: [ ^false ] .

• a Boolean condition can be any expression that results in a value of true or false.
• aBlock of code may be a
  • A single simple message expression
  • A cascading message expression
  • A message expression series.
  • A another nested control type structure such as ifTrue:, timesRepeat: whileFalse: etc.
  • A combination of the above

Top of Page

ifTrue:ifFalse: and ifFalse:ifTrue:

• These 2 methods do exactly the same thing.
• The only difference is the order of the blocks.
• If one block has much more code in it, possibly with additional nested blocks, I would personally use the expression that would allow the complicated code to be in the second block. An example of this is the solution to Exercise 1d.

( aBooleanCondition )
    ifTrue: [ Execute this block of code only if the condition is true ]
    ifFalse: [Execute this block of code only if the condition is false] .

( 'a' = 'a' ) 
    ifFalse: [ ^false ] 
    ifTrue: [ ^true ].

Top of Page

( 4 = 4 ) 
    ifTrue: [ ^true ] .

( 'a' = 'a' )
      ifTrue: [ ^true ]
      ifFalse: [ ^false ] .
^'Another answer'

Top of Page

& (Logical AND)

( (condition 1) & (condition 2) ) 
      ifTrue: [ "Code here is evaluated only if both conditions are true" ]
      ifFalse: [ "Code here is evaluated only if  either one or both conditions are false" ] .

| (Logical OR)

( (condition 1) | (condition 2) ) 
      ifTrue: [ "Code here is evaluated only if either one or both conditions are true" ]
      ifFalse: [ "Code here is evaluated only if  both conditions are false" ] . 

Summarised Rules

When using & (logical AND) both must be true to produce a true answer.

When using | (logical OR) either or both must be true to produce a true answer.

In both the above anything else is false.

 ( (4 < 6) & ('a' > 'c') )

 ( (4 < 6) & (21 > 19) )

( (4 < 6) | (21  > 19) )

 ( (4 < 6) & (21 < 19) ) 

( (4 < 6) | (21  < 19) )

( (4 > 6) & (21 < 19) )

( (4 > 6) | (21 < 19) )

( ('c' > 'a') | (21 < 19) )

( ('c' > 'a') & (21 < 19) )

( ('c' = 'c') | (21 < 19) )

( ('c' == 'c') | (21 < 19) ) 

( (4 < 6) & (21 ~= 19) )

 ( (4 < 6) & (21 < 19) not ) 

( numer1 < number2 ) 
( number3 >= 5 )
( frog1 = frog2 ) 
( aLetter < 'm' )

Top of Page

"T12. Exercise 1 (a)  Using &, ifTrue:ifFalse"
|age|
age := 24.
( (age >= 13) & (age < 19) ) 
    ifTrue: [ ^'You are a teenager' ]
    ifFalse: [ ^'You are NOT a teenager' ] . 
		  

Test the Code.

 (age < 19)
		  

 (age <= 19)
		  

Top of Page

1. In the Workspace Click at the end of code you have just evaluated, to ensure that it is not highlighted.
2. Press the enter key a couple of times to insert 2 blank lines in the Workspace.
3. Copy the corrected code and comment you use in Exercise 1 (a).
4. Paste into the Workspace after the 2 blank lines.
5. Alter the comment of this pasted copy to "T12. Exercise 1 (b). Using &, ifTrue only"
6. Delete only the message selector, ifFalse: not the rest of the code on that line
7. Your code should now look like:

   "T12. Exercise 1b.  Using &, ifTrue"
   |age|
   age := 14.
   ( (age >= 13) & (age <= 19) ) 
       ifTrue: [ ^'You are a teenager' ]
       [ ^'You are NOT a teenager' ] . 
   		  

8. Evaluate and note that deleting ifFalse: has introduced an error.
   • This error is already listed in the list of error message.
   • This error is NOT in the line [ ^'You are NOT a teenager' ]. Remember the message selector has been changed from ifTrue:ifFalse: to ifTrue:
9. Correct this very simple, and very common error.
10. Go to solution for this error
11. Ensure the age is 14, then check that the code is working.
12. At this stage you may be happy with the code, but it is NOT fully tested.
13. Change the age back to 24 and evaluate.
14. The answer BlockClosure [] in Text Editor Evaluation is in the answer pane, this is NOT the answer required
15. Slightly alter the remaining code in the last line so that the correct answer is returned.

• 'You are a teenager'
• 'You are too young to be a teenager'
• 'You are too old to be a teenager'

1. Copy and paste the previous solution code again after a couple of blank lines.
2. Delete the last line.
3. There are many different ways of coding this.
4. The code for the 'You are a teenager' result is already coded
5. As a guide on how to complete this exercise you should use the following in the code you must:
   • Amend the Exercise comment, to something suitable
   • Add 2 additional condition expressions. One for each of the other possible answers.
   • each condition expressions requires an ifTrue structure.
6. Now attempt your solution in the Workspace.
7. Test your solution with all these ages 12, 13, 14, 18, 19, 20. As demonstrated in the last exercise, you are now reaching the stage where, as the code becomes more complicated, testing becomes more important.

Top of Page

Nested conditional expression

1. Starting with the original code

   "T12. Exercise 1 (a)"
   |age|
   age :=24.
   ( (age >= 13) & (age <= 19) ) 
       ifTrue: [ ^'You are a teenager' ]
       ifFalse: [ ^'You are NOT a teenager' ] . 
   		  

2. Amend the comment to the new exercise number etc.
3. Amend 'You are NOT a teenager' to 'You are too old to be a teenager'
4. Place the cursor immediately after the ifFalse: [
5. And press enter. The result should look like

   "T12. Exercise 1d. Nested conditional expression"
   |age|
   age := 24.
   ( (age >= 13) & (age <= 19) ) 
       ifTrue: [ ^'You are a teenager' ]
       ifFalse: [ 
                           ^'You are too old to be a teenager' ] .	  

   This leaves space for your new nested code.
6. Look at the last line, then look at the last 2 lines of solution Exercise 1 (c)
7. The required code is very similar. Remember the code you are about to enter, along with the code that is already ther, will become an expression series, therefore think about the period.
8. In this space you can enter a conditional expression. Because it is within a block it becomes a nested conditional expression. The condition will be the same as one used in Exercise 1 (c) and depending on that expression either an ifTrue: or ifFalse: structure.

Top of Page

Iteration. (Loops)

• Infinitive loops, also known as endless loops should be avoided, they prevent your code from completing.
• You must write your code to prevent this from happening
• This is easy when using timesRepeat:, they repeat a set number of times, but be careful with whileTrue: and whileFalse:
• Tips are given in preventing infinitive loops in the whileTrue: and whileFalse: section below.
• The computer can run out of memory, resulting in a GC flashing cursor
• Use the Ctrl q key combination to break out of an infinative loop.
• Do not use Ctrl Alt Delete combination. You will loose ALL work performed since your last save.
• That when writing your own methods you can accidentally write an infinative loop without using any of the loop structures.
• This is covered later in the course.
• Some TMA questions are carefully written to temp you into making this mistake.

Top of Page

timesRepeat:

number timesRepeat: [ block of code to be repeated ]

• where number can be any expression that results in an integer.
• block of code to be repeated may be a
  • A single simple message expression
  • A cascading message expression
  • A message expression series.
  • A nested control type structure such as ifTrue:, timesRepeat: whileFalse: etc..
  • A combination of the above

| total | 
total := 0. 
4 timesRepeat: [ total := total + 1]. 
total 

4 timesRepeat: [ total := total + 1]. 

Top of Page

| aNumber total count | 
aNumber := 4. 
total := 0. 
count := 0. 
aNumber timesRepeat: [ count := count + 1. 
                                       total := total + count ]. 
total.

| total | 

aNumber := 4. 
total := 0. 
count := 0. 
 

aNumber timesRepeat: 
     [ count := count + 1. 
       total := total + count ].  

total 

  1    := 0 + 1

  1   := 0 + 1

  2    := 1 + 1

  3   := 1 + 2

Top of Page

• How the block has a series of 2 expressions.
• As usual each expression is placed on a separate line.
• Indentation is used to emphasise that both expressions are within the same block. Note how a different indentation/white space style was used in the table, to that in the original code.
  • Take a note as you read course materials, and books how different authors achieve improved readability of code by using white space and indentation.
  • Be consistent on how you develop your own style.
  • Check with your tutor for any recommended style.
• Remember periods. The block has a series of 2 expressions which must be separated by a period.

Top of Page

whileTrue: and whileFalse:

[  a Boolean Condition Block ] 
    whileTrue: [ aBlock of code that is repeatedly executed while the condition remains true ]

• a Boolean Condition Block can be any expression that results in a value of true or false.
  • It must be enclosed in a block.
• aBlock of code may be a
  • A single simple message expression
  • A cascading message expression
  • A message expression series.
  • A nested control type structure such as ifTrue:, timesRepeat: whileFalse: etc..
  • A combination of the above

[ aBooleanConditionBlock  ]
     whileFalse:[ aBlock of code that is repeatedly executed while the condition remains false ]

Top of Page

| aNumber total count | 
aNumber := 4. 
total := 0. 
count := 0. 
    aNumber timesRepeat: [ count :=count + 1. 
                                           total := total + count ]. 
total.

"T12 Exercise 5b    whileTrue:"
 
| aNumber total count | 
aNumber := 4.
total := 0. 
count := 0. 
[ count < aNumber ] 
    whileTrue: [count := count + 1. 
                       total :=total + count]. 
total. 

[ aNumber < 4 ]

[ aNumber := 4 ]

Top of Page

Answers

 ( (4 < 6) & ('a' > 'c') )

( (4 < 6) & (21 > 19) )

( (4 < 6) | (21 > 19) )

 ( (4 < 6) & (21 < 19) )

( (4 < 6) | (21  < 19) )

( (4 > 6) & (21 < 19 ) )

( (4 > 6) | (21 < 19) )

( ('c' > 'a') | (21 < 19) )

 ( ('c' > 'a') & (21 < 19) )

( ('c' = 'c') | (21  < 19) )

'c' = 'c'

( ('c' == 'c') | (21 < 19) ) 

'c' == 'c'

( (4 < 6) & (21 ~= 19) )

21 ~= 19

( (4 < 6) & (21 < 19) not ) 

21 < 19

Top of Page

4 timesRepeat: [ total :=total + 1]. 

4 class. 

| total | 
total := 0. 
4 timesRepeat: [ total := total + 1 ] class. 

0 + 1 = 1

0 + 1 = 1

1 + 1 = 2

1 + 2 = 3

2 + 1 = 3

3 + 3 = 6

3 + 1 = 4

6 + 4 = 10

Top of Page

Exercise Solutions

"T12. Exercise 1a and b  Using &,ifTrue:ifFalse:"

"T12. Exercise 1b.  Using 1 ifTrue:"
|age|
age := 24.
( (age >= 13) & (age < =19) ) 
    ifTrue: [ ^'You are a teenager' ] .
 [^'You are NOT a teenager' ] . 
		  

"T12. Exercise 1b.  Using 1 ifTrue:"
|age|
age := 24.
( (age >= 13) & (age <= 19) ) 
    ifTrue: [ ^'You are a teenager' ] .
^'You are NOT a teenager'. 
		  

"T12. Exercise 1e.  Using 1 ifTrue:"
|age|
age := 24.
( (age >= 13) & (age < =19) ) 
    ifTrue: [ ^'You are a teenager' ] .
 [^'You are NOT a teenager' ] value . 
		  

Top of Page

"T12. Exercise 1c.  Using 3 ifTrue:'s"
|age|
age := 24.
( (age >= 13) & (age <= 19) ) 
    ifTrue: [ ^'You are a teenager' ] .
(age < 13)
     ifTrue: [ ^'You are too young to be a teenager'] .
(age > 19)
     ifTrue: [ ^'You are too old to be a teenager'] .		  

"T12. Exercise 1 (d).   Nested conditional expression"
|age|
age := 24.
( (age >= 13) & (age < =19) ) 
    ifTrue: [ ^'You are a teenager' ] 
    ifFalse: [ (age < 13) 
                             ifTrue: [ ^'You are too young to be a teenager' ] .
                               ^'You are too old to be a teenager' ] .	 

• The nested conditional expression is shown in a red font.
• That there is nothing special about the code for it to be nested.
• It is just typed in, at the required positionwithin the ifFalse: block.

| Previous Tutorial 3. Strings | Next Tutorial 16 Dialog Boxes | Top of Page

Copyright © John McGuinn 2001