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CS 535 Object-Oriented Programming
Fall Semester, 2003
Some Types and Control Structures
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© 2003, All Rights Reserved, SDSU & Roger Whitney
San Diego State University -- This page last updated 10-Sep-03
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Contents of Doc 5, Some Types and Control Structures
References
VisualWorks
Application Developer’s Guide, doc/vwadg.pdf in the VisualWorks
installation. Chapter 3, 5, 16
Smalltalk
Best Practice Patterns, Kent Beck, page 180
Reading
(DevGuide)
Pages 96-97, 311-318 (Up to Dates), 107-109
Simple
Basic Types
Boolean
true
- Unique
instance of True class
false
- Unique
instance of the False class
Smalltalk
uses true and false for boolean values
Boolean
operators (2 > 10) result in true or false
Integers
(0, 1, etc) can not be used for boolean values
nil
Value
of an uninitialized variable
Unique
instance of the UndefinedObject class
Numbers
Integer
Float
Double
Fraction
Fixed-Point
Integer
Smalltalk
supports Integers of arbitrary size
Available
memory dictates integer range
-
536870912to 536870911(29 bits) are handled efficiently
Integers
larger than 29 bits require multiple words
Literal
Forms
1234
1234567890123456789012345678901234567890123456
With
base <base>r<number>
Expression
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Value
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16rFF
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255
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8r11
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9
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3r120
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15
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Examples
1
+ 2
-123
abs
Float
Floating-point
precision numbers
About
8 digits of accuracy
Range
±10^38
Literal
forms
Expression
|
Value
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12.34
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12.34
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12.3e2
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1230.0
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3.14e-10
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3.14e-10
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Double
IEEE
64-bit floating-point numbers
About
14 or 15 digits of accuracy
Range
±10^307
In
scientific notation use d instead of e
Expression
|
Value
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12.34d
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12.34d
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12.34d2
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1234.0d
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12.34
asDouble
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12.340000152588d
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Converting
& Comparing Floating Point Numbers
Why
does 12.34 asDouble result in 12.340000152588d?
12.34
is a decimal number
Most
decimal numbers do not have exact binary representations
If
you want a double start as a double
Exact
comparisons of floating point numbers are dangerous
0.1
* 0.1 = 0.01 is false
Fixed-Point
Numbers
Contain
a fixed number of decimal places
Calculations
are done using specified precision
Examples
- 10s3
- 12.34s
- 12.34s5
- s
indicates this is a fixed-point number
- The
integer after s indicates number of decimal places
- If
no integer after the s, then use precision of the number
Expression
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Result
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2.12s
+ 3.2s2
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5.32s
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2s3
/ 3
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0.667s
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2.11s
+ 2.1s
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4.21s
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0.1s
* 0.1s
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0.0s
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Fraction
Integer
division results in a fraction
Expression
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Result
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1/2
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(1/2)
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(1/2)
+ (1/3)
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(5/6)
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(2r11/16rAA)
* 2
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(3/85)
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Converting
Fractions to Floats
Operations
with floats convert fractions to floats
The
asFloat message converts a fraction to a float
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Expression
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Result
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1/2
asFloat
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0.5
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(1/2)
+ 1.5
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2.0
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(1.0/3)
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0.3333333333333333
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Converting
Between Numbers
Important
messages for any number
- asDouble
- asFoat
- asFixedPoint:
precision
- asInteger
- asRational
- "convert
to fraction'
Examples
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Expression
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Result
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(1/3)
asDouble
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0.33333333333333d
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(1/3)
asFloat
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0.333333
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(1/3)
asFixedPoint: 3
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0.333s
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(1/3)
asRational
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(1/3)
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0.25
asRational
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(1/4)
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0.37
asRational
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(284261/768273)
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0.37s
asRational
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(37/100)
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0.37d
asRational
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(37/100)
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5
asFixedPoint: 3
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5.000s
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5.43
asInteger
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5
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5.43
asDouble
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5.4299998283386d
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5.432d
asFixedPoint: 2
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5.43s
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5.437d
asFixedPoint: 2
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5.44s
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Some
Numerical Methods
Arithmetic
- *
+ - / // \\ abs negated quo: reciprocal rem:
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Result
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division
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4/2
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4/2
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integer
division
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5//2
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2
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modulo
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5\\2
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1
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-3
abs
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3
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5
negated
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-5
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Rounding
4.2
ceiling
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5
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4.2
floor
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4
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3.1523
roundTo: 0.01
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3.15
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4.2
truncated
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4
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Testing
3.2
even
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false
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-3
sign
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-1
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-
- odd,
isZero, negative, positive, strictlyPositive
Others
- arcCos,
arcSin, arcTan, cos, exp,
- floorLog:,
ln, log, log:, raisedTo:,
- sin,
sqrt, squared, tan
Characters
Various
ways to reference a single character
| aChar |
aChar := $a.
aChar := $5.
aChar := Character tab.
aChar := Character value: 65.
aChar := 65 asCharacter.
aChar := 'cat' at: 1. "indexing starts at 1"
Character
class provides class methods for white space characters
backspace
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cr
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del
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esc
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space
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leftArrow
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lf
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del
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tab
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newPage
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Some Character Operations
asciiValue
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digitValue
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<
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=
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>
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isDigit
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isLetter
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isLowercase
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isSeparator
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isUppercase
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isVowel
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asCharacter
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asInteger
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asLowercase
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asSymbol
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asUppercase
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$a isVowel returns true
What
about Strings?
Control
Messages
If
Format
(4 versions)
(boolean expression) ifTrue: trueBlock
(boolean expression) ifFalse: falseBlock
(boolean expression) ifFalse: falseBlock ifTrue: trueBlock
(boolean expression) ifTrue: trueBlock ifFalse: falseBlock
Examples
difference := (x > y)
ifTrue: [ x - y]
ifFalse: [ y - x]
a < 1 ifTrue: [Transcript show: 'hi mom' ]
x sin < 0.5 ifTrue:
[y := x cos.
z := y + 12.
w := z cos]
Boolean
Expressions
Logical
Operations
|
Symbol
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Example
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Or
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a
| b
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And
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&
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a
& b
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Exclusive
OR
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xor:
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a
xor: (b > c)
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Negation
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not
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(a
< b ) not
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Lazy
Logical Operations
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Message
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Example
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Or
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or:
orBlock
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a
or: [b > c]
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And
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and:
andBlock
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a
and: [ c | b ]
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The
orBlock is evaluated only if the receiver of or: is false
The
andBlock is evaluated only if the receiver of and: is true
Performance
Note
To
improve performance the compiler inlines some messages.
Can
I send ifTrue: to a non-Boolean?
Smalltalk
compilers do not check for type usage
Type
usage is check at runtime
If
you send a message to an object that it does not implement a runtime error
results
So
if you execute the following you get a runtime error not a compile error:
5 ifTrue: [1 + 3 ]
Type
Checking & Binding Time
A
Style Issue
Both
of the following have the same effect
Which
to use?
difference := (x > y)
ifTrue: [ x - y]
ifFalse: [ y - x]
(x > y)
ifTrue: [difference := x - y]
ifFalse: [difference := y - x]
Smalltalkers
use and prefer the first version
[1].
isNil
isNil
- Answers
true if receiver is nil otherwise answers false
Common
Usage
x isNil
ifTrue: [ do something]
ifFalse: [ do something else]
Shortcuts
ifNil:ifNotNil:
ifNotNil:ifNil:
ifNil:
ifNotNil:
x
ifNil: [ do something]
ifNotNil: [ do something else]
Blocks
- A
deferred sequence of actions – a function without a name
- Can
have 0 or more arguments
- Executed
when sent the message 'value'
Similar
to
- Lisp
Lambda- Expression
- C
function
- Anonymous
functions
General
Format
[:variable1 :variable2 ... :variableN |
| blockTemporary1 blockTemporary2 ... blockTemporaryK |
expression1.
expression2.
...]
Zero
Argument Block
| block x |
x := 5.
block := [Transcript show: x printString].
x := 10.
block value
Prints
10 in the Transcript window
| block x |
x := 5.
block := [:argument | Transcript show: (x + argument) printString].
x := 10.
block value: 4
Prints
14 in the Transcript window
Blocks
and Return Values
Blocks
return the value of the last executed statement in the block
| block x |
block := [:a :b |
| c |
c := a + b.
c + 5].
x := block value: 1 value: 2.
x
has the value 8
Blocks
and Arguments
[2 + 3 + 4 + 5] value
[:x | x + 3 + 4 + 5 ] value: 2
[:x :y | x + y + 4 + 5] value: 2 value: 3
[:x :y :z | x + y + z + 5] value: 2 value: 3 value: 4
[:x :y :z :w | x + y + z + w] value: 2 value: 3 value: 4 value: 5
Using
the value: keyword message up to 4 arguments can be sent to a block.
[:a :b :c :d :e | a + b + c + d + e ] valueWithArguments: #( 1 2 3 4 5)
[:a :b | a + b ] valueWithArguments: #( 1 2 )
With
the keyword message valueWithArguments: 1 or more arguments can be sent to a
block
The
argument to valueWithArguments: must be an array
#(
1 2 3) creates an array.
More
on arrays soon.
Where
is the Value Message?
In
the message:
difference := (x > y)
ifTrue: [ x - y]
ifFalse: [ y - x]
where
is value sent to the blocks?
In
the False class we have:
ifTrue: trueAlternativeBlock ifFalse: falseAlternativeBlock
^falseAlternativeBlock value
In
the True class we have:
ifTrue: trueAlternativeBlock ifFalse: falseAlternativeBlock
^trueAlternativeBlock value
The
value message is send to the correct block in the True or False class depending
on the value of ( x > y)
Basic
Loops
Format
- aBlockTest
whileTrue
- aBlockTest
whileTrue: aBlockBody
- aBlockTest
whileFalse
- aBlockTest
whileFalse: aBlockBody
The
last expression in aBlockTest must evaluate to a boolean
Examples
| x y difference |
x := 8.
y := 6.
difference := 0.
[x > y] whileTrue:
[difference := difference + 1.
y := y + 1].
^difference
| count |
count := 0.
[count := count + 1.
count < 100] whileTrue.
Transcript clear; show: count printString
More
Loops
Format
- anInteger
timesRepeat: aBodyBlock
- startInteger
to: endInteger do: blockWithArgument
- start
to: end by: increment do: blockWithArgument
Transcript
open;
clear.
3 timesRepeat:
[Transcript
cr;
show: 'Testing!'].
1 to: 3 do:
[ :n |
Transcript
cr;
show: n printString;
tab;
show: n squared printString].
9 to: 1 by: -2 do:
[ :n |
Transcript
cr;
show: n printString].
[1]
See Smalltalk Best Practice Patterns, Kent Beck, Conditional Expression
Pattern, page 180.
Copyright ©, All rights reserved.
2003 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
OpenContent license defines the copyright on this document.
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