Idioms and nuances

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Idioms and nuances
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Idiom

• Whats an idiom?
• Phrase that cannot be understood by looking at
  its parts in isolation
• Specialized phrase
• Example
• currentNumber
• currentNumber currentNumber 1

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For loop

• Purpose
• Idiom for commonprogramming tasks

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For loop fancified while loop

• Scanner stdin new Scanner( System.in )
• System.out.print( "Enter a number " )
• int n stdin.nextInt()
• int j 0
• while ( j lt n )
• System.out.print( j " " )
• j
• System.out.println()

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For loop fancified while loop

• Scanner stdin new Scanner( System.in )
• System.out.print( "Enter a number " )
• int n stdin.nextInt()
• int j 0
• while ( j lt n )
• System.out.print( j " " )
• j
• System.out.println()

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For loop fancified while loop

• Scanner stdin new Scanner( System.in )
• System.out.print( "Enter a number " )
• int n stdin.nextInt()
• int j 0
• while ( j lt n )
• System.out.print( j " " )
• j
• System.out.println()

7
For loop fancified while loop

• Scanner stdin new Scanner( System.in )
• System.out.print( "Enter a number " )
• int n stdin.nextInt()
• int j 0
• while ( j lt n )
• System.out.print( j " " )
• j
• System.out.println()

8
For loop fancified while loop

• Scanner stdin new Scanner( System.in )
• System.out.print( "Enter a number " )
• int n stdin.nextInt()
• while ( int j 0 j lt n )
• System.out.print( j " " )
• j
• System.out.println()

9
For loop fancified while loop

• Scanner stdin new Scanner( System.in )
• System.out.print( "Enter a number " )
• int n stdin.nextInt()
• while ( int j 0 j lt n )
• System.out.print( j " " )
• j
• System.out.println()

10
For loop fancified while loop

• Scanner stdin new Scanner( System.in )
• System.out.print( "Enter a number " )
• int n stdin.nextInt()
• while ( int j 0 j lt n )
• System.out.print( j " " )
• System.out.println()

11
For loop fancified while loop

• Scanner stdin new Scanner( System.in )
• System.out.print( "Enter a number " )
• int n stdin.nextInt()
• while ( int j 0 j lt n j )
• System.out.print( j " " )
• System.out.println()

12
For loop fancified while loop

• Scanner stdin new Scanner( System.in )
• System.out.print( "Enter a number " )
• int n stdin.nextInt()
• for ( int j 0 j lt n j )
• System.out.print( j " " )
• System.out.println()

13
For loop fancified while loop

• Scanner stdin new Scanner( System.in )
• System.out.print( "Enter a number " )
• int n stdin.nextInt()
• for ( int j 0 j lt n j )
• System.out.print( j " " )
• System.out.println()

14
For loop fancified while loop

• Scanner stdin new Scanner( System.in )
• System.out.print( "Enter a number " )
• int n stdin.nextInt()
• for ( int j 0 j lt n j )
• System.out.print( j " " )
• System.out.println()

15
For loop

• Scanner stdin new Scanner( System.in )
• System.out.print( "Enter a number " )
• int n stdin.nextInt()
• for ( int j 0 j lt n j )
• System.out.print( j " " )
• System.out.println()

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While loop

• Scanner stdin new Scanner( System.in )
• System.out.print( "Enter a number " )
• int n stdin.nextInt()
• int j 0
• while ( j lt n )
• System.out.print( j " " )
• j
• System.out.println()

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ForLoop.java
for (int i 0 i lt 3 i) System.out.println(
"i is " i ) System.out.println("all done")
for (int i 0 i lt 3 i) System.out.println(
"i is " i ) System.out.println("all done")
Variable i is local to the for loop. It does
exist not outside of it
i is 0
i is 0 i is 1
i is 0 i is 1 i is 2
i is 0 i is 1 i is 2 all done
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IllegalScope.java

• for (int j 0 i lt n i)
• System.out.println( j " " )
• System.out.println( "j is " j )

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ListRange.java

• int m stdin.nextInt()
• int n stdin.nextInt()
• for (int j m j lt n j )
• System.out.println( j " " )
• System.out.println()

Displays the numbers m n
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SumRange.java

• int m stdin.nextInt()
• int n stdin.nextInt()
• int sum 0
• for ( int j m j lt n j )
• sum sum j
• System.out.println( "Sum from " m " to "
• n " is " sum )

Displays the sum of m n
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NestedLoop.java
int m stdin.nextInt() int n
stdin.nextInt() for (int i 0 i lt m i)
System.out.println("i is " i) for (int j
0 j lt n j) System.out.println(" j is "
j)
int m stdin.nextInt() int n
stdin.nextInt() for (int i 0 i lt m i)
System.out.println("i is " i) for (int j
0 j lt n j) System.out.println(" j is "
j)
int m stdin.nextInt() int n
stdin.nextInt() for (int i 0 i lt m i)
System.out.println("i is " i) for (int j
0 j lt n j) System.out.println(" j is "
j)
int m stdin.nextInt() int n
stdin.nextInt() for (int i 0 i lt m i)
System.out.println("i is " i) for (int j
0 j lt n j) System.out.println(" j is "
j)
// suppose m is 3 // suppose n is 2

• i is 0
• j is 0
• j is 1
• i is 1
• j is 0
• j is 1
• i is 2
• j is 0
• j is 1

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Counts.java

• int counter1 0 int counter2 0 int counter3
  0
• int counter4 0 int counter5 0
• for (int i 0 i lt 5 i)
• counter1
• for (int j 0 j lt 10 j)
• counter2
• for (int k 0 k lt 2 k)
• counter3
• counter4
• counter5
• System.out.println(counter1 " " counter2 "
  "
• counter3 " " counter4 " " counter5)

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50
100
50
5
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Nuances
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Nuance

• Whats a nuance?
• A subtle or find distinction
• The intended message
• Example
• I will not be happy if you do that, said the
  parent

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Shorthand representation
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Examples

• Consider
• String a "excellence
• String b a
• What is the representation?

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Examples

• Consider
• String a "excellence
• String b a
• What is the representation?

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Uninitialized versus null

• Consider
• String dayOfWeek
• Scanner inStream
• What is the representation?

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Uninitialized versus null

• Consider
• String fontName null
• Scanner fileStream null
• What is the representation?

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Quick survey

• What's the memory representation for the
  following definitions?
• String word1 null
• String word2
• String word3 "luminous"

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Assignment

• Consider
• String word1 "luminous"
• String word2 "graceful"
• word1 word2
• Initial representation

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Assignment

• Consider
• String word1 "luminous"
• String word2 "graceful"
• word1 word2
• After assignment

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Testing variables for equality

• Consider
• String a stdin.next()
• String b stdin.next()
• if ( a b )
• System.out.println( "Same" )
• else
• System.out.println( "Different" )

What is the output if the user enters different
words? What about the same words?
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Testing variables for equality

• Consider
• String a stdin.next()
• String b stdin.next()
• if ( a b )
• System.out.println( "Same" )
• else
• System.out.println( "Different" )

What is the output if the user enters different
words? What about the same words?
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Testing objects for equality

• Consider
• String a stdin.next()
• String b stdin.next()
• if ( a.equals( b ) )
• System.out.println( "Same" )
• else
• System.out.println( "Different" )

What is the output if the user enters different
words? What about the same words?
36
Testing objects for equality

• Consider
• String a stdin.next()
• String b stdin.next()
• if ( a.equals( b ) )
• System.out.println( "Same" )
• else
• System.out.println( "Different" )

All objects have an equals() method
Responsibility of the classs designer to make
sure its definition is sensible
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Short-circuit evaluation

• Consider
• a b
• a b
• Facts
• If the left operand of is false, then the
  must be false
• If the left operand of is true, then the
  must be true
• Java uses the facts to make logical operations
  efficient
• If the value of an operation determined from the
  left operand, then the right operand is not
  evaluated
• The operation is short-circuited

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Short-circuit evaluation

• Suppose you are interested in knowing whether
  scoreSum divided by nbrScores is greater than
  value
• The condition can be evaluated only if nbrScores
  is nonzero
• The following expression correctly represents the
  condition
• ( nbrScores ! 0 ) ( ( scoreSum / nbrScores )
  gt value )