1. Conditionals & Boolean Expressions
Lecture 5
Conditionals &
Boolean Expressions
Richard Gesick

2. Topics Conditional Operator Comparing Floating point numbers Objects
Strings
Conditional Operator

3. Comparing Floating-Point Numbers
With IEEE 754 floating-point representation, minor rounding errors can occur in calculations
We compute 11 * .1 two ways
1. Multiplying 11 * .1, the result is 1.1
2. Adding times, the result is …
These values will not compare as equal using the equality operator (==)
We get similar results when assigning the same value to a float variable and to a double variable, then comparing the values.

4. // Part 1: Compute 11 * .1 two ways
double d1 = .0; // add .1 to 0 eleven times
d1 += .1; // 1
d1 += .1; // 2
d1 += .1; // 3
d1 += .1; // 4
d1 += .1; // 5
d1 += .1; // 6
d1 += .1; // 7
d1 += .1; // 8
d1 += .1; // 9
d1 += .1; // 10
d1 += .1; // 11
double d2 = .1 * 11; // compute 11 * .1
C.Wln( "d1 = " + d1 );
C.Wln( "d2 = " + d2 );
if ( d1 == d2 )
C.Wln( "d1 and d2 are equal" );
else
C.Wln( "d1 and d2 are not equal" );
// Part 2: Compare float and double with same value
float piF = f;
double piD = ;
C.Wln( "\npiF = " + piF );
C.Wln( "pid = " + piD );
if ( piF == piD )
C.Wln( "piF and piD are equal" );
C.Wln( "piF and piD are not equal" );

5. Output
d1 = d2 = 1.1 d1 and d2 are not equal piF = pid = piF and piD are not equal

6. Solution
Choose a small threshold value -- how close should the values be to be considered equal?
If the difference between the two values is less than the threshold value, then we will consider the two floating-point numbers to be equal.
Hint: use the Math.Abs method to compute the difference.

7. // Part 1: Same as last example C. Wln( "d1 = " + d1 ); C
// Part 1: Same as last example C.Wln( "d1 = " + d1 ); C.Wln( "d2 = " + d2 ); if ( Math.abs( d1 - d2 ) < ) C.Wln( "d1 and d2 are considered equal" ); else C.Wln( "d1 and d2 are not equal" ); // Part 2: Compare float and double with same value float piF = f; double piD = ; C.Wln( "\npiF = " + piF ); C.Wln( "piD = " + piD ); if ( Math.abs( piF - piD ) < ) C.Wln( "piF and piD are considered equal" ); C.Wln( "piF and piD are not equal" );

8. Output
d1 = d2 = 1.1 d1 and d2 are considered equal piF = pid = piF and piD are considered equal

9. Comparing Floats Problematic given rounding/precision
Pick a tolerance, and if the difference between the numbers is less than this tolerance, consider the numbers equivalent
if ( Math.abs( d1 - d2 ) < THRESHOLD)
C.WL("d1 and d2 are considered equal");
else
C.WL("d1 and d2 are not equal");

10. Comparing Characters What does it mean to “compare” 2 characters?
Is ‘a’ < ‘b’?
Is ‘A’ > ‘z’?

11. Lexicographic Ordering
Because all characters are “encoded” using the Unicode encoding scheme, Unicode values are compared.
Lexicographic ordering is not strictly alphabetical when uppercase and lowercase characters are mixed

12. Comparing Objects
The equality operator ( == ) compares object references.
Example:
If d1 and d2 are two Date object references, then ( d1 == d2 ) evaluates to true only if d1 and d2 point to the same object, that is, the same memory location.
The equality operator does not compare the data (month, day, and year) in those objects.

13. Comparing Object Data With d1 and d2 Date object references:
d1.equals(d2);
Returns true if the month, day, and year of d1 equals the month, day, and year of d2.
Return type
Method name and argument list
boolean
equals( Object obj )
returns true if the data of the object obj is equal to the data in the object used to call the method
5-13
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

14. // instantiate two Date objects with identical data Date d1 = new Date( 4, 10, 2006 ); Date d2 = new Date( 4, 10, 2006 ); // assign object reference d1 to d3 Date d3 = d1; // d3 now points to d1 // instantiate another object with different data Date d4 = new Date( 12, 1, 2006 ); Do not use the equality operators (==, !=) to compare object data; instead, use the equals method.

15. Comparing Date Objects

16. // compare references using equality operator if ( d1 == d2 ) C
// compare references using equality operator if ( d1 == d2 ) C.WL( "d1 and d2 are equal\n" ); else C.WL( "d1 and d2 are not equal\n" ); if ( d1 == d3 ) C.WL( "d1 and d3 are equal\n" ); C.WL( "d1 and d3 are not equal\n" ); // compare object data using the equals method if ( d1.equals( d2 ) ) C.WL( "d1 data and d2 data are equal\n" ); C.WL( "d1 data and d2 data are not equal\n" ); if ( d1.equals( d4 ) ) C.WL( "d1 data and d4 data are equal" ); C.WL( "d1 data and d4 data are not equal" );
Bad
Good

17. The Conditional Operator (?:)
The conditional operator ( ?: ) contributes one of two values to an expression based on the value of the condition.
Some uses are
handling invalid input
outputting similar messages.
Syntax:
( condition ? trueExp : falseExp )
If condition is true, trueExp is used in the expression
If condition is false, falseExp is used in the expression

18. Equivalent Code
The following statement stores the absolute value of the integer a into the integer absValue.
int absValue = ( a > 0 ? a : -a );
The equivalent statements using if/else are:
int absValue;
if ( a > 0 )
absValue = a;
else
absValue = -a;

19. The Conditional Operator
Another example:
C.Wln ("Your change is " + count + ((count == 1) ? "Dime" : "Dimes"));
If count equals 1, then "Dime" is printed
If count is anything other than 1, then "Dimes" is printed

20. Nested if Statements
if statements can be written as part of the true or false block of another if statement.
Typically, you nest if statements when more information is required beyond the results of the first if condition
The compiler matches any else clause with the most previous if statement that doesn't already have an else clause.
You can use curly braces to force a desired if/else pairing.

21. Example
if ( x == 2 )
if ( y == x )
C.Wln( "x and y equal 2" );
else
C.Wln( "x equals 2 but y does not" );
The else clause is paired with the second if , that is: if ( y == x )

22. Another Example
if ( x == 2 ) {
if ( y == x )
C.Wln( "x and y equal 2" ); }
else
C.Wln( "x does not equal 2" );
With curly braces added, the else clause is paired with the first if , that is: if ( x == 2 )

23. The "Dangling else"
A dangling else is an else clause that cannot be paired with an if condition
if ( x == 2 )
if ( y == x )
C.Wln( "x and y equal 2" );
else // paired with ( y == x )
C.Wln( "y does not equal 2" );
else // paired with ( x == 2 )
C.Wln( "x does not equal 2" );
else // no matching if!
C.Wln( "x and y are not equal" );
Generates the compiler error: 'else' without 'if'

24. In the absence of braces,
an else is always paired
with the closest preceding if that doesn’t
already have an else paired with it.

25. Bad Example has output: FAIL
float average;
average = 100.0;
if ( average >= 60.0 )
if ( average < 70.0 )
C.Wln(“Marginal PASS”);
else
C.Wln(“FAIL”);
WHY? The compiler ignores indentation and pairs the else with the second if.
100.0
average

26. To correct the problem, use braces
float average; average = 100.0; if ( average >= 60.0 ) { if ( average < 70.0 ) C.Wln(“Marginal PASS”); } else C.Wln(“FAIL”);
100.0
average

27. The switch Statement
Sometimes the switch statement can be used instead of an if/else/if statement for selection.
Requirements:
we must be comparing the value of a character (char) or integer (byte, short, or int) expression to constants of the same types

28. Syntax of switch
switch ( char or integer expression ) { case constant1: // statement(s); break; case constant2: // statement(s); … default: // optional statement(s); }

29. Operation of switch
The expression is evaluated, then its value is compared to the case constants in order.
When a match is found, the statements under that case constant are executed in sequence until either a break statement or the end of the switch block is reached.
Once a match is found, if other case constants are encountered before a break statement, then the statements for these case constants are also executed.

30. Some Finer Points of switch
The break statements are required. Their job is to terminate execution of the switch statement.
The default label and its statements are optional. They are executed when the value of the expression does not match any of the case constants.
The statements under the case constant are also optional, so multiple case constants can be written in sequence if identical operations will be performed for those values.
You cannot perform relational checks with a switch statement

31. Example: a Simple Calculator
Prompt user for two doubles (num1, num2) and a char (operation), which can be 'a' for addition or 's' for subtraction
switch ( operation ) {
case 'a':
result = num1 + num2;
break;
case 's':
result = num1 - num2; }

32. A Case-Insensitive Calculator
switch ( operation ) { case 'a': case 'A': result = num1 + num2; break; case 's': case 'S': result = num1 - num2; }

33. float weightInPounds = 165.8 f; char weightUnit ;
// user enters letter for desired weightUnit
switch ( weightUnit ) {
case ‘P’ :
case ‘p’ :
C.Wln(weightInPounds + “ pounds “ ) ;
break ;
case ‘O’ :
case ‘o’ :
C.Wln(16.0 * weightInPounds + “ ounces “ ) ;
case ‘K’ :
case ‘k’ :
C.Wln(weightInPounds / “ kilos “ ) ;
default :
C.Wln(“That unit is not handled! “ ) ; }

34. Use Switch for Menus
switch (edit_op) { case ‘D’: case ‘d’: // Delete a substring doc.do_delete (text_string); break; case ‘F’: case ’f’: // Find a substring doc.do_find (text_string); break; case ‘R’: case ‘r’: // Replace a substring doc.do_replace (text_string); break; default: C.Wln(“Invalid edit code entered.”); } // end switch;

35. What’s the output? and Why?
int age =30; if ( age < 18 ) C.Wln(“Do you drive?”); C.Wln(“Too young to vote”);

36. What’s the output? and Why?
boolean code = false; if ( ! code ) C.Wln(“Yesterday”); else C.Wln(“Tomorrow”);

37. What’s the output? and Why?
if (x > y) x = x ; C.Wln(“x bigger” ); else C.Wln(“x smaller” ); C.Wln(“y is “ + y );