CSE 1301

Lecture 8

Conditionals & Boolean Expressions

Figures from Lewis, “C# Software Solutions”, Addison Wesley

Richard Gesick

CSE 1301

Topics

• Comparing– Floating point numbers– Objects– Strings

• Conditional Operator

CSE 1301

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 .1 11 times, the result is 1.0999999…• 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.

// 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.O.Wln( "d1 = " + d1 ); C.O.Wln( "d2 = " + d2 ); if ( d1 == d2 )

C.O.Wln( "d1 and d2 are equal" );

else C.O.Wln( "d1 and d2 are not

equal" );// Part 2: Compare float and

double with same value float piF =

3.141592653589793f; double piD =

3.141592653589793;

C.O.Wln( "\npiF = " + piF ); C.O.Wln( "pid = " + piD ); if ( piF == piD ) C.O.Wln( "piF and piD are

equal" ); else C.O.Wln( "piF and piD are

not equal" );

• 1-4

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Output

d1 = 1.0999999999999999d2 = 1.1d1 and d2 are not equal

piF = 3.1415927pid = 3.141592653589793piF and piD are not equal

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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.

CSE 1301

// Part 1: Same as last example C.O.Wln( "d1 = " + d1 ); C.O.Wln( "d2 = " + d2 ); if ( Math.abs( d1 - d2 ) < .0001 ) C.O.Wln( "d1 and d2 are considered equal" ); else C.O.Wln( "d1 and d2 are not equal" );

// Part 2: Compare float and double with same value float piF = 3.141592653589793f; double piD = 3.141592653589793;

C.O.Wln( "\npiF = " + piF ); C.O.Wln( "piD = " + piD ); if ( Math.abs( piF - piD ) < .0001 ) C.O.Wln( "piF and piD are considered equal" ); else C.O.Wln( "piF and piD are not equal" );

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Outputd1 = 1.0999999999999999d2 = 1.1d1 and d2 are considered equal

piF = 3.1415927pid = 3.141592653589793piF and piD are considered equal

CSE 1301

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");

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Comparing Characters

• What does it mean to “compare” 2 characters?

• Is ‘a’ < ‘b’?• Is ‘A’ > ‘z’?

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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

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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.

CSE 1301

Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-

Wesley

5-13

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

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// 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.

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Comparing Date Objects

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// 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" ); else 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" ); else C.WL( "d1 data and d2 data are not equal\n" );

if ( d1.equals( d4 ) ) C.WL( "d1 data and d4 data are equal" ); else C.WL( "d1 data and d4 data are not equal" );

Bad

Good

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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

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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;

CSE 1301

The Conditional Operator• Another example:

C.O.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

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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.

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Exampleif ( x == 2 ) if ( y == x ) C.O.Wln( "x and y equal 2" ); else

C.O.Wln( "x equals 2 but y does not" );

• The else clause is paired with the second if , that is: if ( y == x )

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Another Exampleif ( x == 2 ){ if ( y == x ) C.O.Wln( "x and y equal 2" );} else C.O.Wln( "x does not equal 2" );

• With curly braces added, the else clause is paired with the first if , that is: if ( x == 2 )

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The "Dangling else"• A dangling else is an else clause that cannot be

paired with an if conditionif ( x == 2 ) if ( y == x ) C.O.Wln( "x and y equal 2" ); else // paired with ( y == x ) C.O.Wln( "y does not equal 2" );else // paired with ( x == 2 ) C.O.Wln( "x does not equal 2" );else // no matching if! C.O.Wln( "x and y are not equal" );

• Generates the compiler error: 'else' without 'if'

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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.

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Bad Example has output: FAIL

float average;

average = 100.0;

if ( average >= 60.0 )if ( average < 70.0 ) C.O.Wln(“Marginal PASS”);

elseC.O.Wln(“FAIL”);

WHY? The compiler ignores indentation and pairs the else with the second if.

average

100.0

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To correct the problem, use braces

float average;

average = 100.0;

if ( average >= 60.0 ){

if ( average < 70.0 ) C.O.Wln(“Marginal PASS”);

}else

C.O.Wln(“FAIL”);

average

100.0

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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

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Syntax of switch

switch ( char or integer expression ) { case constant1: // statement(s);

break; case constant2:

// statement(s); break;

… default: // optional

statement(s); …}

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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.

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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

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Example: a Simple Calculator• Prompt user for two doubles (num1, num2) and a char

(operation), which can be 'a' for addition or 's' for subtractionswitch ( operation ){case 'a':

result = num1 + num2;break;

case 's':result = num1 - num2;break;

}

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A Case-Insensitive Calculatorswitch ( operation ){case 'a':

case 'A':result = num1 + num2;break;

case 's': case 'S':

result = num1 - num2;break;

}

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float weightInPounds = 165.8 f;char weightUnit ;

. . . // user enters letter for desired weightUnitswitch ( weightUnit ){

case ‘P’ :case ‘p’ :

C.O.Wln(weightInPounds + “ pounds “ ) ;break ;

case ‘O’ :case ‘o’ :

C.O.Wln(16.0 * weightInPounds + “ ounces “ ) ;break ;

case ‘K’ :case ‘k’ :

C.O.Wln(weightInPounds / 2.2 + “ kilos “ ) ;break ;

default :C.O.Wln(“That unit is not handled! “ ) ;break ;

}

CSE 1301

Use Switch for Menusswitch (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 substringdoc.do_replace (text_string); break;

default: C.O.Wln(“Invalid edit code entered.”);

} // end switch;

CSE 1301

Write an expression for eachWrite a logical assignment statement for each of the following:• assign a value of true to between if n is in the range of -k to

+k, inclusive; otherwise assign a value of false.

• assign a value of true to uppercase is ch is an uppercase letter; otherwise, assign a value of false.

• assign a value of true to divisor if m is a divisor of n; otherwise assign a value of false.

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What’s the output? and Why?int age =20;

if ( age = 16 ){

C.O.Wln(“Did you get driver’s license?”) ;}

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What’s the output? and Why?

int age =30;

if ( age < 18 )

C.O.Wln(“Do you drive?”); C.O.Wln(“Too young to vote”);

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What’s the output? and Why?boolean code = false;

if ( ! code )

C.O.Wln(“Yesterday”);else

C.O.Wln(“Tomorrow”);

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What’s the output? and Why?

if (x > y) x = x + 10.0;C.O.Wln(“x bigger” );elseC.O.Wln(“x smaller” );C.O.Wln(“y is “ + y );