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Decisions: Handling Infinite
CasesCS 21a: Introduction to
Computing IFirst Semester, 2013-2014
Previously…
►Modularity with procedures and OOP►Generality with variables
Next Set of Lectures…
►Generality with conditionals► For dealing with infinite test cases (when
simple variables and expressions won’t work)
►Generality with loops► For dealing with infinite (or arbitrary
length) processes
►Generality with arrays► For dealing with infinite (or arbitrary-
sized) data
Conditional Execution
►Sometimes a simple, linear, clean formula is enough to solve a general problem.
►Sometimes we need to branch because we are forced to handle some subsets of the input cases slightly differently.
Conditional Execution
►Java structures for conditions►The if-statement►boolean variables, operators,
expressions, and methods►The ? Operator►The switch-statement
The if Statement
►Syntaxif (condition)
statement►Notes►parentheses are required around the
condition►statement means any valid statement
in Java (including if-statements and blocks)
Example 1public class BankAccount{ private double balance; ... public void withdraw( double amount ) { if ( amount <= balance ) balance = balance - amount; System.out.println( "End of Transaction" ); // print statement executed unconditionally } ...}
Example 2public class BankAccount{ private double balance; ... public void withdraw( double amount ) { if ( amount <= balance ) balance = balance - amount; if ( amount > balance )
balance = balance – OVERDRAFT_PENALTY; } ...}
The Optional else Clause
►If-statement syntax revisitedif (condition) statementelse statement
►Use whenever an alternative statement should be executed when the condition is not met
Example 2, revised: Using the else
public class BankAccount{ private double balance; ... public void withdraw( double amount ) { if ( amount <= balance ) balance = balance - amount; else
balance = balance – OVERDRAFT_PENALTY; } ...}
Example 3public class BankAccount
{
private double balance;
...
public void withdraw( double amount )
{
if ( amount <= balance )
balance = balance - amount;
else
System.out.println( "Insufficient Balance" );
System.out.println( "End of Transaction" );
}
...
}
No penalty is appliedbut an error messageis printed when thecondition is not met
Example 4
►Suppose two statements need to be conditionally executed
►Incorrect attemptif ( amount <= balance )
balance = balance - amount;
System.out.println("amount deducted");
►Print statement will be executed unconditionally
Block of Statements
►A block allows us to group several statements into one► place the statements in sequence and
surround them with { }►Correct code
if ( amount <= balance )
{
balance = balance - amount;
System.out.println("amount deducted");
}
Brace Layout, White Spaces,and Indentation
► In Java, spaces, tabs, and extra lines don't affect the meaning of the program
►A program could be written in diff ways; e.g.,► all in one line► such that each word/symbol is in one line► such that words/symbols are separated by
5 spaces each►BUT … Spaces (indentation) help to
clarify intent
Brace Layout
if (amount <= balance){double newBalance = balance – amount;balance = newBalance;
}OR
if (amount <= balance) {double newBalance = balance – amount;balance = newBalance;
}
Indentation and Tabspublic class BankAccount{ private double balance; ... public void withdraw( double amount ) { if ( amount <= balance )` balance = balance - amount; else
balance = balance – OVERDRAFT_PENALTY; } ...}
Two to three spaces per indentation
Indentation and Tabspublic class BankAccount{ private double balance; ... public void withdraw( double amount )
{ if ( amount <= balance ) balance = balance - amount; else
balance = balance – OVERDRAFT_PENALTY;
} ...}
Eight spaces per indentation
Relational Operators
►Compares two (usually numeric) operands
►>, >=, <, <=, == (equal), != (not equal)►Example: >=
► binary operation► returns a boolean result
►true if left operand is greater than or equal to right operand
► false otherwise
Practice Programming Problem
►Implement an absolute value function.
Comparing Floating-Point Numbers
► Consider this code:double r = Math.sqrt(2); double d = r * r - 2; if (d == 0) System.out.println(
"sqrt(2)squared minus 2 is 0" ); else System.out.println(
"sqrt(2)squared minus 2 is not 0 but " + d );
► It prints:sqrt(2)squared minus 2 is not 0but 4.440892098500626E-16
Comparing Floating-Point Numbers
►To avoid round-off errors, don't use == to compare floating-point numbers
►To compare floating-point numbers, test whether they are close enough: |x - y| ≤ ε final double EPSILON = 1E-14;
if (Math.abs(x - y) <= EPSILON)
// x is approximately equal to y
►ε is a small number such as 10-14
Comparing Strings
►Don't use == for strings! if (input == "Y") // WRONG!!!
►Use equals method: if (input.equals("Y"))
►== tests identity, equals() tests forequal contents
►Case insensitive test ("Y" or "y") if (input.equalsIgnoreCase("Y"))
Comparing Strings
►s.compareTo(t) < 0 means:s comes before t in the dictionary
►"car" comes before "cargo" ►All uppercase letters come before
lowercase: "Hello" comes before "car"
Lexicographic Comparison
Practice Programming Problem
►Write a program that reads two words and says if they have the SAME or DIFFERENT starting letter.
►Sample InputCar carrot
►Sample OutputSAME
Comparing Objects
►Since object variables contain references,== tests for identity,not for identical object content
►BankAccount b = new BankAccount( 1000 );BankAccount c = new BankAccount( 1000 );
►Object references are not the same► b != c
►But contents are equal► b.getBalance() == c.getBalance()
Testing for Null► null reference refers means no object
String middleInitial = null; // not setif (…) middleInitial = middleName.substring(0, 1);
► Can be used in tests:if (middleInitial == null) System.out.println(firstName + " " + lastName);else System.out.println(firstName + " " + middleInitial + ". " + lastName);
► Use ==, not equals(), to test for null ► Note that null is not the same as the empty string
""
Sequences of Comparisons
►The statementthat correspondsto the first matching conditionis executed
►Also called anif-else chain
if (condition1)
statement1;
else if (condition2)
statement2;
. . .
else
statement;
Example 5
if ( score >= 92 ) System.out.println( "A" );else if ( score >= 80 ) System.out.println( "B" );else if ( score >= 70 ) System.out.println( "C" );else if ( score >= 60 ) System.out.println( "D" );else System.out.println( "F" );
About the if-else Chain
►Order matters► if (score >= 60)
System.out.println( "D" );else if (score >= 70) System.out.println( "C" );…// most students will get D's
►See Richter's scale example in the textbook for another if-else chain
►The if-else chain is an example of a nested branch
Practice Programming Problem
►Write a program that reads two numbers and tells if the first is bigger, smaller, or equal to the second one. Print out the absolute difference if they’re not equal.
►Sample Input1 2
►Sample Outputsmaller by 1
Nested Branches
►Branch inside another branchif (condition0) if (condition1) statement1a; else statement1b; else statement2;
else portion couldalso contain anested branch; e.g., if (condition2) statement2a; else statement2b;
Example 6
►Movie ticket price example►Premium movie (first week of
release): P120►Regular movie (second week of
release or later): P100►Senior citizen discount: 20%
Example 6public double TicketPrice( int customerAge, int daysShowing ){
double cost;
if (customerAge >= 60)if ( daysShowing < 8 ) cost = 120.0 * 0.80;
else cost = 100.0 * 0.80;
elseif ( daysShowing < 8 ) cost = 120.0;
else cost = 100.0;
return cost; }
The boolean Data Type
►Only two possible values►true and false
►Literals►true, false► lowercase (reserved words in Java)
►Operations► relational operators► logical operators
Logical Operators
►Boolean operands►&& (and), || (or), ! (unary not)►Example
((x>=0) && (x<=9))
►Truth table depicts semantics of the operation►similar to a multiplication/addition
table
! (NOT)
►Unary operation►Returns a boolean result► true when the operand is false► false when the operand is true
►Example►alternative to !=► (a != 5) same as (!(a == 5))
&& (AND)
►Returns a boolean result► true whenever both
operands are true► false otherwise
►Example:► testing whether a
number is between 0 and 9
► if ((num >= 0) && (num <= 9)) ... // inclusive
►Truth Table?
A B A && B
False
False
False
False
True False
True False
False
True True True
|| (OR)
►Returns a boolean result► true when at least one operand
is true► false otherwise
►Example► if ((num % 2 == 0) || (num % 3 == 0)) …
► condition will evaluate to true if the number is a even or if it is a multiple of 3
►Truth Table?
A B A || B
False
False
False
False
True True
True False
True
True True True
Short-Circuit Evaluation
► Sometimes, we know the result of a boolean expression without checking all the conditions
► AND (&&): If first condition is false, then result is false regardless of second condition
( x < y ) && ( a < b )
► OR (||): If first condition is true, then result is true regardless of second condition
( x < y ) || ( a < b )
► Short-circuit evaluation can prevent errors( x < 0 ) || ( Math.sqrt( x ) > 20 )
( x != 0 ) && ( y/x > 20 )
Practice Programming Problem
►Write a program that reads three different numbers and tells if they are in INCREASING order, DECREASING
order, or UNSORTED.
►Sample Input1 3 9
►Sample OutputINCREASING
boolean Variables
►Boolean variables are convenient for long conditions
►Exampleboolean withinRange;
…
withinRange = ( num >= 0 ) && ( num <= 9 );
if ( withinRange ) ...
Methods with boolean Return Values
►Methods can return boolean-type values
►Allows you to encapsulate complex conditions, or conditions that depend on the state of an object
Example 7public class BankAccount
{
...
public static final int MINBAL = 10000;
...
public boolean isBelowMinBalance()
{
if ( balance < MINBAL )
return true;
else
return false;
}...
}
It is common and recommended practice to give boolean-type methods names starting with isXXX
Is there a shorter way to write this?
Example 7, revised
public class BankAccount
{
...
public static final int MINBAL = 10000;
...
public boolean isBelowMinBalance()
{
return (balance < MINBAL);
}
...
}
Calling a boolean-type Method
public class BankSystem
{
public static void main( String args[] )
{
BankAccount b = new BankAccount();
...
boolean below = b.isBelowMinBalance();
if ( below )
System.out.println( "account is below balance" );
else
System.out.println( "account is OK" );
...
}
}
Calling a boolean-type Method within the Same Class
public class BankAccount
{ ...
public void withdraw( double amount )
{
if ( amount <= balance )
{
balance = balance - amount;
if ( isBelowMinBalance() )
balance = balance – 50; // subtract penalty
}
else
System.out.println( "Insufficient Balance" );
} ...
}
Advanced Topics
►Dangling Else►The ? operator►The switch statement
Dangling else
if ( num > 10 ) if ( num > 100 ) System.out.println( "Large" );else System.out.println( "Small" );
►What gets printed out when num = 150? when num = 80? when num = 5?
►Which if does the else clause match?
Dangling else
►Rule in Java: an else clause matches the nearest enclosing if
►Use { } to match the outer ifif ( num > 10 ) { if ( num > 100 ) System.out.println( "Large" );}else System.out.println( "Small" );
The ? operator► Syntax: condition ? then-value : else-value► Use this as a one-line replacement for if► Instead of
String s;if ( x % 2 == 0 ) s = "even";else s = "odd";
► We can sayString s = (x%2 == 0) ? "even" : "odd";
► Commonly used in C programs► Use sparingly, since it can lead to cryptic code
The if-else Chain Revisited
►If the conditions in an if-else chain are exact comparisons (using ==) on integral (or char) values, consider using a switch statement
►Example: print "One" if the value is 1,print "Two" if the value if 2, …
Example 8if (num == 1)
{
System.out.println("One");
}
else if (num == 2)
{
System.out.println("Two");
}
else if (num == 3)
{
System.out.println("Three");
}
else
{
System.out.println("Other number");
}
Example 8, revised;Using a Switch
switch(num)
{
case 1:
System.out.println("One");
break;
case 2:
System.out.println("Two");
break;
case 3:
System.out.println("Three");
break;
default:
System.out.println("Other number");
}
The switch Statement► Use a switch statement whenever
► the conditions in an if-else chain are designed to check for constant values representing a finite number of cases
► Switch-case statement► switch( exp )
► exp must be a "countable" primitive type (i.e., byte, int, char, etc., but not float or double)
► NEW TO JAVA 7: or a String (not allowed in older versions of Java)
► case literal ► serves as "entry-point" label for case when exp==literal
► break;► statement that causes control to exit the block► if there's no break, control "falls-through" until we see a break
Using (or not using) breakswitch( letter ) { case 'a': case 'e': case 'i': case 'o': case 'u': System.out.println("Vowel"); break; default: System.out.println("Consonant");}
Summary
► A program will sometimes need to branch.► Branches often rely on testing if the input
satisfies some condition.► Branches are specified with conditionals.► Conditions are specified with boolean
expressions.► There are several kinds of conditionals in
Java. Learn to choose the most appropriate for the problem at hand.
Next Time…
►Loops, or finite algorithms for infinite processes.
