cmpt 125: lecture 4 conditionals and loopstamaras/conditionalsandloops/conditionals... · cmpt 125:...

CMPT 125: Lecture 4

Conditionals and Loops

Tamara Smyth, [email protected]

School of Computing Science,

Simon Fraser University

January 17, 2009

1

mailto:[email protected]

http://www.cs.sfu.ca

http://www.sfu.ca

Flow of Control

• The order in which statements are executed is calledthe flow of control.

• Unless otherwise specified, the flow of control islinear, that is, statements are executed in the order inwhich they occur.

• Java begins a program on the first statement of themain method.

• A change in the flow of control can be accomplishedby:

– invoking a method

– conditional statements

– loops

or some combination.

CMPT 125: Conditionals and Loops, Lecture 4 2

Conditional Statements

• A conditional statement allow us to choose whichstatement will be executed next.

• Java has three conditional statements:

1. if-else

2. else if

3. switch

• These statements allow us to decide which statementto execute next based on a condition, that is, theoutcome of a boolean expression.


Boolean Expressions

• Recall that a boolean is a data type that evaluates toeither true or false.

• A boolean statement also evaluates to true or false,and may consist solely of a single variable, or a largerstatement involving a logical operator.

boolean hungry = false; //hungry is a boolean

if (hungry)

System.out.println(’’Let’s go eat!’’);

else

system.out.println(’’Let’s go for a drink instead.’’);

• The if statement allows a program to choosewhether to execute a particular statement.

if (num > 20)

System.out.println(’’Count is over 20.’’);

else

system.out.println(’’Count is less than or equal to 20.’’);


Evaluating Boolean Expressions

• Boolean expressions are evaluated using:

– equality operators:== test whether two values ARE equal!= test whether two values ARE NOT equal

– relational operators:> test whether one value is greater than another< test whether one value is less than another>= greater than or equal<= less than or equal

– logical operators:! logical NOT&& logical AND|| logical OR

• Logical operators take boolean operands


Logical NOT

• The logical NOT is unary, and has only two possiblevalues: true or false.

boolean hungry = false;

if (!hungry)

system.out.println(’’Let’s go for a drink instead.’’);

OUTPUT:

Let’s go for a drink instead.

• The ! is used to perform the logical NOT, or logical

complement, that is, it returns the opposite value.

boolean hungry = true;

if (!hungry)

System.out.println(’’Let’s go for a drink instead.’’);

else

System.out.println(’’Let’s go eat.’’);

OUTPUT:

Let’s go eat.


Logical AND, OR

• Logical AND, OR are binary operators that use twooperands, both of which are boolean.

• The result of a logical AND operation is true if andonly if both operands are true, but false otherwise.

• The result of a logical OR operation is true if one, orboth, of the operands are true, but false otherwise.

• There are four possible combinations of two logicaloperands a and b:

a b a && b a || b

false false false falsefalse true false truetrue false false truetrue true true true


Precedence of Operators

• Operators in order of precedence:

1. logical NOT (highest)

2. logical AND

3. logical OR

if (!done && (count > MAX))

System.out.println(‘‘Completed’’);

• To print or not to print. Let’s look at the truth table

which tells us all the possible combinations of valuesfor the variables involved.

done count > MAX !done !done && (count > MAX)

false false true false

false true true true

true false false false

true true false false


Example code using if-else, else if

public class testNum

{

public static void main(String[] args)

{

int num = Integer.parseInt(args[0]);

if (num > 5)

System.out.println(num + " > 5");

else if (num > 4)


else if (num > 2)


else

System.out.println(num + " is 2 or less");

}

}


Short Circuiting Operations

• The AND and OR operators may be “short-circuited”meaning that is the left operand is sufficient to decidethe boolean result, the right operand is not evaluated.

• You can use this to your advantage by ensuringvariables will not result in an expression that isundefined, e.g. preventing a division by zero.

if (count != 0 && total/count > MAX)

System.out.println(‘‘Testing’’);

• An equivalent way of achieving the same result wouldinvolve nesting the if statements:

if (count != 0)

if (total/count > MAX)

System.out.println(‘‘Testing’’);


Block Statement

• An if statement consists of the reserved word if

followed by a boolean expression, followed by one ormore statements. That is, a single statement can bereplaced with a block statement, a collection ofstatements enclosed in braces.

if (guess == answer)

System.out.println(‘‘You’re right!’’);

else

{

System.out.println(‘‘I’m sorry.’’);

System.out.println(‘‘The correct answer is ’’ + answer);

}

• It is common practice, and very important to thehuman reader, to indent the statement(s) followingthe boolean expression to show they are part of theif statement.


A Loop, or Repetitive Statement

• A loop, or repetitive statement, allows us to execute aprogramming statement more than once.

• Like a conditional, a loop is based on the evaluationof a boolean expression

• Java has three loop statements:

1. while

2. do

3. for


The while Statement

• A while statement is followed by a booleanexpression, and evaluates a statement, or a block ofcode, repeatedly, while the condition remains true.


{

int sum = 0, value, count = 0;

Scanner scan = new Scanner(System.in);

System.out.println("Enter an integer (0 to quit): ");

value = scan.nextInt();

while(value != 0)

{

count++;

sum += value;

System.out.println("The sum so far is " + sum);

System.out.println("Enter an integer (0 to quit): ");

value = scan.nextInt();

}

if (count==0)

System.out.println("No values were entered.");

else

{

DecimalFormat fmt = new DecimalFormat("0.###");

System.out.println("The average is "

+ fmt.format((double)sum/count));

}

}

• Avoid infinite loops!


The Conditional Operator

• When using the conditional operator, a booleancondition is followed by the ? operator which isfollowed by two expressions separated by the :

operator.

: 2nd expressionboolean condition ? 1st expression

Figure 1: The conditional operator.

• The first of the two expression is evaluated if theboolean condition is true, and if false, the secondexpression is evaluated.

• The following is an example use of the conditionaloperator

• A ternary operator because it requires three operands.

(total > MAX) ? total + 1 : total * 2;

• The Conditional operator returns a value, andtherefore might be used as follows:

total = (total > MAX) ? total + 1 : total * 2;


Comparing Floats

• Two floating point values are equal, according to the== operator, only if all the binary digits of theirunderlying representations match.

• Since this is unlikely to be the case (perhaps due torounding errors), it is best not to use this operator forcomparing floating point values.

• A better way computes the absolute value of thedifference between the two, and then compare theresult to some tolerance level.

if (Math.abs(f1-f2) < TOLERANCE)

System.out.println(‘‘Effectively equal.’’);


Comparing Characters

• Equality and relational operators may also be used oncharacter data.

• Because of the Unicode character set, it is possible todetermine if one is less than another.

if (ch1 > ch2)

System.out.println(ch1 + ‘‘ is greater than ‘‘ + ch2);

else

System.out.println(ch1 + ‘‘ is NOT greater than ‘‘ + ch2);


Comparing Objects

• The String class contains an equals method thatreturns true if two strings have exactly the samecharacters, and false otherwise.

if (name1.equals(name2))

System.out.println(‘‘The names are the same.’’);

else

System.out.println(‘‘The names are NOT the same.’’);

• It is valid to test two String objects using using theequality operator, but it will test whether theyreference the same object:

String name1 = new String(‘‘string’’);

String name2 = new String(‘‘string’’);

if (name1 == name2)

System.out.println(‘‘These variables contain ‘‘ +

‘‘the same address.’’);

else

System.out.println(‘‘These variables contain ‘‘ +

‘‘different addresses.’’);

OUTPUT:

These variables contain different addresses.


The switch Statement

• The switch statement evaluates an expression todetermine a value.

• It then matches that value with one of severalpossible cases.

switch(grade)

{

case ’A’:

System.out.println(‘‘Excellent Work!’’);

break;

case ’B’:

System.out.println(‘‘Good Work.’’);

break;

case ’C’:

System.out.println(‘‘Satisfactory Work.’’);

break;

default:

System.out.println(‘‘Error in identifying your grade!’’);

}

• If no case matches the evaluated expression,execution continues with the default case.

• When a break statement is encountered, theprogram exits the switch statement, and continueswith the next statement.


Nested Loops

• The body of a loop can contain another loop.

public class Palindrome

{


{

String str, another = "y";

int left, right;

Scanner scan = new Scanner(System.in);

while(another.equalsIgnoreCase("y"))

{

System.out.println("Enter a palindrome");

str = scan.nextLine();

left = 0;

right = str.length() - 1;

while(str.charAt(left) == str.charAt(right) &&

left < right)

{

left++;

right--;

}

System.out.println();

if (left<right)

System.out.println("The string is NOT a palindrome.");

else

System.out.println("The string IS a palindrome.");

System.out.print("Test another palindrome (y/n)?");

another = scan.nextLine();

}

}


}


Iterators

• An iterator is an object that has methods that allowyou to process a collection of items one at a time.

• Since it is a repetitive process, it is related to loops.

• An iterator in Java is defined using the Iterator

interface.

• The Scanner class has methods such as hasNext,hasNextInt, hasNextDouble, to determine if thenext input token is of a particular type.

• The Scanner class as an iterator is useful whenprocessing a data file or processing parts of acharacter string.

• A delimiter is used to separate tokens in a Scanner

object.

fileScan.useDelimeter(‘‘;’’);


The do statement

• The do loop executes the statement in the loop untilthe condition, written at the end of of the loop,becomes false.

int count = 0;

do

{

count++;

System.out.println(count);

} while (count < 5);

• A do statement executes its loop body at least once.This may give you reason to choose a do, rather thana while loop.


The for statement

• The do and while loops are usually used when youdon’t know exactly how many times you want toexecute the loop body.

• The for loop is suited to situations where you knowprecisely how many times the loop should beexecuted.

for (int count = 0; count < 10; count++)


• The header of a for loop contains three partsseparated by semicolons:

1. initialization: executed before the loop begins.

2. boolean condition: evaluated before the loopbody; if true the loop is executed.

3. increment: executed after each loop iteration.

• It is also possible to decrement a value:

for (int count = 10; count > 0; count--)



Iterators and for loops

• A variation of the for loop lets us process the itemsin an iterator with a different syntax. Consider theiterator Booklist that manages Book objects:

for(Book myBook : BookList)

System.out.println(myBook);

is equivalent to

Book mybook;

while (BookList.hasNext())

{

myBook = BookList.next();

System.out.println(myBook);

}

• This version can also be used on arrays.


cmpt 125: lecture 4 conditionals and loopstamaras/conditionalsandloops/conditionals... · cmpt 125:...

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