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CS 177Week 4: Conditional Execution

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Conditional execution

So far we have only considered Java programs that do one thing after another, in sequence

Our programs have not had the ability to choose between different possibilities

Now, they will!

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The if-statement

The if-statement:

“x is small” will only print if x is less than 5

In this case, we know that it is, but x could come from user input or a file or elsewhere

int x = 4;

if( x < 5 )System.out.println(“x is small!”);

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The if part

Any boolean expression

Any single executable statement

Anatomy of an if

if( condition ) statement;

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Conditions in the if

Any expression that evaluates to a boolean is legal in an if-statement

Examples: x == y mass > 21.75 Character.isDigit(gender) s.equals(“help”) && (z < 4)

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Comparison

The most common condition you will find is a comparison between two things

In Java, that comparison can be: == equals != does not equal < less than <= less than or equal to > greater than >= greater than or equal to

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Equals

You can use the == operator to compare any two things of the same type

Different numerical types can be compared as well (3 == 3.0)

Be careful with double types, 0.33333333 is not equal to 0.33333332

int x = 3;if( x == 4 )System.out.println(“This doesn’t print”);

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Not Equals

Any place you could have used the == operator, you can use the != operator

If == gives true, the != operator will always give false, and vice versa

If you want to negate a condition, you can always use the ! as a not

is the same as

if( x != 4 )

if( !(x == 4) )

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= != ==

Remember, a single equal sign (=) is the assignment operator (think of a left-pointing arrow)

A double equals (==) is a comparison operator

if( y = 6 ) //compiler error!

if( y == 6 ) //how to test if y is 6!

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Less Than (or Equal To)

Inequality is very important in programming

You may want to take an action as long as a value is below a certain threshold

For example, you might want to keep bidding at an auction until the price is greater than what you can affordif( x <= 4 )

if( x < 4 ) // what is the difference?

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Greater Than (or Equal To)

Just like less than or equal to, except the opposite

Note that the opposite of <= is > and the opposite of >= is <

Thus, !( x <= y ) is equivalent to ( x > y ) !( x >= y ) is equivalent to ( x < y )

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Either/Or

Sometimes you have to make a decision

If a condition is true, you go one way, if not, you go the other

For example: If I pass CS177,▪ Then I throw a party to celebrate

Otherwise,▪ I take it again and go to class next time

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Exclusivity

Notice the nature of this kind of condition

Both outcomes cannot happen Either a party gets thrown or you

take CS 177 again For these situations, we use the else

construct

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Anatomy of an if-else

Two different

outcomes

if( condition ) statement1;

elsestatement2;

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else example

double balance = …;

if( balance < 0 )System.out.println(“You are in debt!”);

elseSystem.out.println(“You have $” + balance);

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What if you need to do several things conditionally?

No problem Use braces to treat a group of

statements like a single statement

if( x == 4 ){System.out.println(“I hate 4”);System.out.println(“Let’s change x.”);x = 10;

}

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An if with multiple statements

if( condition ){

statement1;statement2;…statementn;

}

A whole bunch of

statements

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Nesting

Sometimes you want to make one set of decisions based on another set of decisions

if-statements can be nested inside the bodies of other if-statements

You can put if-statements inside of if-statements inside of if-statements… going arbitrarily deep

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Nested ifs

if( condition1 ){

statement1;if( condition2 ) {

if( condition3 )statement2;

…}

}

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An example using quadrants

For the next example, recall the 4 quadrants of the Cartesian coordinate system

x-x

y

-y

(0,0)

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3 4

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Nesting example

Find which quadrant the point (x,y) is inif( x >= 0.0 )

{if( y >= 0.0 )

System.out.println(“Quadrant 1”);else

System.out.println(“Quadrant 4”);}else{if( y >= 0.0 )

System.out.println(“Quadrant 2”);else

System.out.println(“Quadrant 3”);}

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if and else if

You can list a sequence of exclusive possibilities using nesting:

if( index == 1 )System.out.println(“First”);

else if( index == 2 )System.out.println(“Second”);

else if( index == 3 )System.out.println(“Third”);

elseSystem.out.println(“Oops. Problem”);

}

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Watch out!

Now you are controlling the flow of execution in your program

There is a wider range of mistakes you can make when giving instructions

Huge chunks of code can be executed or skipped by mistake

Here are a few things to watch out for…

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Empty statements

Remember that an if-statement is not an executable statement

It does not end with a semicolon

if( balance < 0 ); // empty statement{ // this block always runsSystem.out.println(“You owe a fee!”);balance -= 15;

}

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Confusing indentation

In some languages, indentation actually matters

Java ignores whitespace

“Negotiate!” prints no matter what

if( enemies > 2 ) System.out.println(“Run away!”);

elsedefense = true;System.out.println(“Negotiate!”);

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Imprecise conditions

It’s easy to make logical errors when writing conditions

If an airline allows two or fewer bags on the plane, someone might code that as:

But this is too restrictive. It should be:

if( bags < 2 ) // only allows 1 or 0boarding = true;

if( bags <= 2 )boarding = true;

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Reversed conditions

Sometimes it’s easy to get a condition backwards

Try not to assume you wrote the condition correctly

Always test your code

if( number % 3 == 0 ) System.out.println(“Not divisible by 3!”);

elseSystem.out.println(“Divisible by 3!”);

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Speed limit

Sometimes you probably break the speed limit

But, there’s one speed limit you can never break

The speed of light c is about 3 x 108 miles/second

Given a variable named speed of type double, what’s an if-statement that will print an error message if speed is larger than c?

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Speed limit answer

The simplest answer:

What if we want to add a message if the speed is legal?

if( speed > 3.0e8 )System.out.println(“Not so fast!”);

if( speed > 3.0e8 )System.out.println(“Not so fast!”);

elseSystem.out.println(“That speed is fine.”);

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DNA

Assume that you have a variable called base of type char

Let base contain one of: ‘A’, ‘C’, ‘G’, ‘T’ Write a series of if- and else-statements

that will print out the chemical name of the base denoted by the corresponding character A -> Adenine C -> Cytosine G -> Guanine T -> Thymine

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Printing DNA bases

What if you want to take care of upper and lower cases?

if( base == ‘A’ )System.out.println(“Adenine”);

else if( base == ‘C’ )System.out.println(“Cytosine”);

else if( base == ‘G’ )System.out.println(“Guanine”);

else if( base == ‘T’ )System.out.println(“Thymine”);

elseSystem.out.println(“Base is not correct”);

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Upper and lower case bases using logic

Is there a simpler way?

if( base == ‘A’ || base == ‘a’ )System.out.println(“Adenine”);

else if( base == ‘C’ || base == ‘c’ )System.out.println(“Cytosine”);

else if( base == ‘G’ || base == ‘g’ )System.out.println(“Guanine”);

else if( base == ‘T’ || base == ‘t’ )System.out.println(“Thymine”);

elseSystem.out.println(“Base is not correct”);

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Upper and lower case bases using character conversion

base = Character.toUpperCase( base );

if( base == ‘A’ )System.out.println(“Adenine”);

else if( base == ‘C’ )System.out.println(“Cytosine”);

else if( base == ‘G’ )System.out.println(“Guanine”);

else if( base == ‘T’ )System.out.println(“Thymine”);

elseSystem.out.println(“Base is not correct”);

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if statements are okay…

But, didn’t that DNA example seem a little clunky?

Surely, there is a cleaner way to express a list of possibilities

Yes! It is the switch statement

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Anatomy of a switch statement

switch( data ){

case value1:statements 1;

case value2:statements 2;

…case valuen:

statements n;default:

default statements;}

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DNA hittin’ switches

switch( base ){

case‘A’: System.out.println(“Adenine”);break;

case‘C’: System.out.println(“Cytosine”);break;

case‘G’: System.out.println(“Guanine”);break;

case‘T’: System.out.println(“Thymine”);break;

default: System.out.println(“Base is not correct”);break; // unnecessary

}

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Peculiarities of switch

int data = 3;switch( data ){

case 3:System.out.println(“Three”);

case 4:System.out.println(“Four”);break;

case 5:System.out.println(“Five”);

}

Both “Three” and “Four” are

printed

The break is optional

The default is

optional too

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Rules for switch

1. The data that you are performing your switch on must be either an int or a char

2. The value for each case must be a literal3. Execution will proceed to the case that

matches4. Execution will continue until it hits a break5. If no case matches, it will go to default6. If there is no default, it will skip the whole

switch block

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DNA with upper and lower case

switch( base ){ case‘A’: case‘a’:

System.out.println(“Adenine”);break;

case‘C’: case‘c’:

System.out.println(“Cytosine”);break;

case‘G’: case‘g’:

System.out.println(“Guanine”);break;

case‘T’: case‘t’:

System.out.println(“Thymine”);break;

default: System.out.println(“Base is not correct”);break; // unnecessary

}

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A caution about switch

Using if-statements is usually saferif-statements are generally clearer

and more flexibleswitch statements are only for long

lists of specific cases Be careful about inconsistent use of break

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Input

Input can be a messy thing So far, the only input you have used

are the arguments passed into the program

Sometimes a program needs to get input several times from a user

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StdIn

To do this, we are going to use the StdIn library, created by the author of the textbook

Read pages 126 – 130 in Sedgewick Java provides several other ways of

doing input However, StdIn is simpler than most The only painful thing is that you

have to have another class file in the directory with your program to make it work

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Downloading StdIn

The location of the StdIn library is: http://www.cs.princeton.edu/introcs/15inout/StdIn.java

This is just a regular Java file Look through it if you are curious When you compile code that uses StdIn methods, StdIn.java will automatically be compiled, as long as it is in the same directory

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Purpose of StdIn

The purpose of StdIn is to let you read stuff that the user types in

If the user is running the program from the command line, he or she will enter input there

If the user is running the program from DrJava, he or she will enter input in the Interactions pane

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DrJava vs. Command Line

DrJava

Command Line

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StdIn methods

For now, we are only going to focus on a few methods in StdIn

Method Use

int readInt() Read in the next int

double readDouble() Read in the next double

boolean readBoolean() Read in the next boolean

char readChar() Read in the next char

String readString() Read in the next String

String readLine() Read in a whole line

String readAll() Read in all remaining input

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Now we can make decisions before we read in more data

System.out.println(“How many sides does your shape have?”);int sides = StdIn.readInt();if( sides == 3 ){

System.out.println(“Enter the base and the height:”);double base = StdIn.readDouble();double height = StdIn.readDouble();System.out.println(“The area of your triangle is “ + (.5 * base * height ));

}else if( sides == 4 ){

System.out.println(“Enter the length and the width:”);double length = StdIn.readDouble();double width = StdIn.readDouble();System.out.println(“The area of your rectangle is “ + (length * width));

}else

System.out.println(“I don’t know what the area “ + “of your shape is.”);

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A few notes about StdIn

StdIn is pretty smart It can ignore whitespace when

looking for the next int or double It is not foolproof If you try to read in an int and the

user types “pigs”, your program will crash