Week 4.pptx

CS 177 Week 4: Conditional Execution 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! 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!?); Anatomy of an if if( condition ) statement; The if part Any boolean expression Any single executable statement 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) 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 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?); 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) ) = != == 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! 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 afford if( x <= 4 ) if( x < 4 ) // what is the difference? 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 ) 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 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 Anatomy of an if-else Two different outcomes if( condition ) statement1; else statement2; else example double balance = ?; if( balance < 0 ) System.out.println(?You are in debt!?); else System.out.println(?You have $? + balance); 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; } An if with multiple statements if( condition ) { statement1; statement2; ? statementn; } A whole bunch of statements 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 Nested ifs if( condition1 ) { statement1; if( condition2 ) { if( condition3 ) statement2; ? } } An example using quadrants For the next example, recall the 4 quadrants of the Cartesian coordinate system x -x y -y (0,0) 1 2 3 4 Nesting example Find which quadrant the point (x,y) is in if( 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?); } 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?); else System.out.println(?Oops. Problem?); } 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? 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 runs System.out.println(?You owe a fee!?); balance -= 15; } Confusing indentation In some languages, indentation actually matters Java ignores whitespace ?Negotiate!? prints no matter what if( enemies > 2 ) System.out.println(?Run away!?); else defense = true; System.out.println(?Negotiate!?); 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 0 boarding = true; if( bags <= 2 ) boarding = true; 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!?); else System.out.println(?Divisible by 3!?); 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? 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!?); else System.out.println(?That speed is fine.?); 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 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?); else System.out.println(?Base is not correct?); 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?); else System.out.println(?Base is not correct?); 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?); else System.out.println(?Base is not correct?); 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 Anatomy of a switch statement switch( data ) { case value1: statements 1; case value2: statements 2; ? case valuen: statements n; default: default statements; } 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 } 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?); } The default is optional too Both ?Three? and ?Four? are printed The break is optional Rules for switch The data that you are performing your switch on must be either an int or a char The value for each case must be a literal Execution will proceed to the case that matches Execution will continue until it hits a break If no case matches, it will go to default If there is no default, it will skip the whole switch block 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 } A caution about switch Using if-statements is usually safer if-statements are generally clearer and more flexible switch statements are only for long lists of specific cases Be careful about inconsistent use of break 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 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 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 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 DrJava vs. Command Line DrJava Command Line 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 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.?); 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