1 Conditions and control statements. 1.1 Conditional statement if

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1 1 Conditions and control statements In the previous problems we have solved program execution has been an uninterrupted flow of statements. A sequence of statements executed in the order written within the source code. We have been unable to make decisions or change the behavior of the program depending on user input and actions. A collection of such statements are described as a sequence of statements. Sequence: Start: statement 1 -> statement 2 -> statement 3 -> statement 4 End. We are now about to get acquainted with a new way of describing the order in with statements are to be executed. Based on a decision, a condition, a java program is able to branch into two different sequences. This is called selection. 1.1 Conditional statement if A conditional statement is a construction which allows us to decide between two different statement sequences, or, to put it another way, select one of two possible paths, depending on a condition. if statement form: if ( logic_expression ) statement / statement block else statement / statement_block or if only one possible branch is possible if ( logic_expression ) statement / statement block The logic expression, expresses a condition, either true or false. The first statement or statement block is executed if the logic expression is evaluated to true, the second (if present ) is executed if the statement is false. The second branch (including else) is an optional part of the if statement. A few basic examples: int numberofpets; System.out.println("How many pets do you have?"); numberofpets = keyb.nextint(); System.out.println("So you have "+ numberofpets+ " pets"); if ( numberofpets > 5 ) System.out.println("Oh that s a lot!"); The statements above have to be enclosed within a main( ) method. This program now has two possible scenarios. 1. The condition is true, the user input a number larger than 5.

2 How many pets do you have? 12 So you have 12 pets Oh that's a lot! 2. The condition is false. The user inputs a value less than, or equal to, 5. How many pets do you have? 2 So you have 2 pets The above example doesn t use the else clause. A statement and not a statement block is used in the above example. The if statement, when evaluated to true, is only branching to the statement immediately following the condition. Note the indentation following the if (condition). The statement System.out.println( "Oh that s a lot" ); is typed 2 to 4 spaces inside of the if statement. This indentation is the same as have been seen following the class declaration as well as the main method declaration. The purpose of the indentation is to make the program code easy to follow. The structure of a program becomes apparent when the indentation is consistent. The indentation should be 2 to 4 spaces. Be consistent! Select a number of spaces you are comfortable with and stick with it. A program quickly becomes very hard to read if the indentation changes. See Statement blocks To be able to execute an entire branch of statements within one condition a statement block is used. A statement block is a sequence enclosed by { and bracket. The main method also describes a sequence beginning with { and ending with. This marks the beginning and the end of the application. A similar example int numberofpets; System.out.println("How many pets do you have?"); numberofpets = keyb.nextint(); if ( numberofpets > 5 ){ System.out.println("Oh really " + numberofpets + " pets"); System.out.println("that s a lot!"); else{ System.out.println("Ok, you have " + numberofpets + " pets"); System.out.println("that s not too many"); Note the indentation across the entire statement block. The program will after evaluating the condition select one of the two blocks Exercise: Create a complete java program containing the statements above. Is the result what you expected? Check the indentation of your source code. Note: the NetBeans environment has a feature "Reformat Code" accessible from right clicking the editor screen. If you have indented your code correctly, the command will not make any changes.

3 1.1.3 Common errors A common error is to include a single statement between the condition and the statement block, the compiler cannot catch this error. Another common error is to start the statement block before the if statement, or before the else statement, the compiler will catch these errors. {if ( numberofpets > 5 ) // ERROR, block starting before if statement System.out.println("Oh really " + numberofpets + " pets"); System.out.println("that s a lot!"); else System.out.println("Ok, you have " + numberofpets + " pets"); { // ERROR else statement only contains a single statement. System.out.println("that s not too many"); An if statement with a semicolon immediately after the if condition counts as an empty statement, and does not produce an error. if ( length < 30.7 ); // ERROR! System.out.println("a short length"); The consequence of this construct is that the if statement will not affect the execution at all. Another mistake is to enclose the else statement within the controlled statement block. This is invalid syntax and will produce a compiler error. boolean turnedon = false; if ( turnedon ){ System.out.println("On!"); else System.out.println("Off!"); The correct syntax being boolean turnedon = false; if ( turnedon ){ System.out.println("On!"); else{ System.out.println("Off!"); 1.2 Conditions The if statement contains a new kind of expression, the Boolean expression. A Boolean expression is evaluated according to the same rules as other expressions but, always has a Boolean return type; hence Boolean expression.

4 Boolean binary operators == Equals!= Not equals < Less than <= Less or equal > Greater than >= Greater or equal && Boolean AND Boolean OR You can, as with other value types, use Boolean declared variables to store Boolean values, as in the previous if example. This is however not very common. The common context for Boolean operators and Boolean expressions is within a control statements logical expression. An unnecessarily verbose Boolean expression. boolean manypets; manypets = numberofpets > 5; if ( manypets ) // statement block as before with the same meaning as before. The variable manypets can be assigned the values true or false. Most logic expression behaves according to the common algebraic rules. Follwing are some samples: Assume variables are declare with a sensible simple type. ( weight < 60.0 ) ( numberofdogs== 2 ) ( numberofwhales!= 2 ) ( count*2 >= 52 ) // the arithmetic * is evaluated before >= The && ( AND ) and (OR) are used to combine Boolean expressions ( boolean ) && ( boolean ): boolean ( boolean ) ( boolean ): boolean Both the left hand side and the right hand side are required to be of Boolean type. No promotion is inferred. ( count == 2 && weight < 20.0 )// both conditions needs to be // true for the entire expression to be true ( antal == 2 vikt < 20.0 ) // if any of the conditions are true // the entire expression is true You can also negate your logic expression by applying the unary! operator. (!( 3 > 4 ) ) // is true (!( 7 == 12 ) ) // is true (!(4==4) ) // is false Warning: when checking for equality between float and double values, caution is advised. The values compare needs to be equal to a very large precision. I.e. a value displayed as 1.0 doesn t have to be equal to another value displayed as 1.0. Complex expressions can be tricky to make sense of

5 int howold; System.out.println("What is your age?"); howold = keyb.nextint(); // check if teen if ( 12 < howold < 20 ) // ERROR! { System.out.println("You are a teen"); If the expression is expanded, one notices that the second expression will be evaluated as (boolean)<(int). This is not a valid expression. The correct way to do it is // check if teen if ( 12 < howold && howold< 20 ) // CORRECT! { System.out.println("You re a teen");