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Decision Structures: The If Statement, Else-If Statement, and Relational Operators. CS0007: Introduction to Computer Programming. Review. Scope refers to… where in a program an entity can be accessed by name. Three kinds of comments in Java:
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Decision Structures: The If Statement, Else-If Statement, and Relational Operators CS0007: Introduction to Computer Programming
Review • Scope refers to… • where in a program an entity can be accessed by name. • Three kinds of comments in Java: • Single-Line – begin with // and the entire line is ignored • Multi-Line – begin with /* and end with */, and everything between is ignored • Documentation Comments – special comments that can be used to make attractively formatted HTML files that document the source code. • Programming Style refers to… • the way a programmer uses spaces, indentation, blank lines, and punctuation characters. • The standard input device is normally the… • keyboard
Review • The Java object that refers to the standard input device is… • System.in • The class that the Java API provides to allow us to take in input as primitive types or strings is named… • Scanner
Decision Structures • So far our programs have been very sequential: int a = 2, b = 3, c, d; c = b + a; d = b – a; • All of these statements will be executed, in order, from top to bottom • This is what is called a sequence structure, because the statements are executed in order, without branching. • However, it is often the case that we require a program to execute some statements only under certain circumstances. • We can do this with decision structures.
Decision Structures • In a decision structure’s simplest form certain statements are executed only when a specific condition exists. If the condition does not exist, the statements are not executed. • It is said that the statements inside of the decision structure are conditionally executed. • We will go over many decision structures, some of which are more complex than this. Is it cold outside? Boolean Expression Condition Yes True True Wear a coat. Statement 1 Statement 1 False False No Statement 2 Wear a coat. Statement 2 Statement 3 Wear a coat. Statement 3
The if Statement • The most basic decision structure in Java is the if statement. if(BooleanExpression) statement; if(BooleanExpression) { statement1; statement2; ... } • BooleanExpression– a boolean expression. • A boolean expression is one that is either true or false. • If the boolean expression is true, the statement that follows will be executed • In the multiple statement caseif the boolean expression is true, all the statements in the block will be executed. • A block is a collection of statements that are organized to be together physically. • In this case (and in most) the statements in the brackets are a block. • Otherwise the statement is skipped
If Statement Example 1 • New Topics: • If Statements • Dead Code
Relational Operators • Often we want to compare values in order to make a decision. • We can do this in Java with relational operators • Relational Operators determine whether a specific relationship exist between two values. • All relational operators resolve to either true or false.
Relational Operators • Let’s look at an example: length < width • If length is less than width the whole expression resolves to… • true • If length is greater than width the whole expression resolves to… • false • If length is equal to width the whole expression resolves to… • false • Another example: length >= width • If length is less than width the whole expression resolves to… • false • If length is greater than width the whole expression resolves to… • true • If length is equal to width the whole expression resolves to… • true
Relational Operators Example 1 • New Topics: • Relational Operators
Programming Style and the if Statement • Even if an if there is only one conditionally executed statement, it is still acceptable to put brackets around it. if(BooleanExpression){ statement; } • However, there are two rules you should always follow: • The first conditionally executed statement should be on the next line after the if statement. • The conditionally executed statement should be indented one level from the if statement. • Note: There is NO semicolon after an if statement.
Flags • A Flag is a boolean variable that signals when some condition exists in a program. • When a flag is set to true, it means some condition exists • When a flag is set to false, it means some condition does not exist. if(score > 95) highscore = true; • Here, highscore is a flag indicating that the score is above 95. • Right now, we don’t have any situations where these are terribly useful, but for now, just know we can and will use them.
Comparing Characters • You can also use relational operators on character data as well: if(ch == 'A') System.out.println("The character is A"); • Equal to and not equal to are the most natural for this data type, but you can use any relational operators. 'A' <'B' • Resolves to true 'a' < 'B' • Resolves to false • Why? • Remember, all characters are represented as Unicode numbers. • What Java does when comparing characters is compare the Unicode values
Character Comparison Example • New Topic: • Character Comparison
The if-else Statement • There is an expansion of the if statement called the if-else statement. if(BooleanExpression) statement or block 1 else statement orblock 2 • Just like the if statement, BooleanExpressionis evaluated. • If it resolves to true, then statement or block 1 is executed • If it resolves to false, then statement or block 2 is executed
if-else Flowchart Statement 4 Statement 5 Boolean Expression True False Statement 6 Statement 1 Statement 2 Statement 3
else-if Example • New Topic: • else-if statement
