Decision Statements

Decision Structures: The If Statement,
Else-If Statement, and Relational
CS0007: Introduction to Computer Programming
 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
 Programming Style refers to…
 the way a programmer uses spaces, indentation, blank lines, and
punctuation characters.
 The standard input device is normally the…
 keyboard
 The Java object that refers to the standard input device is…
 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
 We will go over many decision structures, some of which are more complex
than this.
Is it cold
Wear a coat.
Statement 1
Statement 1
Wear a coat.
Statement 2
Statement 2
Wear a coat.
Statement 3
Statement 3
The if Statement
 The most basic decision structure in Java is the if statement.
if(BooleanExpression) {
 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 case if the boolean expression is true, all the statements in the block will be
 A block is a collection of statements that are organized to be together physically.
o 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
 All relational operators resolve to either true or false.
Relational Operators (in
Order of Precedence)
Greater than
Less than
Greater than or equal to
Less than or equal to
Equal to
Not equal to
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
 Even if an if there is only one conditionally executed
statement, it is still acceptable to put brackets around it.
 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.
 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
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 or block 1
statement or block 2
 Just like the if statement, BooleanExpression is evaluated.
 If it resolves to true, then statement or block 1 is
 If it resolves to false, then statement or block 2 is
if-else Flowchart
Statement 4
Statement 1
Statement 5
Statement 2
Statement 6
Statement 3
else-if Example
 New Topic:
 else-if statement
Group Programming: Fraction to
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