Alice Control Statements: Introduction

• Control structures/statements allow you to change the order in which state-ments are executed in a program

• The order is sometimes referred to as flow of control

• We’ve already seen two such structures:

1. do in order

2. do together

• do in order represents sequential control or processing

– Statements are executed one after the other, in the order they are listed

• do together represents parallel control or processing

– Statements are executed at the same time, in tandem

• This chapter introduces two types of control statements:

1. Selection

2. Iteration (repetition, looping)

• To help visualize flow of control, flow charts are used

– A flow chart is made of symbols, each with a special meaning

– By following the arrows through the chart, you see the order in whichstatements are executed

– A few flow chart symbols:

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Alice Control Statements: Introduction (2)

– The flow chart for sequential control is

– The flow chart for parallel control is

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Alice Control Statements: Boolean Expressions

• The Boolean data type has two values:

1. true

2. false

• Alice variables can be of type Boolean, and they are assigned Boolean expres-sions

– A Boolean expression is one that evaluates to true/false

• Types of Boolean expressions:

1. Boolean literal (true/false)

2. Boolean variable

3. Relational expression

– constructed from relational operators:

==, !=, <, >, >=, <=

4. Logical expression

– Logical expressions are constructed from three logical operators:

(a) && (and)

(b) || (or)

(c) ! (not)

– Arguments must be Boolean

– Semantics are represented by truth tables:Graphical way of expressing value of boolean expression

A B A && B A || B !A

true true true true false

true false false true false

false true false true true

false false false false true

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Alice Control Statements: Boolean Expressions (2)

• Boolean variables

– Often have names like isDone, isZero, isValid

– This also holds for Boolean functions (those returning boolean values)

• Cautions

1. English does not translate directly to Boolean expressions

– ”If the color is red or blue, ....”

– Wrong: color == RED OR BLUE ...

– Right: (color == RED) OR (color == BLUE)

2. Strings cannot be compared with the relational operators; they use specialfunctions (p 143)

– str1.contentEquals(str2)

– str1.equalsIgnoreCase(str2)

– str1.startsWith(str2)

– str1.endsWith(str2)

– str1.contains(str2)

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Alice Control Statements: Selection

• Selection deals with making a choice

• If/Else is the basic selection statement

• Syntax:

if <boolean_expression>

<if_statements>

else

<else_statements>

• Semantics:

– if Boolean expression is true, execute if statements after if

– if Boolean expression is false, execute else statements following else (if theelse is included)

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Alice Control Statements: Selection (2)

• To add to program:

1. Drag from control statement list in editor window

2. Select a condition (Boolean expression)

– This value can be replaced by a Boolean expression of your choice later

3. If there is nothing to do when the condition is false, simply add nothing tothe else part

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Alice Control Statements: Selection - Nested If/Else Statements

• If/Else statements can be nested

• This is done by just dropping if statement in the drop statement here locationof another if

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Alice Control Statements: Selection - To Nest or Not to Nest If/Else Statements?

• Example:

• Proper use of nested if-else:

if (avg >= 90)

grade = "A"

else

if (avg >= 80)

grade = "B"

else

if (avg >= 70)

grade = "C"

else

if (avg >= 60)

grade = "D"

else

grade = "F"

– Improper use of nested if-else:

if (age >= 65)

exemptions = exemptions + 1;

else

if (spouseAge >= 65)

exemptions = exemptions + 1;

else

if (dependents > 0)

exemptions = exemptions + dependents;

else

if (married AND (filingStatus == 2))

exemptions = exemptions + 1;

else

NULL

NOTE: NULL means no statement; it is NOT an Alice3 statement

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Alice Control Statements: Selection - Issues When Writing Nested If/ElseStatements (2)

– Proper version of above - do not nest:

...

if (age >= 65)

exemptions = exemptions + 1;

else

NULL

if (spouseAge >= 65)

exemptions = exemptions + 1;

else

NULL

if (dependents > 0)

exemptions = exemptions + dependents;

else

NULL

if (married AND (filingStatus == 2))

exemptions = exemptions + 1;

else

NULL

This is an example of sequential if/else statements

• Usage rules of thumb:

– Use sequences of ifs for independent situations

– Use nested ifs for dependent, mutually-exclusive situations

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Alice Control Statements: Iteration - Introduction

• Iteration/repetition/looping: Executing a sequence of instructions multi-ple times

• Loop components:

– Condition: determines whether loop should continue

– Body (process): instructions being repeated

– Loop control variable (LCV): Variable whose value determines when loopstops

• Basic steps of loop control:

Initialize Process

Initialize LCV

BEGIN LOOP

Update Process

Update LCV

END LOOP

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Alice Control Statements: Iteration - Loop Control

• Main issue with loops is loop control

– Determining

1. How many times the loop body should execute, and

2. The methods for controlling this

• 2 general types of loop control:

1. Definite iteration:

– Loop executes a fixed number of times

2. Indefinite iteration:

– Loop executes an indeterminate number of times

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Alice Control Statements: Iteration - Definite Iteration

• Sometimes called count-controlled iteration

• Body will execute a fixed number of times

• LCV is an example of a variable called a counter

– It counts how many times something occurs

• Example:

– count is the LCV

– count is a counter variable

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Alice Control Statements: Iteration - Definite Iteration (2)

• Alice 3 uses a count control structure for implementing definite iteration

– It appears as a for loop in the code when the programming language pref-erence is set to Java

– Dragging count from the control structure list will create a for loop with asystem-generated LCV (whose name can be changed with a right-click)

– You will be asked what value to count to

– Note that it counts from zero

– If you have added variables to your code (or parameters), you have theoption of using one of them as the value to count to

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Alice Control Statements: Iteration - Definite Iteration (3)

– What the count loop can’t do:

∗ Start at a value other than zero

∗ Count by increments other than one

∗ Count backwards

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Alice Control Statements: Iteration - Indefinite Iteration

• Used when don’t know ahead of time how many times loop should execute

• Types of control:

1. Count controlled: Stop when an event has occurred a predefined numberof times

2. Sentinel: Stop when a particular value (or value from a set of values) isencountered

• Count controlled

– Iterations determined by occurrence of some event a specified number oftimes

– Loop may need to iterate more than this number

– Example:

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Alice Control Statements: Iteration - Indefinite Iteration (2)

• Sentinel control

– Sentinel is one or more values that signal the end of iteration

– They are frequently values that are outside of range of acceptable valuesused for processing

∗ For example:

1. Temperatures below absolute 0

2. Ages below 0

3. Numeric grades below 0

4. Letter grades other than A, B, C, D, F

– Example:

– Requires initialization of the LCV prior to entering loop

– A Boolean variable that is used for sentinel control is sometimes called aflag

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Alice Control Statements: Iteration - While Loops

• Pretest loop

• Most general loop construct

– All other loop types can be written as while loops

• General structure:

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Alice Control Statements: Iteration - While Loops in Alice

• These are created like any other control structure

• The while loop in Alice works exactly as described above

• Example:

• When initially added to a program, you select an initial condition

• Afterwards, you can construct as complicated Boolean expression as you likeby dragging and dropping

– The expressions are constructed in the same way as they are for if/elsestatements

• The following for and while are equivalent

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Alice Control Statements: Iteration - Potential Problems

1. Infinite loop

• Loop that never ends

• Can result due to many reasons:

– Failure to initialize LCV

– Failure to update LCV properly

– Poor choice of termination condition

– Failure to update process

2. Off-by-one error

• Count-controlled loop that iterates one too many or one too few times

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Alice Control Statements: Iteration - Nested Loops

• Can place one loop inside of another

• Must make sure that initializations and updates occur for each loop and in theappropriate places

• Example:

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