coit29222 structured programming slide 1 coit29222-structured programming lecture week 06 reading:...

COIT29222 Structured Programming Slide 1

COIT29222-Structured Programming Lecture Week 06

Reading: Study Guide Book 2, Modules 9 & 10 Textbook (4th Ed.), Chapter 2 Textbook (6th Ed.), Chapters 4 & 5

This week, we will cover the following topics:More on Selection

-switch statement -selection (ternary) operators

More on Loops-while vs. do-while-for vs. while-nested loops


Selection statementsWe have been using one C++ selection

statement – if-else. In this class we explore other selection

statements – statements that allow us to perform different tasks, depending on the input data.

switch statementternary operators


if-else – a reviewLet’s start out by reviewing what we know about the if-else

statement.

if statementif (Age < 18){

cout << “A child!“ << endl;}

if-else statementif (Age < 18){

cout << “A child!“ << endl;}else {

cout << “An adult!“ << endl;}


if-else-if statementif (Age < 13){

cout << “A child!“ << endl;}else if ((Age >= 13 ) && (Age <= 17)){

cout << “A teenager!“ << endl;}else{

cout << “An adult!“ << endl;}


Bracesif (Age < 13)

cout << “A child!“ << endl;else if ((Age >= 13 ) && (Age <= 17))

cout << “A teenager!“ << endl;else

cout << “An adult!“ << endl;

Braces are not required if branch has only one statement.We recommend you always use braces in this course. Sometimes we omit them to fit our examples on a slide.


Nested if/elseif (Gender == ‘M’) if (Age < 18) cout << “A male child!“ << endl; else cout << “A man!“ << endl;else if (Age < 18) cout << “A female child!“ << endl; else cout << “A woman!“ << endl;


Nested if/elseif (Gender == ‘M’) if (Age < 18) cout << “A male child!“ << endl; else cout << “A man!“ << endl;else if (Gender == ‘F’) if (Age < 18) cout << “A female child!“ << endl; else cout << “A woman!“ << endl;else cout << “Unknown gender!“ << endl;


Other selection statementsSelection statements allow us to perform different tasks in our programs, depending on the input data.

The if-else statement is the only selection statement we need. However, most programming languages provide other selection statement for convenience.

In C++, the switch statement is an alternative to the if-else-if statement

For example, a menu-driven program might start like this...


Other selection statementsData Processing Application===========================Select from the menu below: 1 – Load input data from disk 2 – Enter input data 3 – Save input data to disk 4 – Process input data 5 – Display output data 6 – Clear input data 0 – Exit Enter your selection ==> 2:

The main function of this program may include the following if-else-if statement...


int MenuSelection = 1;while (MenuSelection != 0){ DisplayMenuAndObtainSelection( MenuSelection ); if ( MenuSelection == 1 ) LoadInputDataFromDisk( InputData ); else if ( MenuSelection == 1 )

EnterInputData( InputData ); else if ( MenuSelection == 3 )

SaveInputDataToDisk( InputData ); else if ( MenuSelection == 4 )

ProcessInputData( InputData, OutputData ); else if ( MenuSelection == 5 )

DisplayOutputData( OutputData ); else if ( MenuSelection == 6 )

ClearInputData( InputData ); else if ( MenuSelection != 0 )

cout << ”Invalid menu selection!";}

Or, we can use a switch statement...

functions


int MenuSelection = 1;while (MenuSelection != 0){ DisplayMenuAndObtainSelection( MenuSelection ); switch ( MenuSelection ) { case 1: LoadInputDataFromDisk( InputData ); break;

case 2: EnterInputData( InputData ); break;

case 3: SaveInputDataToDisk( InputData ); break;

case 4: ProcessInputData( InputData, OutputData ); break;

case 5: DisplayOutputData( OutputData ); break;

case 6: ClearInputData( InputData ); break;

case 0: break; default: cout << ”Unknown menu selection!"; }}

notes: a little easier to read than if-else-if









notes: on break jump to end of switch statement









notes: default handles cases not handled above


The switch statementswitch is a convenient replacement for simple if-else-if statements

However, switch can only be used when the selection depends on the value of a variable of type integer or char (characters are stored as an integer, using ASCII coding system)

switch ( <integer or char variable> ) {...}


The switch statementor, more fully…

switch ( <integer or char variable> ) { case < constant or literal integer or char >: < statements to handle this case> break; : < other cases > : default: < statements to handle cases not handled above> }

notes: must be a variable of type integer or char




notes: integer or char literal or constant – eg: 1, 'A', EXIT




notes: don’t forget the colon




notes: break; is optional


Optional break;?switch ( CharMenuSelection ){ case 'a': case 'A': ProcessSelectionA; break; case 'b': case 'B': ProcessSelectionB; break; :}


Ternary operatorC++ also has a selection operator – an operator that selects between one of two expression, depending on the input data operators are applied to expressions to produce values of interest:

(FahrenheitTemp - 32) / 1.8

Like switch, the ternary operator is simply a convenience – the role it plays can be performed by if-else...


Ternary operator

The following example outputs “Pass” or “Fail”, depending on value of Mark : cout << (( Mark >= 50 ) ? "Pass" : "Fail ");

syntax: (( <condition> ) ? <expression 1> : <expression 2>)

same as: if ( Mark >= 50 ) cout << "Pass"; else cout << "Fail";

if condition is True, expression 1 is evaluated; otherwise, expression 2 is evaluated


More on LoopsWe have been using one C++ repetition (loop)

statement – while while ( <condition> )

{ < while statements >

}

NbrTimesTold = 0;while (NbrTimesTold < NbrTimesToTell){

cout << “ No new taxes!“ << endl;NbrTimesTold = NbrTimesTold + 1;

}


while – a Review

Common looping errors are: loops that fail to stop (continue forever) loops that stop one repetition too early loops that perform one repetition too

many loops that fail to start

Normal while-loop structure is…


while – a Review

< initialise variables in while-condition > while ( <condition> ){ < while statements > < update variables in while-condition > }

NbrTimesTold = 0;while (NbrTimesTold < NbrTimesToTell){

cout << “ No new taxes!“ << endl;NbrTimesTold = NbrTimesTold + 1;

}


Activity

Number = 5;while (Number > 0){ Sum = Sum + Number ; Number = Number - 1 ;} cout << “The sum is “ << Sum << endl;

What will the following code output?


Activity Feedback

Number = 5;while (Number > 0){ Sum = Sum + Number ; Number = Number - 1 ;} cout << “The sum is “ << Sum << endl;

Output depends on initial value of Sumif Sum is zero at start: “The sum is 15”

must always initialise a sum to zero!


Activity

Sum = 0 ;while (Number > 0){ Sum = Sum + Number ; Number = Number - 1;}cout << “The sum is “ << Sum << endl;



Activity Feedback

Output depends on initial value of Number

must initialise variables in while condition!

Sum = 0 ;while (Number > 0){ Sum = Sum + Number ; Number = Number - 1;}cout << “The sum is “ << Sum << endl;


Activity

Sum = 0 ;Number = 5;while (Number > 0){ Sum = Sum + Number ; Number++;}cout << “The sum is “ << Sum << endl;



Activity Feedback

Loop will never end – an infinite loop

This is a logic error! Always check loop conditions carefully.

Sum = 0 ;Number = 5;while (Number > 0){ Sum = Sum + Number ; Number++;}cout << “The sum is “ << Sum << endl;


while vs do-while

The while statement tests a condition at the start of the loop

The do-while statement tests a condition at the end of the loop


while vs do-while

The while statement tests a condition at the start of the loop

cout << “Select a direction – N,S,E or W ==> “;cin >> Char;while ((Char != ‘N’) && (Char != ‘S’) && (Char != ‘E’) && (Char != ‘W’)){ cout << “Invalid direction!” << endl; cout << “Select a direction – N,S,E or W ==> “; cin >> Char;}


do-while loops

If a task must be performed at least once, we can perform the test at the end of the loop using do-whiledo { cout << “Select a direction – N,S,E or W ==> “; cin >> Char; if ((Char != ‘N’) && (Char != ‘S’) && (Char != ‘E’) && (Char != ‘W’)) cout << “Invalid direction!” << endl;} while ((Char != ‘N’) && (Char != ‘S’) && (Char != ‘E’) && (Char != ‘W’));


Activity

What are advantages and disadvantages of this design (compared to using while)? do { cout << “Select a direction – N,S,E or W ==> “; cin >> Char; if ((Char != ‘N’) && (Char != ‘S’) && (Char != ‘E’) && (Char != ‘W’)) cout << “Invalid direction!” << endl;} while ((Char != ‘N’) && (Char != ‘S’) && (Char != ‘E’) && (Char != ‘W’));

Can you suggest an improvement?


Activity Feedback

One advantage of do-while is that there is only one copy of prompt and input lines



Activity Feedback

One advantage of do-while is that there is only one copy of prompt and input lines

do { cout << “Select a direction – N,S,E or W ==> “; cin >> Char; if ((Char != ‘N’) && (Char != ‘S’) && (Char != ‘E’) && (Char != ‘W’)) cout << “Invalid direction!” << endl;} while ((Char != ‘N’) && (Char != ‘S’) && (Char != ‘E’) && (Char != ‘W’));


Activity Feedback

One disadvantage of do-while is that the loop condition appears twice

do { cout << “Select a direction – N,S,E or W ==> “; cin >> Char; if ((Char != ‘N’) && (Char != ‘S’) && (Char != ‘E’) && (Char != ‘W’)) cout << “Invalid direction!” << endl;} while ((Char != ‘N’) && (Char != ‘S’) && (Char != ‘E’) && (Char != ‘W’));


Activity Feedback

One disadvantage of do-while is that the loop condition appears twice



Activity Feedback

Repetition of complex loop conditions can be avoided using a Boolean variable… WaitingForDirection = true;do { cout << “Select a direction – N,S,E or W ==> “; cin >> Char; if ((Char != ‘N’) && (Char != ‘S’) && (Char != ‘E’) && (Char != ‘W’)) cout << “Invalid direction!” << endl; else WaitingForDirection = false;} while ( WaitingForDirection );


Activity

Is the following logic OK? Sum = 0 ;do{ cout << “Enter a number (-9 to quit) ==> "; cin >> Number; Sum = Sum + Number;} while (Number != -9);cout << “Sum = “ << Sum;

if not, fix it.


Activity Feedback

The problem with the logic is that it will include –9 in the sum – it should be:

Sum = 0 ;do{ cout << “Enter a number (-9 to quit) ==> "; cin >> Number; if (Number != -9) Sum = Sum + Number;} while (Number != -9);cout << “Sum = “ << Sum; note: you will often see

loop conditions repeated in do-while statements


for vs while cout << “Number of marks in exam ==> “;

cin >> NbrMarks; cout >> “Number of students ==> “ cin >> NbrStudents; NbrLoops = 0;while (NbrLoops < NbrStudents){

cout << “Student’s mark ==> “;cin >> StudentMark;Percentage = 100 * StudentMark / NbrMarks;cout << “ Student’s percentage: “;cout << Percentage;NbrLoops = NbrLoops +1;

}

notes: initialise loop control variable




cout << “Student’s mark ==> “;cin >> StudentMark;Percentage = 100 * StudentMark / NbrMarks;cout << “ Student’s percentage: “;cout << Percentage;NbrLoops = NbrLoops +1;

}

notes: loop condition




cout << “Student’s mark ==> “;cin >> StudentMark;Percentage = 100 * StudentMark / NbrMarks;cout << “ Student’s percentage: “;cout << Percentage;NbrLoops++;

}

notes: modify loop control variable (to avoid looping forever)


cout << “Number of marks in exam ==> “; cin >> NbrMarks; cout >> “Number of students ==> “ cin >> NbrStudents; for (NbrLoops = 0; NbrLoops < NbrStudents; NbrLoops++){

cout << “Student’s mark ==> “;cin >> StudentMark;Percentage = 100 * StudentMark / NbrMarks;cout << “ Student’s percentage: “;cout << Percentage;

}

for vs while

notes: initialise loop control variable




}

for vs while

notes: loop condition




}

for vs while

notes: modify loop control variable (to avoid looping forever)


for ( < loop initialisation statement > ; < loop condition > ; < loop completion statement > ){

< for statements >}

for Loop Syntax

notes: parentheses around for clause



< for statements >}

for Loop Syntax

notes: statement performed once before entering loop for first time



< for statements >}

for Loop Syntax

notes: semi-colons after loop initialisation and loop condition



< for statements >}

for Loop Syntax

notes: condition tested at the start of each loop – including the very first loop



< for statements >}

for Loop Syntax

notes: statement performed at the end of each loop


for Loop Operation

for

loopstatements

loopconditio

n

false

true

loopinitialisestateme

nt

loopcompletio

nstatemen

t


Activity

for ( int Counter = 0; Counter < 5; Counter++ ) {

cout << “Counter = “ << Counter << endl;}

What output is produced by the following code:


Activity Feedback

Counter = 0Counter = 1Counter = 2Counter = 3Counter = 4

The output produced by this code is:


Loop Control VariablesUsually an integer, but can be a character can be

declared within the for clause – instead of...int NbrLoops;cout << “Number of marks in exam ==> “; cin >> NbrMarks; cout >> “Number of students ==> “ cin >> NbrStudents; for (NbrLoops = 0; NbrLoops < NbrStudents; NbrLoops++){cout << “Student’s mark ==> “;cin >> StudentMark;Percentage = 100 * StudentMark / NbrMarks;cout << “ Student’s percentage: “;cout << Percentage;

}


cout << “Number of marks in exam ==> “; cin >> NbrMarks; cout >> “Number of students ==> “ cin >> NbrStudents; for (int NbrLoops = 0; NbrLoops < NbrStudents; NbrLoops++){


}

Loop Control Variables


int NbrLoops;cout << “Number of marks in exam ==> “; cin >> NbrMarks; cout >> “Number of students ==> “ cin >> NbrStudents; for (int NbrLoops = 0; NbrLoops < NbrStudents; NbrLoops++){


}

Loop Control Variables

notes: if you try to declare the same variable twice, you get a compilation error


Activity

for ( int Counter = 0; Counter != 5; Counter++ ) {

cout << “Counter = “ << Counter << endl; Counter = Counter + 1;}

What output is produced by the following code?


Activity Feedback

Counter = 0Counter = 2Counter = 4Counter = 6Counter = 8:( until you terminate the program! )

The output produced by this code is:


Activity

Write the for clause of a for loop to produce this output:

Counter = 15Counter = 14Counter = 13Counter = 12Counter = 11Counter = 10


Activity Feedback

The for clause of a for loop to produce this output is:

for ( int Counter = 15; Counter >= 10; Counter-- )


Nested LoopsWe saw that an if-else statement can be embedded

inside another if-else statement

Likewise, a for statement can be nested inside another for statement

In fact, any selection statement (if-else, switch) or repetition statement (while, do-while, for) can be embedded inside another selection or repetition statement

With a nested loop, one performs one or more trips around the inner loop for each trip around the outer loop.Here’s an example...


Nested Loops

for ( int RowNbr = 1; RowNbr <= 5; RowNbr++ ){ cout << endl << “Row “ << RowNbr << “:“; for ( int ColNbr = 1; ColNbr <= 3; ColNbr++ ) { cout << " Col " << ColNbr ; }}

inner loop


Activity


activity: what does this code produce as output


Activity Feedback


feedback: this code produces the following output… Col 1 Col 2 Col3


Nested Loops


outer loop

inner loop


Nested Loops


note: here, three trips around the inner loop are performed for each trip around the outer loop


Activity


activity: what is the output produced by this code?


Activity Feedback


feedback: the output produced by this code isRow 1: Col 1 Col 2 Col 3 Row 2: Col 1 Col 2 Col 3 Row 3: Col 1 Col 2 Col 3Row 4: Col 1 Col 2 Col 3


SummaryThe switch statements provide a convenient

alternative to simple if-else-if statements

The ternary operator provides some useful additional flexibility to the power of expressions

The task in the “do-while” loop is performed at least once

The for loop is useful when the number of required trips around a loop is known before entering the loop

Consequently, the for loop is useful when using arrays – the topic of a future class

coit29222 structured programming slide 1 coit29222-structured programming lecture week 06 reading:...

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