Fundamental Programming 310201 Fundamental Programming More on Selection.

Fundamental Programming 310201

Fundamental Programming

More on Selection


Selection statements

We have been using one C++ selection statement – if-then-else. In this class we explore other selection statements – statements that allow us to perform different tasks, depending on the input data.

– switch – a specialised version of ‘if ‘– ternary operators (three parts )


If-then-else – a review

Let’s start out by reviewing what we know about the if-then-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;

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/else is different from if-else-if...


Nested if/else vs if-then-else-if

Gender = ‘M’

else

Age< 18

else elseAge< 18

malechild

man femalechild

woman

Age< 13 Age >= 13

&&Age <= 17

Age> 17

child

teenager

adult


ActivityCan our nested if/else example be re-code as an if-then-else-if?

If so, do so!

If so, can a nested if/else always be re-coded as an if-then-else-if?

If so, why not always use if-then-else-if instead of a nested if/else?


Activity Break


Activity Feedbackif ((Gender == ‘M’) && (Age < 18)) cout << “A male child!“ << endl;else if (Gender == ‘M’) cout << “A man!“ << endl;else if (Age < 18) cout << “A female child!“ << endl;else cout << “A woman!“ << endl;


Activity FeedbackNested if/else can always be re-coded as an if-then-else-if.

We do not always use if-then-else-if instead of a nested if/else because:

• conditions become more complicated

• we may wish to perform some common processing for all cases in a sub-branch


Activity Feedbackif (Gender == ‘M’) { NbrMales = NbrMales + 1; if (Age < 18) cout << “A male child!“ << endl; else cout << “A man!“ << endl; }else { : }


Handling other casesOne must always be aware of the need to handle other cases..

What are the cases that we are expected to be able to handle?

Do we need to consider the possibility of encountering other cases?

If so, how should we handle them?


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 statements

Selection 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.


Other selection statementsIn C++, the switch statement is an alternative to the if-then-else-if statement

if-then-else-if is more powerful than switch – there are tasks you can handle with if-then-else-if that you can’t handle with switch

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:

This main function of this program may include the following if-then-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-then-else-if









notes: on break jump to end of switch statement









notes: default handles cases not handled above


ActivityFor previous slide:

what is effect of removing the following line? case 0: break;

what is effect of removing second line below?

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


Activity Break


Activity Feedbackeffect of removing the following line is that the program will say “Unknown menu selection” before exiting case 0: break;

effect of removing second line below, is that the program will process data AND display output data when user selects 4

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


The switch statementswitch is a convenient replacement for simple if-then-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


Ternary operator

As you know, the power of expression comes from the ability to nest simple expressions inside more complex expressions

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


Summary

Nested if/else statements provide a convenient alternative to if-then-else-if statements

The switch statements provide a convenient alternative to simple if-then-else-if statements

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

Fundamental Programming 310201 Fundamental Programming More on Selection.

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