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1 SELECTION using IF and IF-ELSE Chapter 4

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Page 1: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

1

SELECTION using IF and IF-ELSE

Chapter 4

Page 2: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

2

Agenda

Background One Way selection (if)

Two Way selection (if-else)

Compound Statements

Nested if-else

Logical operators

Common pitfalls

Page 3: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

3

Review:Standard Program Structure

To solve most problems, your main() program will generally look like this (conceptually)

1. Declare variables for input, result and intermediate data

2. Ask for data (cout)3. Input data (cin)4. Calculate result5. Output result (cout)

Page 4: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

4

Simple Flow of ControlFlow of control

The order in which statements are executed

SequentialThe normal default flow…one line after the other

Conditional (or Branch, or Selection)Lets program choose between one or more

alternatives

Loop (or Iteration)Lets program repeat a block of statements many times

Page 5: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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Which way to go?

Conditional statements allow execution of certain statements only if a particular condition is satisfied

Consider these statements as some kind of a gate keeper that allows entry only if a condition is satisfied

Page 6: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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Who are these gatekeepers ?

There are two types of conditional

statements :

The if Statement

The if…else Statement

Page 7: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

7

Agenda

Background

One Way selection (if) Two Way selection (if-else)

Compound Statements

Nested if-else

Logical operators

Common pitfalls

Page 8: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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The plain old if…The if statement allows conditional execution What does this mean ?

Consider this ….You get an award only if you get a perfect score on a test

if (score == 100)cout<<“Congratulations!”;

Page 9: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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if statement syntaxif (condition) statement ;

Condition – is a logical expression likescore == 100 x+y>10 ans!=‘y’

Statement – is any executable statement like

cout<<“Congrats!”; OR x = x + 50;

Page 10: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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How it works The if statement first evaluates the condition… for

example…

if (score == 100)

cout<<“Congratulations!”;

if score is 100 then the condition evaluates to true

so it displays a Congratulations! message

if score is not 100, it skips to the next line…no output

Page 11: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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One-Way Selection Flowchart (if)

score==100

true

false CONGRATS!

Next statement

Page 12: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

12

Logical Expressions

Logical expressions are expressions that are either true or false(4<3)

(hours>40)

(Length>=Width)

relational operators such as ‘>’ (greater than) are used to compare variables and/or numbers

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Relational Operators

The Six Relational operators

(No spaces allowed between the symbols!)• < less than

• > greater than

• <= greater than or equal to

• >= less than or equal to

• == equal or equivalent (Only use on int or char)

• != is not equal to (Only use on int or char) (not suitable with floats

due to roundoff error)Common source of errors, don’t use one =!

Page 14: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

14

Q1) One-Way Selection What is output from following:

if (score > 65)cout<<“Pass! ”;

cout<<“Congrats!”;

Given a) score = 80 ____________b) score = 40 ____________ c) score = 65 ____________

Page 15: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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Agenda

Background

One Way selection (if)

Two Way selection (if-else)

Compound Statements

Nested if-else

Logical operators

Common pitfalls

Page 16: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

16

The if…else statementThe if…else statement selects between one of

two statements. Here is the syntax of if-else:

if (condition)

statement1;

else

statement2;

So how does this work ….. ?

Notice: no condition here

Page 17: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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How it worksAgain, condition is a logical expression like

score > 65

statement1 and statement2 are executable, like

cout<<“Congrats!”; OR x = x + 50;

If condition is true then statement1 will execute

Otherwise (i.e. condition is false) statement2 will execute

Page 18: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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if…else Example

To calculate hourly wages there are two choicesRegular time ( up to 40 hours)

• gross_pay = rate * hours;

Overtime ( over 40 hours) gets $50 bonus• gross_pay = rate * 40 + 50;

The program must choose which of these expressions to use

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19

Conditional Flowchart (if-else)

hours>40truefalse

calc paywith overtime

calc payregular

display payto screen

Page 20: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

20

Designing the Conditional

Determine if (hours >40) is trueIf it is true, then use gross_pay = rate * 40 + 50;

If it is not true, then use gross_pay = rate * hours;

Page 21: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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Implementing the Branch

The actual C++ to do this: if (hours > 40) gross_pay = rate * 40 + 50;

else

gross_pay = rate * hours;

Notice, one condition, for two statements

Notice: no condition here!“else” means “condition was false”

Page 22: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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Full Application: Calculating Pay// pay.cppint main(){ float hours, pay, rate;cout << "enter hours and rate: ";cin >> hours >> rate;

if (hours > 40) pay = rate * 40 + 50;

else pay = rate * hours;cout << "pay is " << pay;

}

This is Step4. Calc Result

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Q2) Two-Way SelectionWhat is output from the following:

if (score > 60)

cout<<“Pass--”;

else

cout<<“Fail--”;

cout<<“Have a nice day!”;

Given a) score = 80 ____________

b) score = 40 ____________

c) score = 60 ____________

Page 24: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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Q3) Two-Way SelectionWhat is output from the following:

if (score = 100)

cout<<“Congrats!”;

else

cout<<“Nice try”;

Given a) score = 100 ____________

b) score = 80 ____________

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Application of 2-Way Selection//cost.cpp see p67-68 Use for Problem 10 int number, cost; cout << "Number purchased: "; cin >> number; if (number < 5) cost = 12 * number; else cost = 10 * number; cout << number << " baseballs cost $" << cost;

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26

Agenda

Background

One Way selection (if)

Two Way selection (if-else)

Compound Statements Nested if-else

Logical operators

Common pitfalls

Page 27: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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Compound StatementsA compound statement is more than one statement enclosed in { } Branches of if or if-else statements often need to execute more that one statementExample: if (condition) { statements for true } else {

statements for false }

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Example with two if-else statements

if (hours > 40.0)pay = 40.0 * rate + 50;

elsepay = hours * rate;

cout << "pay is " << pay;

if (hours > 40.0)cout << " overtime worked";

elsecout << " no overtime worked";

Page 29: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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Above redone (simplified) with compound statements

if (hours > 40.0){

pay = 40.0 * rate + 50;cout << " pay is " << pay << " overtime

worked";}else{

pay = hours * rate;cout << " pay is " << pay << " no overtime";

}

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Q4) What’s wrong with this code?

if (hours > 40.0)

pay = 40.0 * rate + 50;

cout << " pay is " << pay << " overtime worked";

else

{

pay = hours * rate;

cout << " pay is " << pay << " no overtime";

}

Page 31: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

31

Agenda

Background

One Way selection (if)

Two Way selection (if-else)

Compound Statements

Nested if-else Logical operators

Common pitfalls

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32

Nesting and Multi-Way BranchesHow to select from more than two possibilities?How to check a variable is inside a range of values?Answer: put an if-else inside if or else block …nestingif (expr1) statement1; // do if expr1 is trueelse // do if expr1 is false

if (expr2) statement2; // expr1 false, expr2 true

elsestatement3; // both expr1 and expr2 false

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Nested if-else (multi-way branch)

if (expr1) { block1 }else if (expr2) { block2 } else if (expr3) {block 3} else {block4}

Syntax:

if( grade>=90) cout<<"Your GPA is A."<<endl;else if (grade>=80) cout<<"Your GPA is B."<<endl;else if (grade>=70) cout<<"Your GPA is C."<<endl;else if (grade>=60) cout<<"Your GPA is D."<<endl;else {cout<<"Your GPA is F."<<endl; cout<<"You will cry!"<<endl;}

Example:

Page 34: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

34

Q5) What does this code produce:if( grade>=90) cout<<"GPA is A."<<endl;else if (grade>=70) cout<<"GPA is C."<<endl;else if (grade>=80) cout<<"GPA is B."<<endl;

Given a) grade = 93 ____________

b) grade = 85 ____________

c) grade = 75 ____________

d) grade = 45 ____________

Page 35: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

35

Application – Multiway Branch // suit.cpp See p75 Use for Problem 11 char suit; // 'C', 'D', 'H', or 'S' cout << "First letter of suit (C,D,H, or S): "; cin >> suit; if (suit == 'C') cout << "clubs"; else if (suit == 'D') cout << "diamonds"; else if (suit == 'H') cout << "hearts"; else if (suit == 'S') cout << "spades"; else cout << "Invalid suit";

Page 36: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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Time for Break!

Download Lab3If.cpp

Work on 1-6, 11

You need to do 1 thru 10 for 10 points

Call me over if you need help

Page 37: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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Application: How to tell if n is in the correct range

cout<<“Enter a number between 1 and 10”;cin>>n;if (n>=1)

if (n<=10) cout<<“OK, n is between 1 and 10!”;

elsecout<<“n is too big”;

elsecout<<“n is too small”;

Page 38: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

38

A better way using &&

cout<<“Enter a number between 1 and 10”;

cin>>n;

if (n>=1 && n <=10)

cout<<“OK, n is between 1 and 10!”;

else

cout<<“illegal value of n”;

Page 39: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

39

Agenda

Background

One Way selection (if)

Two Way selection (if-else)

Compound Statements

Nested if-else

Logical operators Common pitfalls

Page 40: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

40

Logical Operators

AND operator: p && q

OR operator:p || q

NOT operator:!p

Both p and q are relational expressions (like a<5) or boolean (true/false) expressions

Page 41: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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Compound Conditions

Logical AND means true only if both true

AND operator: && p && q

p q p && q

T T T

T F F

F T F

F F F

Page 42: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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Compound Conditions

Logical OR means true if at least 1 is true

OR operator: ||p || q

p q p || q

T T T

T F T

F T T

F F F

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Compound Conditions

NOT operator: ! !p

p !p

T F

F T

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44

Compound Condition Exampleint main()

{

int n1, n2, n3;

cout << “Enter three integers: “;

cin >> n1 >> n2 >> n3;

if (n1 <= n2 && n1 <= n3) cout << “Minimum is “ << n1 << endl;

if (n2 <= n1 && n2 <= n3) cout << “Minimum is “ << n2 << endl;

if (n3 <= n1 && n3 <= n2) cout << “Minimum is “ << n3 << endl;

}

Page 45: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

45

OR Exampleint main()

{

char ans;

cout << “Are you enrolled (y/n): ”;

cin >> ans;

if ( ans == 'Y' || ans == 'y')

cout <<“You are enrolled.\n”;

else cout << “You are not enrolled.\n”;

}

This will accept both ‘Y’ and

‘y’ as an answer

Page 46: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

46

YOUR TURN6. T/F ( 3 < 2 ) && (5 > 7)

7. T/F ! ( 2 > 3 )

8. T/F (25 = = 25 ) || ( 2 > 3 )

 

9. the expression: ( number > 10 && number < 40 )

 is TRUE, when

a)      number is larger than 40

b)      number is smaller than 10

c)      number is between 10 and 40

d) Never

Page 47: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

47

Agenda

Background

One Way selection (if)

Two Way selection (if-else)

Compound Statements

Nested if-else

Logical operators

Common pitfalls

Page 48: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

48

Common Pitfalls in Selection/Logic

Dangling else

Using = instead of ==

Forgetting { } around multiple statement blocks

Not testing your proposed solution using simple test data values

Trying to solve all at once…a mess!Better: build up solution step by step: see p79-82

Read/steal code examples from text noted on handout

If stuck, start small, do drill exercises, then tackle programs

Page 49: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

49

Dangling Else

A subtle bug possibility

When you nest a single-choice if in a 2-choice if:

if (cond1)

if (cond2)

statement1;

else

statement2;

Q: Which if does the else belong to?

A: the closest one above it

Page 50: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

50

Dangling else…continued

Compiler really interprets above like this:

if (cond1)

if (cond2)

statement1;

else

statement2;

Single choice if

Two choice if

No matter how you type it!

Page 51: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

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Dangling else…endA good idea to avoid confusion, put { } around both choices of two-choice if

if (cond1){

if (cond2)statement1;

}else{ statement2;}

Now it’s clear how to interpret it!

Page 52: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

52

Go back home proud ! You’re a C++ programmer !

Page 53: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

53

A1) One-Way Selection What is output from following:

if (score > 65)cout<<“Pass! ”;

cout<<“Congrats!”;

Given a) score = 80 Pass! Congrats! b) score = 40 Congrats!c) score = 65 Congrats!

Page 54: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

54

A2) Two-Way SelectionWhat is output from the following:

if (score > 60)

cout<<“Pass--”;

else

cout<<“Fail--”;

cout<<“Have a nice day!”;

Given a) score = 80 Pass--Have a nice day!

b) score = 40 Fail--Have a nice day!

c) score = 60 Fail--Have a nice day!

Page 55: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

55

A3) Two-Way SelectionWhat is output from the following:

if (score = 100)

cout<<“Congrats!”;

else

cout<<“Nice try”;

Given a) score = 100 Congrats!

b) score = 80 Congrats! (due to bug)

Bug! Use ==

Page 56: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

56

A4) What’s wrong with this code?

if (hours > 40.0)

pay = 40.0 * rate + 50;

cout << " pay is " << pay << " overtime worked";

else

{

pay = hours * rate;

cout << " pay is " << pay << " no overtime";

}

Need { }

Page 57: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

57

A5) What does this code produce:if( grade>=90) cout<<"GPA is A."<<endl;else if (grade>=70) cout<<"GPA is C."<<endl;else if (grade>=80) cout<<"GPA is B."<<endl;

Given a) grade = 93 A

b) grade = 85 C

c) grade = 75 C

d) grade = 45 nothing

Page 58: 1 SELECTION using IF and IF-ELSE Chapter 4. 2 Agenda Background One Way selection (if) Two Way selection (if-else) Compound Statements Nested if-else

58

YOUR TURN6. F ( 3 < 2 ) && (5 > 7)

7. T ! ( 2 > 3 )

8. T (25 = = 25 ) || ( 2 > 3 )

 

9. the expression: ( number > 10 && number < 40 )

 is TRUE, when

a)      number is larger than 40

b)      number is smaller than 10

c)      number is between 10 and 40 d) Never


SAP ABAP A short introduction. SAP ABAP • Selectie: – IF …… ENDIF. – IF …. ELSE … ENDIF. – IF …. ELSEIF …. ELSEIF …. ELSE …. ENDIF. – IF condition_1

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