SELECTION
using IF and IF-ELSE
Chapter 4
1
Agenda
Background 
One Way selection (if)
Two Way selection (if-else)
Compound Statements
Nested if-else
Logical operators
Common pitfalls
2
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 result
5. Output result (cout)
3
Simple Flow of Control
Flow of control
The order in which statements are executed
Sequential
The 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
4
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
5
Who are these gatekeepers ?
There are two types of conditional
statements :
The if Statement
The if…else Statement
6
Agenda
Background
One Way selection (if) 
Two Way selection (if-else)
Compound Statements
Nested if-else
Logical operators
Common pitfalls
7
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!”;
8
if statement syntax
if (condition) statement ;
Condition – is a logical expression like
score == 100
x+y>10
ans!=‘y’
Statement – is any executable statement like
cout<<“Congrats!”;
OR
x = x + 50;
9
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
10
One-Way Selection Flowchart (if)
true
score==100
false
CONGRATS!
Next statement
11
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
12
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 =!
13
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 ____________
14
Agenda
Background
One Way selection (if)
Two Way selection (if-else) 
Compound Statements
Nested if-else
Logical operators
Common pitfalls
15
The if…else statement
The if…else statement selects between one of
two statements. Here is the syntax of if-else:
if (condition)
statement1;
Notice: no condition here
else
statement2;
So how does this work ….. ?
16
How it works
Again, 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
17
if…else Example
To calculate hourly wages there are two
choices
Regular 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
18
Conditional Flowchart (if-else)
false
true
hours>40
calc pay
with overtime
calc pay
regular
display pay
to screen
19
Designing the Conditional
Determine if (hours >40) is true
If it is true, then use
gross_pay = rate * 40 + 50;
If it is not true, then use
gross_pay = rate * hours;
20
Implementing the Branch
The actual C++ to do this:
if (hours > 40)
gross_pay = rate * 40 + 50;
Notice: no condition here!
else
“else” means
gross_pay = rate * hours;
“condition was false”
Notice, one condition, for two statements
21
Full Application: Calculating Pay
// pay.cpp
int main()
{ float hours, pay, rate;
cout << "enter hours and
cin >> hours >> rate;
rate: ";
This is Step4. Calc Result
if (hours > 40)
pay = rate * 40 + 50;
else
pay = rate * hours;
cout << "pay is " << pay;
}
22
Q2) Two-Way Selection
What 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 ____________
23
Q3) Two-Way Selection
What is output from the following:
if (score = 100)
cout<<“Congrats!”;
else
cout<<“Nice try”;
Given
a) score = 100 ____________
b) score = 80 ____________
24
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;
25
Agenda
Background
One Way selection (if)
Two Way selection (if-else)
Compound Statements 
Nested if-else
Logical operators
Common pitfalls
26
Compound Statements
A compound statement is more than one
statement enclosed in { }
Branches of if or if-else statements often need to
execute more that one statement
Example:
if (condition)
{
statements for true
}
else
{
statements for false
}
27
Example with two if-else statements
if (hours > 40.0)
pay = 40.0 * rate + 50;
else
pay = hours * rate;
cout << "pay is " << pay;
if (hours > 40.0)
cout << " overtime worked";
else
cout << " no overtime worked";
28
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";
}
29
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";
}
30
Agenda
Background
One Way selection (if)
Two Way selection (if-else)
Compound Statements
Nested if-else 
Logical operators
Common pitfalls
31
Nesting and Multi-Way Branches
How 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 …nesting
if (expr1)
statement1; // do if expr1 is true
else
// do if expr1 is false
if (expr2)
statement2; // expr1 false, expr2 true
else
statement3; // both expr1 and expr2 false
32
Nested if-else (multi-way branch)
Syntax:
if (expr1)
{ block1 }
else
if (expr2)
{ block2 }
else
if (expr3)
{block 3}
else
{block4}
Example:
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;}
33
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 ____________
34
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";
35
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
36
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!”;
else
cout<<“n is too big”;
else
cout<<“n is too small”;
37
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”;
38
Agenda
Background
One Way selection (if)
Two Way selection (if-else)
Compound Statements
Nested if-else
Logical operators 
Common pitfalls
39
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
40
Compound Conditions
Logical AND means true only if both true
AND operator: &&
p && q
p
q
p && q
T
T
F
F
T
F
T
F
T
F
F
F
41
Compound Conditions
Logical OR means true if at least 1 is true
OR operator: ||
p || q
p
q
p || q
T
T
F
F
T
F
T
F
T
T
T
F
42
Compound Conditions
NOT operator: !
!p
p
!p
T
F
F
T
43
Compound Condition Example
int main()
{
int n1, n2, n3;
cout << “Enter three integers:
cin >> n1 >> n2 >> n3;
if (n1 <= n2 && n1 <= n3) cout
“ << n1 << endl;
if (n2 <= n1 && n2 <= n3) cout
“ << n2 << endl;
if (n3 <= n1 && n3 <= n2) cout
“ << n3 << endl;
}
“;
<< “Minimum is
<< “Minimum is
<< “Minimum is
44
OR Example
int 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
45
YOUR TURN
6. T/F
7. T/F
8. T/F
( 3 < 2 ) && (5 > 7)
!(2>3)
(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
46
Agenda
Background
One Way selection (if)
Two Way selection (if-else)
Compound Statements
Nested if-else
Logical operators
Common pitfalls 
47
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
48
Dangling Else
A subtle bug possibility
When you nest a single-choice if in a 2-choice if:
if (cond1)
if (cond2)
Q: Which if does
statement1;
the else belong to?
else
statement2;
A: the closest one
above it
49
Dangling else…continued
Compiler really interprets above like this:
if (cond1)
Single choice if
if (cond2)
Two choice if
statement1;
else
statement2;
No matter how you type it!
50
Dangling else…end
A good idea to avoid confusion, put { } around
both choices of two-choice if
if (cond1)
{
if (cond2)
statement1;
Now it’s clear how
}
to interpret it!
else
{
statement2;
}
51
Go back home proud !
You’re a C++ programmer !
52
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!
53
A2) Two-Way Selection
What 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!
54
A3) Two-Way Selection
What is output from the following:
if (score = 100)
cout<<“Congrats!”;
else
cout<<“Nice try”;
Bug! Use ==
Given
a) score = 100
b) score = 80
Congrats!
Congrats! (due to bug)
55
A4) What’s wrong with this code?
if (hours > 40.0)
pay = 40.0 * rate + 50;
cout << " pay is " << pay << " overtime worked";
else
{
Need { }
pay = hours * rate;
cout << " pay is " << pay << " no overtime";
}
56
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
57
YOUR TURN
6. F
7. T
8. T
( 3 < 2 ) && (5 > 7)
!(2>3)
(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
58