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## Chapter 2 - Control Structures

Outline
2.1 Introduction
2.2 Algorithms
2.3 Pseudocode
2.4 Control Structures
2.5 The if Selection Structure
2.6 The if/else Selection Structure
2.7 The while Repetition Structure
2.8 Formulating Algorithms: Case Study 1
(Counter-Controlled Repetition)
2.9 Formulating Algorithms with Top-Down, Stepwise Refinement:
Case Study 2 (Sentinel-Controlled Repetition)
2.10 Formulating Algorithms with Top-Down, Stepwise Refinement:
Case Study 3 (Nested Control Structures)
2.11 Assignment Operators
2.12 Increment and Decrement Operators
2.13 Essentials of Counter-Controlled Repetition
2.14 The for Repetition Structure
2.15 Examples Using the for Structure

2

## Chapter 2 - Control Structures

Outline
2.16 The switch Multiple-Selection Structure
2.17 The do/while Repetition Structure
2.18 The break and continue Statements
2.19 Logical Operators
2.20 Confusing Equality (==) and Assignment (=) Operators
2.21 Structured-Programming Summary

3

2.1 Introduction

## Before writing a program:

Have a thorough understanding of problem
Carefully plan your approach for solving it
While writing a program:
Know what building blocks are available
Use good programming principles

4

2.2 Algorithms

## All computing problems

can be solved by executing a series of actions in a specific
order
Algorithm
A procedure determining the
Actions to be executed
Order in which these actions are to be executed
Program control
Specifies the order in which statements are to executed

5

2.3 Pseudocode

Pseudocode
Artificial, informal language used to develop algorithms
Similar to everyday English
Not actually executed on computers
Allows us to think out a program before writing the code
for it
Easy to convert into a corresponding C++ program
Consists only of executable statements

6

## 2.4 Control Structures

Sequential execution
Statements executed one after the other in the order written
Transfer of control
When the next statement executed is not the next one in
sequence
Bohm and Jacopini: all programs written in terms
of 3 control structures
Sequence structure
Built into C++. Programs executed sequentially by default.
Selection structures
C++ has three types - if, if/else, and switch
Repetition structures
C++ has three types - while, do/while, and for

7

## 2.4 Control Structures

C++ keywords
Cannot be used as identifiers or variable names.
C++ Keywords
Keywordscommontothe
CandC++programming
languages
auto break case char const
continue default do double else
enum extern float for goto
if int long register return
short signed sizeof static struct
switch typedef union unsigned void
volatile while
C++onlykeywords

## asm bool catch class const_cast

delete dynamic_cast explicit false friend
inline mutable namespacenew operator
private protected public reinterpret_cast
static_cast template this throw true
try typeid typename using virtual
wchar_t

8

## 2.4 Control Structures

Flowchart
Graphical representation of an algorithm
Drawn using certain special-purpose symbols connected by
arrows called flowlines.
Rectangle symbol (action symbol)
Indicates any type of action.
Oval symbol
indicates beginning or end of a program, or a section of code
(circles).
single-entry/single-exit control structures
Connect exit point of one control structure to entry point of
the next (control-structure stacking).
Makes programs easy to build.

9

## 2.5 The if Selection Structure

Selection structure
used to choose among alternative courses of action
Pseudocode example:
If students grade is greater than or equal to 60
Print Passed
If the condition is true
print statement executed and program goes on to next
statement
If the condition is false
print statement is ignored and the program goes onto the next
statement
Indenting makes programs easier to read
C++ ignores whitespace characters

10

## Translation of pseudocode statement into C++:

if ( grade >= 60 )
cout << "Passed";
Diamond symbol (decision symbol)
indicates decision is to be made
Contains an expression that can be true or false.
Test the condition, follow appropriate path
if structure is a single-entry/single-exit structure

11

any expression.

nonzero - true
Example:
false
3 - 4 is true

12

## 2.6 The if/else Selection Structure

if
Only performs an action if the condition is true
if/else
A different action is performed when condition is true and
when condition is false
Psuedocode
if students grade is greater than or equal to 60
print Passed
else
print Failed
C++ code
if ( grade >= 60 )
cout << "Passed";
else
cout << "Failed";

13

false true

## Ternary conditional operator (?:)

Takes three arguments (condition, value if true, value if false)
Our pseudocode could be written:
cout << ( grade >= 60 ? Passed : Failed );

14

## 2.6 The if/else Selection Structure

Nested if/else structures
Test for multiple cases by placing if/else selection
structures inside if/else selection structures.
if students grade is greater than or equal to 90
Print A
else
if students grade is greater than or equal to 80
Print B
else
if students grade is greater than or equal to 70
Print C
else
if students grade is greater than or equal to 60
Print D
else
Print F
Once a condition is met, the rest of the statements are skipped
15

## 2.6 The if/else Selection Structure

Compound statement:
Set of statements within a pair of braces
Example:
if ( grade >= 60 )
cout << "Passed.\n";
else {
cout << "Failed.\n";
cout << "You must take this course
again.\n";
}
Without the braces,
cout << "You must take this course again.\n";
would be automatically executed
Block
Compound statements with declarations

16

## 2.6 The if/else Selection Structure

Syntax errors
Errors caught by compiler
Logic errors
Errors which have their effect at execution time
Non-fatal logic errors
program runs, but has incorrect output
Fatal logic errors
program exits prematurely

2.7 The while Repetition Structure
17

Repetition structure
Programmer specifies an action to be repeated while some
condition remains true
Psuedocode
while there are more items on my shopping list
Purchase next item and cross it off my list
while loop repeated until condition becomes false.
Example
int product = 2;
while ( product <= 1000 )
product = 2 * product;

18

## Flowchart of while loop

true
product <= 1000 product = 2 * product

false

2.8 Formulating Algorithms (Counter-
19

Controlled Repetition)
Counter-controlled repetition
Loop repeated until counter reaches a certain value.
Definite repetition
Number of repetitions is known
Example
A class of ten students took a quiz. The grades (integers in
the range 0 to 100) for this quiz are available to you.
Determine the class average on the quiz.

2.8 Formulating Algorithms (Counter-
20

Controlled Repetition)
Pseudocode for example:
Set total to zero
While grade counter is less than or equal to ten
Set the class average to the total divided by ten
Print the class average

## Following is the C++ code for this example

1 // Fig. 2.7: fig02_07.cpp 21
2 // Class average program with counter-controlled repetition Outline
3 #include <iostream>
4
5 using std::cout; 1. Initialize Variables
6 using std::cin;
7 using std::endl;
2. Execute Loop
8
9 int main()
10 { 3. Output results
11 int total, // sum of grades
14 average; // average of grades
15
16 // initialization phase
17 total = 0; // clear total
18 gradeCounter = 1; // prepare to loop
19 The counter gets incremented each time
20 // processing phase the loop executes. Eventually, the
21 while ( gradeCounter <= 10 ) { // loop 10counter
times causes the loop to end.
22 cout << "Enter grade: "; // prompt for input
26 }
27
28 // termination phase
29 average = total / 10; // integer division
30 cout << "Class average is " << average << endl;
31
32 return 0; // indicate program ended successfully
22
Outline
Class average is 81

2.9 Formulating Algorithms with Top-Down, 23

Repetition)

## Suppose the problem becomes:

Develop a class-averaging program that will process an
arbitrary number of grades each time the program is run.
Unknown number of students - how will the program know
to end?
Sentinel value
Indicates end of data entry
Loop ends when sentinel inputted
Sentinel value chosen so it cannot be confused with a regular
input (such as -1 in this case)

2.9 Formulating Algorithms with Top-Down, 24

Repetition)

## Top-down, stepwise refinement

begin with a pseudocode representation of the top:
Determine the class average for the quiz
Divide top into smaller tasks and list them in order:
Initialize variables
Input, sum and count the quiz grades
Calculate and print the class average

2.9 Formulating Algorithms with Top-Down,
25

Stepwise Refinement
Many programs can be divided into three phases:
Initialization
Initializes the program variables
Processing
Inputs data values and adjusts program variables accordingly
Termination
Calculates and prints the final results.
Helps the breakup of programs for top-down refinement.
Refine the initialization phase from
Initialize variables
to
Initialize total to zero
Initialize counter to zero

2.9 Formulating Algorithms with Top-Down,
26

Stepwise Refinement
Refine
Input, sum and count the quiz grades
to
Input the first grade (possibly the sentinel)
While the user has not as yet entered the sentinel
Input the next grade (possibly the sentinel)
Refine
Calculate and print the class average
to
If the counter is not equal to zero
Set the average to the total divided by the counter
Print the average
Else
1 // Fig. 2.9: fig02_09.cpp 27
2 // Class average program with sentinel-controlled repetition. Outline
3 #include <iostream>
4
1. Initialize Variables
5 using std::cout;
6 using std::cin;
7 using std::endl; 2. Get user input
8 using std::ios;
9 2.1 Perform Loop
10 #include <iomanip>
11
12 using std::setprecision;
13 using std::setiosflags;
14
15 int main()
Data type double used to represent
16 {
decimal numbers.
17 int total, // sum of grades
20 double average; // number with decimal point for average
21
22 // initialization phase
23 total = 0;
25
26 // processing phase
27 cout << "Enter grade, -1 to end: ";
29
30 2000 Prentice
while Hall, Inc.!=All-1
) { reserved.
31 total = total + grade; 28
33 cout << "Enter grade, -1 to end: ";
35 } 3. Calculate Average
36

## 3.1 Print Results

37 // termination phase
38 if ( gradeCounter != 0 ) {
39 average = static_cast< double >( total ) / gradeCounter;
40 cout << "Class average is " << setprecision( 2 )
41 << setiosflags( ios::fixed | ios::showpoint )
42 << average << endl;
43 }
44 else setiosflags(ios::fixed | ios::showpoint) - stream
static_cast<double>() - treats total as a
manipulator
45 cout << "No grades were entered" << endl;
46 double temporarily.
47 return 0; // indicateios::fixed
program ended - output numbers with a fixed number of decimal
successfully
48 }Required because dividing two integers truncates the
points.
remainder.
Program Output
ios::showpoint - forces decimal point and trailing zeros, even if
is an75int,unnecessary:
but it gets promoted to
66 printedsetprecision(2)
as 66.00 - prints only two digits
Enter grade, -1 to end: 94
Enterdouble.
grade, -1 to end: 97 past decimal point.
Enter grade, -1 to end: 88
Enter grade, -1 to end: 70
| - separates multiple option.
Enter grade, -1 to end: 64
Programs that use this must include <iomanip>
Enter grade, -1 to end: 83
Enter grade, -1 to end: 89
Enter grade, -1 to end: -1
Class average is 82.50

29

## 2.10 Nested control structures

Problem:
A college has a list of test results (1 = pass, 2 = fail) for 10
students. Write a program that analyzes the results. If more
than 8 students pass, print "Raise Tuition".
We can see that
The program must process 10 test results. A counter-
controlled loop will be used.
Two counters can be usedone to count the number of
students who passed the exam and one to count the number
of students who failed the exam.
Each test result is a numbereither a 1 or a 2. If the number
is not a 1, we assume that it is a 2.
Top level outline:
Analyze exam results and decide if tuition should be raised

30

## 2.10 Nested control structures

First Refinement:
Initialize variables
Input the ten quiz grades and count passes and failures
Print a summary of the exam results and decide if tuition
should be raised
Refine
Initialize variables
to
Initialize passes to zero
Initialize failures to zero
Initialize student counter to one

31

## 2.10 Nested control structures

Refine
Input the ten quiz grades and count passes and failures
to
While student counter is less than or equal to ten
Input the next exam result
If the student passed
Else
Refine
Print a summary of the exam results and decide if tuition should be raised
to
Print the number of passes
Print the number of failures
If more than eight students passed
Print Raise tuition
1 // Fig. 2.11: fig02_11.cpp 32
2 // Analysis of examination results Outline
3 #include <iostream>
4
1. Initialize variables
5 using std::cout;
2. Input data and
count passes/failures
6 using std::cin;
7 using std::endl;
8
9 int main()
10 {
11 // initialize variables in declarations
12 int passes = 0, // number of passes
13 failures = 0, // number of failures
14 studentCounter = 1, // student counter
15 result; // one exam result
16
17 // process 10 students; counter-controlled loop
18 while ( studentCounter <= 10 ) {
19 cout << "Enter result (1=pass,2=fail): ";
20 cin >> result;
21
22 if ( result == 1 ) // if/else nested in while
23 passes
2000 Prentice = passes
Hall, Inc. All rights + 1;
reserved.
24 else 33
25 failures = failures + 1; Outline
26
27 studentCounter = studentCounter + 1; 3. Print results
28 }
29
30 // termination phase
31 cout << "Passed " << passes << endl;
32 cout << "Failed " << failures << endl;
33
34 if ( passes > 8 )
35 cout << "Raise tuition " << endl;
36
37 return 0; // successful termination
38 }

## Enter result (1=pass,2=fail):

Enter result (1=pass,2=fail):
1
1
Program Output
Enter result (1=pass,2=fail): 1
Enter result (1=pass,2=fail): 1
Enter result (1=pass,2=fail): 2
Enter result (1=pass,2=fail): 1
Enter result (1=pass,2=fail): 1
Enter result (1=pass,2=fail): 1
Enter result (1=pass,2=fail): 1
Enter result (1=pass,2=fail): 1
Passed 9
Failed 1
Raise tuition

34

## Assignment expression abbreviations

c = c + 3; can be abbreviated as c += 3; using the
Statements of the form
variable = variable operator expression;
can be rewritten as
variable operator= expression;
Examples of other assignment operators include:
d -= 4 (d = d - 4)
e *= 5 (e = e * 5)
f /= 3 (f = f / 3)
g %= 9 (g = g % 9)

35

## 2.12 Increment and Decrement Operators

Increment operator (++) - can be used instead of c
+= 1
Decrement operator (--) - can be used instead of c
-= 1
Preincrement
When the operator is used before the variable (++c or c)
Variable is changed, then the expression it is in is evaluated.
Posincrement
When the operator is used after the variable (c++ or c--)
Expression the variable is in executes, then the variable is changed.
If c = 5, then
cout << ++c; prints out 6 (c is changed before cout is
executed)
cout << c++; prints out 5 (cout is executed before the
increment. c now has the value of 6)
36

## When Variable is not in an expression

Preincrementing and postincrementing have the same effect.
++c;
cout << c;
and
c++;
cout << c;
have the same effect.

2.13 Essentials of Counter-Controlled
37

Repetition
Counter-controlled repetition requires:
The name of a control variable (or loop counter).
The initial value of the control variable.
The condition that tests for the final value of the control
variable (i.e., whether looping should continue).
The increment (or decrement) by which the control variable
is modified each time through the loop.
Example:
int counter =1; //initialization
while (counter <= 10){ //repetition
condition
cout << counter << endl;
++counter; //increment
}

2.13 Essentials of Counter-Controlled
38

Repetition
The declaration
int counter = 1;
Names counter
Declares counter to be an integer
Reserves space for counter in memory
Sets counter to an initial value of 1

39

## The general format when using for loops is

for ( initialization; LoopContinuationTest;
increment )
statement
Example:
for( int counter = 1; counter <= 10; counter++ )
cout << counter << endl;
Prints the integers from one to ten
No
semicolon
after last
statement

40

## 2.14 The for Repetition Structure

For loops can usually be rewritten as while loops:
initialization;
while ( loopContinuationTest){
statement
increment;
}
Initialization and increment as comma-separated lists
for (int i = 0, j = 0; j + i <= 10; j++, i++)
cout << j + i << endl;

41

## 2.15 Examples Using the for Structure

Program to sum the even numbers from 2 to 100
1 // Fig. 2.20: fig02_20.cpp
2 // Summation with for
3 #include <iostream>
4
5 using std::cout;
6 using std::endl;
7
8 int main()
9 {
10 int sum = 0;
11
12 for ( int number = 2; number <= 100; number += 2 )
13 sum += number;
14
15 cout << "Sum is " << sum << endl;
16
17 return 0;
18 }

Sum is 2550

42

## 2.16 The switch Multiple-Selection Structure

switch
Useful when variable or expression is tested for multiple values
Consists of a series of case labels and an optional default case

true
case a case a action(s) break

false

true
case b case b action(s) break

false

.
.
.

true
case z case z action(s) break
false

default action(s)

1 // Fig. 2.22: fig02_22.cpp 43
2 // Counting letter grades Outline
3 #include <iostream>
4
5 using std::cout; 1. Initialize variables
6 using std::cin;
7 using std::endl;
8 2. Input data
9 int main()

## 2.1 Use switch loop to

10 {
12 aCount = 0, // number of A's update count
13 bCount = 0, // number of B's
14 cCount = 0, // number of C's
15 dCount = 0, // number of D's
16 fCount = 0; // number of F's
17
18 cout << "Enter the letter grades." << endl
19 << "Enter the EOF character to end input." << endl;
20
21 while ( ( grade = cin.get() ) != EOF ) {
22 Notice how the case statement is used
23 switch ( grade ) { // switch nested in while
24
25 case 'A': // grade was uppercase A
26 case 'a': // or lowercase a
27 ++aCount;
28 break; // necessary to exit switch
29
30 case 'B': // grade was uppercase B
31 case 'b': // or lowercase b
32 ++bCount;
33 break;
34
35 case 'C': // grade was uppercase C 44
36
37
case 'c': // or lowercase c
++cCount;
Outline
38 break;
39 2.1 Use switch loop to
update
40 case 'D': // grade was uppercase D
41 case 'd': // or lowercase d
break causes switch to end and count
42 ++dCount; the program continues with the first
statement after the switch structure.3. Print results
43 break;
44
45 case 'F': // grade was uppercase F
46 case 'f': // or lowercase f
47 ++fCount;
48 break;
49
50 case '\n': // ignore newlines,
51 case '\t': // tabs,
52 Notice
case ' ': // and spaces in input the default statement.
53 break;
54
55 default: // catch all other characters
56 cout << "Incorrect letter grade entered."
57 << " Enter a new grade." << endl;
58 break; // optional
59 }
60 }
61
62 cout << "\n\nTotals for each letter grade are:"
63 << "\nA: " << aCount
64 << "\nB: " << bCount
65 << "\nC: " << cCount
66 << "\nD: " << dCount
67 << "\nF: " << fCount << endl;
68
69 return 0;
45
Outline
Enter the EOF character to end input.
a
B
Program Output
c
C
A
d
f
C
E
D
A
b

A: 3
B: 2
C: 3
D: 2
F: 1

46

## The do/while repetition structure is similar to the

while structure,
Condition for repetition tested after the body of the loop is
executed
Format:
do {
statement
} while ( condition );
action(s)

## Example (letting counter = 1):

do {
cout << counter << " "; true
condition
} while (++counter <= 10);
This prints the integers from 1 to 10 false

47

## 2.18 The break and continue Statements

Break
Causes immediate exit from a while, for, do/while or
switch structure
Program execution continues with the first statement after the
structure
Common uses of the break statement:
Escape early from a loop
Skip the remainder of a switch structure

48

## 2.18 The break and continue Statements

Continue
Skips the remaining statements in the body of a while,
for or do/while structure and proceeds with the next
iteration of the loop
In while and do/while, the loop-continuation test is
evaluated immediately after the continue statement is
executed
In the for structure, the increment expression is executed,
then the loop-continuation test is evaluated

49

## 2.19 Logical Operators

&& (logical AND)
Returns true if both conditions are true
|| (logical OR)
Returns true if either of its conditions are true
! (logical NOT, logical negation)
Reverses the truth/falsity of its condition
Returns true when its condition is false
Is a unary operator, only takes one condition
Logical operators used as conditions in loops
Expression Result
true && false false
true || false true
!false true

2.20 Confusing Equality (==) and
50

## Assignment (=) Operators

These errors are damaging because they do not
ordinarily cause syntax errors.
Recall that any expression that produces a value can be used in
control structures. Nonzero values are true, and zero values
are false
Example:
if ( payCode == 4 )
cout << "You get a bonus!" << endl;
Checks the paycode, and if it is 4 then a bonus is awarded
If == was replaced with =
if ( payCode = 4 )
cout << "You get a bonus!" << endl;
Sets paycode to 4
4 is nonzero, so the expression is true and a bonus is awarded,
regardless of paycode.
2.20 Confusing Equality (==) and
51

## Assignment (=) Operators

Lvalues
Expressions that can appear on the left side of an equation
Their values can be changed
Variable names are a common example (as in x = 4;)
Rvalues
Expressions that can only appear on the right side of an
equation
Constants, such as numbers (i.e. you cannot write 4 = x;)
Lvalues can be used as rvalues, but not vice versa

2.21 Structured-Programming Summary
52

Structured programming
Programs are easier to understand, test, debug and, modify.
Rules for structured programming
Only single-entry/single-exit control structures are used
Rules:
1) Begin with the simplest flowchart.
2) Any rectangle (action) can be replaced by two rectangles
(actions) in sequence.
3) Any rectangle (action) can be replaced by any control
structure (sequence, if, if/else, switch, while, do/while or for).
4) Rules 2 and 3 can be applied in any order and multiple times.

2.21 Structured-Programming Summary
53

## Representation of Rule 3 (replacing any rectangle with a control structure)

Rule 3

Rule 3 Rule 3

2.21 Structured-Programming Summary
54

## All programs can be broken down into

Sequence
Selection
if, if/else, or switch
Any selection can be rewritten as an if statement
Repetition
while, do/while or for
Any repetition structure can be rewritten as a while
statement
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