Chapter 2 - Flow of Control Part I.pdf

Fundamentals of
Computer Programming
Chapter 2
Flow of Control Part I
(Selection Statement)
Chere L. (M.Tech)
Lecturer, SWEG, AASTU
1

Outline
 Introduction to flow control
 Branching flow controls (selection statements)
 One-way selection
 Two-way selection
 Multiple selection
 switch statement
Chapter 2
2

Objectives
 Learn how to use selection flow control
 Learn how to form Boolean expressions and examine
relational and logical operators
 Design and develop program using selection
statements
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3

1. Introduction to flow controls
 Flow of control is the order in which a program statements are
executed (performs actions).
 The term flow control reflects the fact that the currently executing
statement has the control of the CPU and handed over (flow) to
another statement when it’s execution completed.
 Typically, the flow control in a program is sequential, which is the
most common and straight-forward.
 However, usually a program execution is not limited to a sequential
 Programmers can control the order of instruction execution
 Accordingly, most programming language including C++ provides
control structures that serve to specify what has to be done by our
program, when and under which circumstances.
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1. Introduction to flow controls (cont’d)
 Generally there are three basic program flow control
1. Sequential – execution of instruction sequentially one after an other
2. Selection/branching – allow alternative actions based up on conditions
that are evaluated at run time.
3. Iteration/loop – allows to execute a statement or group of statements
multiple times
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2. Recalling relational and logical operators
 Logical/Boolean expressions are a fundamental part of control
statements and are formed with combinations of two kinds of
operators:
 The expression of both types of operators are evaluated to
true/false
(a) Relational operators (b) Logical operators
Chapter 2
Boolean
expression
example
6

2. Selection/Branching statements
 The selection statements include the following
 if statement ----> One-way selection
 if...else statement ---> Two-way selection
 Conditional operator
 nested if --- else statement
 else if…else statement ---> Multiple selection
 switch statement
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2. Selection statements (cont’d)
 One-way selection --- > if statement
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if (expression) {
statement (s);
}
next statement(s);
 Expression
 A condition that must evaluated to true/false (i.e. Boolean expression)
 One or more relational expression can be combined with logical operators
 if condition is TRUE the statement(s) or the block that follow the selection
is executed and then next statement(s) get executed.
 Otherwise nothing will be executed and execution continues with the next
statement in the program.

 Two-way selection -- > if ….. else statement
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9
 Expression -- similar to that of one-selection statement
 if condition is TRUE the statement1 or block1 is that follow the if selection
is executed and then next statement(s) get executed.
 Otherwise the statement2 or block2 is executed and the execution
continues with the next statement in the program.
if (expression){
statement1 / block1
}
else{
statement2 / block2;
}
next statement(s);

 Another Syntax ----- > without the block { }
Chapter 2
 Can be used when there is only one statement
 Not suggested (it causes dangling Else Problem)
Example
if (condition)
<statement_true>;
else
<statement_false>;
if (condition)
<statement_true>;
if (mark >= 50) {
cout << "Congratulation!" << endl;
cout << "Keep it up!" << endl;
}
else {
cout << “Failed, try harder!" << endl;
}
if (work_hrs > 40) {
OT = work_hrs – 40 * 120;
cout << “Over Time " <<OT<<endl;
}
10

 Conditional operator instead of if …. else statement
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11

 Nested if/else statement
 Refers to using within another selection statement
 Match each else with the last unmatched if
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12

 Multiple selection -- > if ….. else if statement
Allows for conditional execution based up on more than two alternatives
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13
 Expression -- similar to that of one-selection statement
 if expression1 returns TRUE the statement1 or block1 is that follow the if
selection is executed and then next statement(s) get executed.
 Otherwise expression2 of each else if part is evaluated and the statement2 or
block2 of the selection that returns TRUE is executed and the execution continues
with the next statement in the program.
if (expression1){
statement1 / block1
}
else if (expression2){
statement2 / block2;
}
. . . . .
else {
statement-N / block-N; }
next statement(s);

 if ….. else if Vs. nested if … else statement
 What is the difference between nested if – else and else if selection
structure?
 Evaluate the below two examples
(a) (b)
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 switch statement
It is similar to if … else if combination, it enables you to test several cases
generated by a given expression
The value of a variable/expression determine where the program will branch
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switch (expression)
{
case constant-1:
group of statements 1;
break;
case constant-2:
break;
case constant-3:
break;
-
-
default:
default group of statements
}

 How it works
 The switch expression is evaluated once
 The value of the expression is compared with values of each case
 If there is a match, the associated block of statements is executed
 The statements following the matched case will be executed until
a break statement is reached
 When a break statement is reached, the switch terminates, and the flow
of control jumps to the next line following the switch statement.
 A switch statement can have an optional default case, which usually
appears at the end of the switch.
 The default case can be used for performing a task when none of cases is
true.
 Note
 The expression must evaluated to literal value (integral/character/enum)
 Each case is followed by the value to be compared to and a colon.
 The expression of each case statement in the block must be unique.
 If no break appears, the flow of control will fall through to subsequent
cases until a break is reached.
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16

 Example 1:
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17
Additional notes
 The case statement expression
cannot be variable and also range
 Switch can only be used to
compare an expression against
constants
 It is not necessary to include
braces {} surrounding the
statements for each of the cases
 Even if it is usually necessary to
include a break statement at the
end of each case, there are
situations in which it makes sense
to have a case without a break

Example 2: switch statement Vs. if…else if statement
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18
Which is selection statement is best fit
for a problem which has range selection?

Chapter 2
19
Dangling else problem
 What does it display for x=4?
 The problem is that it displays “4 is an odd number” message for positive even
numbers and zero
 Reason is that, although indentation says the reverse, else belongs to second
(inner) if
 else belongs to the most recent if
 Solution: using brace {} as follow

Chapter 2
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Short-circuit Evaluation
 Evaluate the first (leftmost) Boolean sub-expression.
 If its value is enough to judge about the value of the entire expression, then stop
there. Otherwise continue evaluation towards right.
 Example: if (count != 0 && scores/count < 60)
{
cout<<"low average";
}
 In this example, if the value of count is zero, then first sub-expression becomes
false and the second one is not evaluated.
 In this way, we avoid “division by zero” error (that would cause to stop the
execution of the program)
 Alternative method to avoid division by zero without using short-circuit
evaluation: if (count != 0){
if (scores/count < 60){
cout<<"low average";
}
}

Reading Resources/Materials
Chapter 5 & 6:
 Diane Zak; An Introduction to Programming with C++ [8th
Edition], 2016 Cengage Learning
Chapter 4:
 Gary J. Bronson; C++ For Engineers and Scientists [3rd
edition], Course Technology, Cengage Learning, 2010
Chapter 2 (section 2.4):
 Walter Savitch; Problem Solving With C++ [10th edition],
University of California, San Diego, 2018
Chapter 4:
 P. Deitel , H. Deitel; C++ how to program, [10th edition],
Global Edition (2017)
21

Thank You
For Your Attention!!
22

Chapter 2 - Flow of Control Part I.pdf

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Chapter 2 - Flow of Control Part I.pdf