Lecture 3 control_structures_i

OBJECTIVES
• In this Lecture you will learn how to:
– Make decisions
• Using :
– simple if
– If…else statement
– If…else if statement
– Use logical operators
2
Intro to Programming with C++
Sir Noble

Program Control and Control Structures
– An algorithm is a procedure for solving a
problem in terms of
• the actions to execute and
• the order in which the actions execute
– Program control
• Specifying the order in which statements (actions)
execute.
– Control Structures
• Simple control statements
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Sir Noble

The Simple if Statement
• Allows statements to be conditionally executed or
skipped over
• Models the way we mentally evaluate situations:
– "If I do not have the minimum CGPA required, I will not
graduate."
– "If I have codeblocks installed on my machine, I will be able
to write and debug simple c++ programs."
• If condition is true, body of the if statement
executes
• If condition is false, body of the if statement does
not execute
4
Sir Noble

The Logical flow of Simple If
Decision
5
Sir Noble

Decision Making: Equality and Relational
Operators
• The if Statement
– Syntax
– Boolean expression is any C++
expression that can be evaluated
– Statement is any C++ statement or block
of statements that you want to execute
when the boolean expression evaluates
as true.
6
if ( boolean expression )
Statement;
Sir Noble

7
Simpe If Statement
Sir Noble

8
Example of Simpe If Statement
Sir Noble

9
Example of Simpe If Statement
Sir Noble

Expanding the if Statement
• To execute more than one statement as part of an if
statement, enclose them in curly braces{ }:
if (gpa >= 3.75)
{
cout << “ GPA is ”<< << gpa << endl;
cout <<“Congrats! You are a first class
student” << endl;
}
• { } creates a block of code
10
Sir Noble

Nested if Statements
• An if statement that is nested inside another
if statement
• Nested if statements can be used to test more
than one condition
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Sir Noble

Nested if Statements - Example
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Sir Noble

The if/else statement
• Provides two possible paths of execution
• Performs one statement or block if the
expression is true, otherwise performs
another statement or block.
13
Sir Noble

The if/else statement
• General Format:
if (expression)
statement1; // or block
else
statement2; // or block
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True
False
Sir Noble

if/else-What Happens
To evaluate:
if (expression)
statement1;
else
statement2;
• If the expression is true, then statement1 is executed
and statement2 is skipped.
• If the expression is false, then statement1 is skipped
and statement2 is executed.
• For example, consider this pseudocode
if student’s grade is greater than or equal to 40
Print “Passed”
else
Print “Failed”
15
Sir Noble

16
Example If - else Statement
Sir Noble

Blocks
• Blocks/Compound Statement
– Set of statements within a pair of braces
– Used to include multiple statements in an if body
– Example
– Without braces
cout << "You must take this course again" << endl;
always executes
17
if ( studentGrade >= 60 )
cout << "Passed“ << endl;
else
{
cout << "Failed“ << endl;
cout << "You must take this course
again“ << endl;
}
Sir Noble

The if/else if Statement
• Tests a series of conditions until one is found to
be true
• Often simpler than using nested if/else
statements
• Can be used to model thought processes such as:
If I have ‘chaw’ money, I will buy pizza,
else, if it is not ‘chaw’, I will buy kenkey and fish,
else, no ‘chop chop’
18
Sir Noble

if/else if Format
19
Sir Noble

The if/else if Statement in
Program
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Sir Noble

Logical Operators
• Logical operators
– Allows for more complex conditions
• Combines simple conditions into complex conditions
• Used to create relational expressions from other relational
expressions
• C++ logical operators
– && (logical AND)
– || (logical OR)
– ! (logical NOT)
21
Sir Noble

Logical Operators
• Logical AND (&&) Operator
– It takes two expressions as operands and creates an
expression that is true only when both sub expressions
are true.
– Consider the following if statement
if ( temperature < 20 && minutes > 12 )
cout << “The temperature is in the danger
zone” << endl;
• Combined condition is true
– If and only if both simple conditions are true
• Combined condition is false
– If either or both of the simple conditions are false
22
Sir Noble

23
You must provide complete expressions on both sides of the &&
operator. For example, the following is not correct because the condition
on the right side of the && operator is not a complete expression.
temperature > 0 && < 100
The expression must be rewritten as
temperature > 0 && temperature < 100
Sir Noble

Logical Operators
24
expression1 expression2 expression1 && expression2
false false false
false true false
true false false
true true true
Sir Noble

Logical Operators
• Logical OR (||) Operator
– It takes two expressions as operands and creates an
expression that is true when either of the sub-
expressions are true.
if ( temperature < 20 || temperature > 100 )
cout << “The temperature is in the danger zone” << endl;
• Combined condition is true
– If either or both of the simple conditions are true
• Combined condition is false
– If both of the simple conditions are false
25
Sir Noble

Logical Operators
26
expression1 expression2 expression1 || expression2
false false false
false true true
true false true
true true true
Sir Noble

The logical || Operator in
Program
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Sir Noble

The logical || Operator in
Program
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Sir Noble

Logical Operators
• Logical NOT (!) Operator
– It takes an operand and reverses its truth or
falsehood.
– If the expression is true, the ! Operator returns false
and if the expression is false, it returns true
if ( !(temperature > 100) )
cout << “You are below the maximum temperature” << endl;
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Sir Noble

Logical Operators-Examples
int x = 5, y = -7, z = 9;
(x > y) && (y > z) True/False
(x > y) && (z > y) True/False
(x <= z) || (y == z) True/False
(x <= z) || (y != z) True/False
!(x >= z) True/False
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Sir Noble

Exercise – if-else…if-else
31
• A program that calculates the charges for
membership in a Sanhedrin Gym. The gym has three
membership packages to choose from: bronze
membership, silver membership and gold
membership. The program presents a menu that
allows the user to choose the desired package and
then calculates the cost of the membership.
Membership rates
Bronze membership: GHC 30.00 per month
Silver membership: GHC 40.00 per month
Gold membership: GHC 50.00 per month
Sir Noble

Lecture 3 control_structures_i

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