Lecture03 - Copy

Conditional Operators
Syntax:
exp1 ? exp2 : exp3
Where exp1,exp2 and exp3 are expressions
Working of the ? Operator:
Exp1 is evaluated first, if it is nonzero(1/true) then the expression2 is evaluated and this becomes the value of the expression,
If exp1 is false(0/zero) exp3 is evaluated and its value becomes the value of the expression
Ex: m=2;
n=3
r=(m>n) ? m : n;

Increment & Decrement Operators
C++ supports 2 useful operators namely
Increment ++
Decrement- -operators
The ++ operator adds a value 1 to the operand
The -- operator subtracts 1 from the operand
++a or a++
--a or a--

Rules for ++ & -- Operators
These require variables as their operands.
When postfix either ++ or -- is used with the variable in a given expression, the expression is evaluated first and then it is incremented or decremented by one.
When prefix either ++ or – is used with the variable in a given expression, it is incremented or decremented by .one first and then the expression is evaluated with the new value

Examples for ++ and -- Operators
Let the value of a =5 and b=++a then
a = b =6
Let the value of a = 5 and b=a++ then
a =6 but b=5
i.e.:
1. a prefix operator first adds 1 to the operand and then the result is assigned to the variable on the left
2. a postfix operator first assigns the value to the variable on left and then increments the operand.

Shorthand Assignment Operators

Objectives
To use the selection structure to choose among the alternative actions
if selection statement
Double Selection (if else ) statements
Nested Selection statements

Control structures
Control structure
A control statement and the statements whose execution it controls.
a control statement is any statement that alters the linear flow of control of a program. C++ examples include: if-else, while, for, break, continue and return statement
Sequential execution
Statements executed in order
Transfer of control
Next statement to be executed may be other than the next one in sequence.

Control structures
Bohm and Jacopini’s control structures
Bohm and Jacopino’s work demonstrate that all programs could be written in terms of only three control structures.
Sequence structures, selection structures and repetition 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 – for, while and do while

Control structures
if Selection Structure
Perform an action if condition is true.
.Skip the action if condition is false.
If/else Selection Structure
Perform an action if condition is true.
Performs a different action if the condition is false.

We are going to discuss if and if/else selection structures and switch first and will explain the repetition structures later. It was just an introduction to these structures.

Decision making structure/ selection structure
Selection structure/decision structure
allows the program to make a decision or comparison and then select one of two paths, depending on the result of the comparison.
Condition
condition is a logical expression that evaluates to true or false. It could be a relational or Boolean expression. In other words
Specifies the decision you are making
Must result in either a true or false answer
IF Structure
One statement or a block of statement enclosed in braces {} i.e. (compound statement), to be processed If condition met otherwise it is skipped.
Uses equality and relational operators

Equality and Relational operators
Conditions in if structures can be formed by using the equality and relational operators
Equality operators
Same level of precedence
Associated left to right.
Relational operators
Same level of precedence.
Associated left to right.
Equality operators precedence is lower then precedence of relational operators.

Equality and relational operators

Boolean Expressions
Boolean expressions are expressions that are
either true or false
comparison operators such as '>' (greater than)
are used to compare variables and/or numbers
(grade >= 60) Including the parentheses, is the
boolean expression from the grade example
A few of the comparison operators that use two
symbols (No spaces allowed between the symbols!)
> greater than
!= not equal or inequality
== equal or equivalent
<= less than or equal to
etc

If Selection Structure
The primary C++ selection structure statement used to perform a single-alternative selection.
Choose among alternative courses of action
If the condition is true
statement Print statement executed, program continues to next
If the condition is false
Print statement ignored, program continues

Flow chart
Graphical representation of an algorithm or a portion of algorithm.
Drawn using certain special-purpose symbols connected by arrows called flow lines.
Special purpose symbols.
Following are some symbols to draw a flow chart and there purpose.
Rectangles are action symbols to indicate any type of ACTION, including a calculation or an input output operation.
Diamonds are conditional or decision symbols. It indicates a decision to be made.
Ovals or rounded rectangles, indicate start or end of the program usually containing the word "Start, begin” or "End“.

Flow chart
Parallelogram are the input/output symbols
Arrows, showing what's called "flow of control“. An arrow coming from one symbol and ending at another symbol represents that control passes to the symbol the arrow points to. Arrows are also called flow lines.

If selection structure flow chart

If selection structure
example:
Pseudocode
If student’s grade is greater than or equal to 60
Print “Passed”
C++ code
if ( grade >= 60 ) cout << "Passed";

If selection structure
Flow chart for the pseudocode statement.

Example
Example is using the relational and equality operators.
Following example used six if statements to compare two numbers input by the user.
If the condition in any of these if statements is satisfied, the output statement associated with if is executed.
If the condition with the if statement is false the output statement associated with that if statement is skipped.
Observe the different outputs for various inputs at the end of the program.

Input is 35 and 30
Input is 25 and 25
Input is 10 and 20

The if-else statement
The if statement by itself will execute a single statement, or a group of statements , when conditions following if is true.
It does nothing when the conditions is false.
Can we execute one group of statements if the condition is true and another group of statements if the condition is false?
Of course this is what is the purpose of else statement, which is demonstrated in the following example:

Ques: In a company an employee is paid as under:
If his basic salary is less than $1500, then HRA = 10% of basic salary and DA=90% of basic. If his salary is either equal to or above $1500, then HRA = $500 and DA=98% of basic salary. If the employee’s salary is input through keyboard write a program to find his gross salary.

If/else selection structure
Different actions if conditions true or false
Syntax
A single statement for each alternative
if (Boolean_expression)
yes_statement;
else
no_statement;
A sequence statement for each alternative
if (Boolean_expression)
{ yes_statement1;
yes_statement2;
yes_statement last; }
else
{ no_statement1;
no_statement2;
no_statement last; }

If/else selection structure flow chart
Action if true
true
false

If/else selection structure
Example
Pseudocode
if student’s grade is greater than or equal to 60
print “Passed”
else
print “Failed”
C++ code
if ( grade >= 60 ) cout << "Passed";
else
cout << "Failed";

If/else selection structure
Flow chart for the pseudocode

Compound Statement
Compound statement
Set of statements within a pair of braces also known as BLOCK
if ( grade >= 60 )
cout << "Passed.\n";
else {
cout << "Failed.\n";
cout << "You must take this course again.\n";
}
Without braces,
cout << "You must take this course again.\n";
always executed

Another simple Example
//program to determine if user is ill
#include
using namespace std;
int main()
{
const double NORM = 98.6; // degree Fahranheit;
double temperature;
cout<<"Enter your temperature\t";
cin>>temperature;
if (temperature>NORM) //if temperature is greater than NORM print following statement
cout<<"\n\nYou are sick\n"<<"take rest and drink lots of fluids";
else //if temperature is <= NORM print following statement
cout<<"\n\nYou are perfectly fine";
return 0;
}

output
Input is 100
100 > NORM
Input is 98
98

Example of if and if/else selection structure drawn in flow chart form
If selection structure(program 1)
If/else selection structure(program2)

c++ code for the flow chart of if selection structure (program 1).
//program 1
//program to find the part prices for the given part number.
#include
using namespace std;
int main()
{
int part_number, ;
float price;
cout<<"enter part number"<
cin>>part_number;
cout<<"enter price"<
cin>>price;
if(part_number==203)
price = price*1.1;
cout<<"price is "<
return 0;
}

Output of program 1
Part_number is equal to 203
Part_number is not equal to 203

c++ code for the flow chart of if/else selection structure (program 2).
//program to calculate the sales commission
#include
using namespace std;
int main()
{
float sales, commission;
cout<<"enter the sales"<
cin>>sales;
if (sales>1500)
commission = sales*0.2;
else
commission = sales*0.1;
cout<<"commission is"<
return 0;
}

Output of program 2
Sales is 1500
Sales is greater than 1500

Example of if/else
Write a program to calculate the gross pay of an employee.
The employee is paid at his basic hourly rate for the first 40 hours worked during a week. Any hours worked beyond 40 is considered overtime.
overtime is paid at the 1.5 times the hourly rate.
Pseudocode
If total number of hours >40
Gross pay = rate*40 + rate*1.5(hours – 40)
Else
Gross pay = rate * hours

C++ code
#include
using namespace std;
int main()
{
int hour; //declaration
double gross_pay, rate;
cout<<"enter the hourly rate of pay\t"<
cin>>rate; //reading data
cout<<"enter the number of hours worked \t"<
cin>>hour; //readin data
if(hour>40) //condition if working hours is greater than 40
gross_pay = rate*40 + 1.5*rate*(hour - 40); //gross pay including extra hour
else //if working hour is <=40 then use this formula to get gross pay
gross_pay = rate*hour;
cout<<" Gross pay is \t"<
return 0;
}

output
Hour<40
Hour>40

Nested if/else structure
if structure that rests entirely within another if structure, within either the if or the else clause
One inside another, test for multiple cases
Once condition met, other statements skipped

Simple example to understand the nested if/else
Consider an example of a program segment that accepts a gender code and print gender according to that code.
Pseudocode
if the gender code is F
print female
else
if the gender code is M
print male
else
print invalid code

Simple example to understand the nested if/else
Flow chart

Simple example to understand the nested if/else
C++ code
//F and M are correct codes
//f and m are invalid codes

Nested if/else structure
following pseudocode is example of nested if/else
if student’s grade is greater than or equal to 90
Print “A”
else if student’s grade is greater than or equal to 80
Print “B”
else if student’s grade is greater than or equal to 70 Print “C”
else if student’s grade is greater than or equal to 60 Print “D”
else
Print “F”

Nested if/else structure
C++ code
if ( grade >= 90 ) // 90 and above
cout << "A";
else
if ( grade >= 80 ) // 80-89
cout << "B";
else
if ( grade >= 70 ) // 70-79
cout << "C"; else
if ( grade >= 60 ) // 60-69
cout << "D";
else // less than 60
cout << "F";
if grade>=90, first four conditions will be true. But only the cout statement after the first test will be executed. After that cout is executed, the else part of the outer if/else statement is skipped.

Avoiding Common Pitfalls with if Statements
Forgetting that C++ is case sensitive
Assuming that indentation has a logical purpose
Adding an unwanted semicolon
Forgetting curly braces
Using = instead of == (explained with example on next slide)
Making unnecessary comparisons