Control structure C++

Control Structures
ANIL KUMAR
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Control Structures
• Programs are written using three basic structures
– Sequence
• a sequence is a series of statements that execute one
after another
– Repetition(loop or iteration)
• repetition (looping) is used to repeat statements while
certain conditions are met.
– Selection(branching)
• selection (branch) is used to execute different
statements depending on certain conditions
• Called control structures or logic structures

Sequence
• The sequence structure
directs the computer to
process the program
instructions, one after
another, in the order
listed in the program

Selection
(Branch)
• Selection structure:
makes a decision
and then takes an
appropriate action
based on that
decision
– Also called the
decision structure

Repetition (Loop)
• Repetition structure:
directs computer to
repeat one or more
instructions until some
condition is met
– Also called a loop or
iteration

Flow Control
• How the computer moves through the
program
• Many keywords are for "flow control"
int, float, double, char,
break, case, continue, default, do, else, for,
goto, if, return, switch, while

Normal flow
Statement 1;
Statement 2;
Statement 3;
Statement 4;

Flow control
 Selection (Branch)
 if
 if-else
 nested if
 (goto)
 switch
 Repetition (Loop)
 while
 do-while
 For
 nesting

Conditional Statements
• if
• if else
• nested if (if – else if – else if – else)
• statement blocks ({…})
• (goto)
• switch (case, default, break)

if
• if(condition)statement
if (x==100)
cout<<“x is 100”;
 If the condition is true, statement is
executed.
 If the condition is false, statement is not
executed.

if flow
Condititional Statementif(…)
true
false

if else
 if (condition)statement1
else statement2
if (x==100)
cout<<“x is 100”;
else
cout<<“x is not 100”;
 If the statement1 is true, then print out on the screen x is
100
 If the statement2 is true, then print out on the screen x is
not 100

if else flow
Condititional Statementif(…)
else Statement
true
false

Statement Blocks
 If we want more than a single instruction is executed, we must
group them in a in a block of statements by using curly brackets
({…})
if (x>0)
{
cout<<“x is positive”;
}
else
{
cout<<“x is negative”;
}

Nested if statements
 When if statement occurs with in another if statement, then such type of if statement is called nested if statement.
 if (condition1)
if (condition2)
statement-1;
else
statement-2;
else
statement-3;

Nested if statements
 In this program the
statement if(a>c) is nested
with in the if(a>b).
 if (a>b) is true only then
the second if statement
if(a>c) is executed.
 If the first if condition is
false then program control
shifts to the statement
after corresponding else
statement.
if(a>=b && a>=c)
cout<<“a is
biggest”;
elseif(b>=a &&
b>=c)
cout<<“b is
biggest”;
Else
cout<<“c is
biggest”;

Nested if flow
Condititional Statement 2else if
else if Condititional Statement 3
Else Statement
Condititional Statement 1if true
true
true
false
false
false

goto
• It allows making an absolute jump to another point in the
program.
• "Go to" part of a program
#include<iostream.h>
int main()
{
int n=10;
loop:
cout<<n<<“,”;
n--;
if(n>0) goto loop;
cout << “FIRE!“;
return 0;
}

Switch Statement
• Like the goto statement but more structured
– Structure is good - less confusing
– Its objective is to check several possible constant
values for an expression and Similar to if-elseif-
elseif-else but a little simpler.
– So the switch statement is better than goto !

Switch flow
Condititional Statement 2case 2
case 3 Condititional Statement 3
Condititional Statement 1case 1
switch

Switch statement
switch(expression) {
case constant1:
block of instructions 1
break;
case constant2:
block of instructions 2
break;
default:
default block of instructions
}

Switch statement
 Switch evaluates expression and checks if it is equivalent to
constant1, if it is, it executes block of instructions 1 until it
finds the break keyword, then the program will jump to the
end of the switch selective structure.
 If expression was not equivalent to constant1, it will check if
expression is equivalent to constant2. if it is, it executes block
of instructions 2 until it finds the break keyword.
 Finally if the value of expression has not matched any of the
specified constants, the program will execute the instructions
included in the default: section, if this one exists, since it is
optional.

Switch statement
switch(x) {
case 1:
cout<<“x is 1”;
break;
case 2:
cout<<“x is 2”;
break;
default:
cout<<“value of x is unknown”;
}

Loops and iterations
• Loops have as objective to repeat a certain number of
times or while a condition is fulfilled. When a single
statement or a group of statements will be executed
again and again in the program then such type of
processing is called loop. Loop is divided into two parts
Body of loop
Control of loop

 Control of loop is divided into two parts:
 Entry control loop- in this first of all condition is checked
if it is true then body of the loop is executed. Otherwise
we can exit from the loop when the condition becomes
false. Entry control loop is also called base loop.
Example- While loop and for loop
 Exit control loop- in this first body is executed and then
condition is checked. If condition is true, then again body
of loop is executed. If condition is false, then control will
move out from the loop. Exit control loop is also called
Derived loop. Example- Do-while

• The loops in statements in C++ language are-
While loop
Do loop/Do-while loop
For loop
Nested for loop

While Loop
 while (expression) statement
 While loop is an Entry control loop
 And its function is simply to repeat statement while
expression is true.
Condititional Statementswhile(…)
true
false

While loop
int main()
{
int n;
cout<<“Enter the starting number”;
cin>>n;
while(n>0){
cout<<n<<“,”;
n--;
}
return 0;
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For loop
• For loop is an Entry control loop when action is to be
repeated for a predetermined number of times.
• Most used – most complicated
• Normally used for counting
• Four parts
– Initialise expression
– Test expression
– Body
– Increment expression
for(initialization;condition;increment)

for loop
body
statements
continuation
test
initialisation
increment
true
false

For loop
int main()
{
int count;
for (count=1; count<=10; count++)
{
cout <<count<<“,”;
}
cout<<“FIRE”;
return 0;

Do-While Loop
• do statement while (expression)
• Do-while is an exit control loop. Based on a condition,
the control is transferred back to a particular point in the
program.
– Similar to the while loop except that condition in
the do while is check is at end of loop not the start
do {
action1;
}
while (condition is true);
action2;

Do Flow
condititional statements
while(…)
true
false
do

Nesting of Loops
• A loop can be inside another loop. C++ can
have at least 256 levels of nesting.
for(init;condition;increment)
{
for(init;condition;increment)
}
statement(s);
}
statement(s);

Break Statement
 Goes straight to the end of a do, while or for
loop or a switch statement block,
int main(){
int n;
for(n=10;n>0;n--){
cout<<n<<“,”;
if(n==5){
cout<<“count down aborted!”;
break;}}
return 0;}
O/P: 10,9,8,7,6,5,count down
aborted!

Continue Statement
 Goes straight back to the start of a do, while or
for loop,
int main()
{
int n;
for(n=10;n>0;n--){
if(n==5)continue;
cout<<n<<“,”;}
return 0;}
O/P: 10,9,8,7,6,4,3,2,1,FIRE!

Summary
 Programs: step-by-step instructions that tell a
computer how to perform a task
 Programmers use programming languages to
communicate with the computer
 First programming languages were machine languages
 High-level languages can be used to create procedure-
oriented programs or object-oriented programs
 Algorithm: step-by-step instructions that accomplish a
task (not written in a programming language)
 Algorithms contain one or more of the following control
structures: sequence, selection, and repetition

Summary (continued)
• Sequence structure: process the instructions,
one after another, in the order listed
• Repetition structure: repeat one or more
instructions until some condition is met
• Selection structure: directs the computer to
make a decision, and then to select an
appropriate action based on that decision

Control structure C++

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Control structure C++