Control flow stataements

© 2015UPESJuly 2015 Department. Of Civil Engineering
DECISION CONTROL AND
LOOPING STATEMENTS

DECISION CONTROL STATEMENTS
 Decision control statements are used to alter the flow of a sequence of instructions.
 These statements help to jump from one part of the program to another depending on whether
a particular condition is satisfied or not.
 These decision control statements include:
If statement
If else statement
If else if statement
Switch statement

IF STATEMENT
 If statement is the simplest form of decision control statements that is frequently used in
decision making. The general form of a simple if statement is shown in the figure.
 First the test expression is evaluated. If the test expression is true, the statement of if block
(statement 1 to n) are executed otherwise these statements will be skipped and the execution
will jump to statement x.
SYNTAX OF IF STATEMENT
if (test expression)
{
statement 1;
..............
statement n;
}
statement x;
Test
Expression
Statement Block 1
TRUE
Statement x
FALSE

IF ELSE STATEMENT
 In the if-else construct, first the test expression is evaluated. If the expression is true, statement block 1 is
executed and statement block 2 is skipped. Otherwise, if the expression is false, statement block 2 is
executed and statement block 1 is ignored. In any case after the statement block 1 or 2 gets executed the
control will pass to statement x. Therefore, statement x is executed in every case.
SYNTAX OF IF STATEMENT
if (test expression)
{
statement_block 1;
}
else
{
statement_block 2;
}
statement x;
Test Expression
Statement Block 1
TRUE
Statement x
FALSE
Statement Block 2

// Program to demonstrate the use of if-statement
#include<stdio.h>
int main()
{
int x=10;
if ( x>0)
x++;
printf("n x = %d", x);
return 0;
}
PROGRAMS TO DEMONSTRATE THE USE OF
IF STATEMENT

 // PROGRAM TO FIND WHETHER A NUMBER IS EVEN OR ODD
 #include<stdio.h>
 main()
 {
 int a;
 printf("n Enter the value of a : ");
 scanf("%d", &a);
 if(a%2==0)
 printf("n %d is even", a);
 else
 printf("n %d is odd", a);
 return 0;
 }
PROGRAMS TO DEMONSTRATE THE USE OF
if elseSTATEMENT

IF ELSE IF STATEMENT
 C language supports if else if statements to test additional conditions apart from the initial test
expression. The if-else-if construct works in the same way as a normal if statement.
SYNTAX OF IF-ELSE STATEMENT
if ( test expression 1)
{
statement block 1;
}
else if ( test expression 2)
{
statement block 2;
}
...........................
else if (test expression N)
{
statement block N;
}
else
{
Statement Block X;
}
Statement Y;
Test Expression 1
Statement Block 1
TRUE FALSE
Test Expression 2
Statement Block 2
TRUE
FALSE
Statement Block X
Statement Y

// PROGRAM TO CLASSIFY A NUMBER AS POSITIVE, NEGATIVE OR
ZERO
#include<stdio.h>
main()
{
int num;
printf("n Enter any number : ");
scanf("%d", &num);
if(num==0)
printf("n The value is equal to zero");
else if(num>0)
printf("n The number is positive");
else
printf("n The number is negative");
return 0;
}

SWITCH CASE
 A switch case statement is a multi-way decision statement. Switch statements are used:
1. When there is only one variable to evaluate in the expression. 2. When many conditions are
being tested for.
 Switch case statement advantages include:1. Easy to debug, read, understand and maintain. 2.
Execute faster than its equivalent if-else construct .
switch(grade)
{case 'A':
printf("n Excellent");
break;
case 'B':
printf("n Good");
break;
case 'C':
printf("n Fair");
break;
default:
printf("n Invalid Grade");
break; }

 int number = 10;
if(number == 1)
{
printf("Given number is 1n");
}
else if((number == 2)
{
}
else if((number ==3)
{
}
else if((number ==4)
{
}
else if (number ==5)
{
}
else
{
if(number<0)
printf("Given number is negativen");
else
printf("Given number is greater than 5n");

 // PROGRAM TO PRINT THE DAY OF THE WEEK
 #include<stdio.h>
 int main( )
 { int day;
 printf(“n Enter any number from 1 to 7 : “);
 scanf(“%d”,&day);
 switch(day)
 { case 1: printf(“n SUNDAY”); break;
 case 2 : printf(“n MONDAY”); break;
 case 3 : printf(“n TUESDAY”); break;
 case 4 : printf(“n WEDNESDAY”); break;
 case 5 : printf(“n THURSDAY”); break;
 case 6 : printf(“n FRIDAY”); break;
 case 7 : printf(“n SATURDAY”); break;
 default: printf(“n Wrong Number”);
 } return 0; }

The ?: Operator
 ?:- is known as conditional operator and is useful for making two way
decisions.
 It takes three arguments.
 Conditional expression? Expression1:exprssion2
 The conditional expression is evaluated first. If the result is nonzero,
expression1 is evaluated and is returned as the value of conditional expression.
Otherwise expression2 is evaluated and its value is returned.
If (x<0)
flag=0;
else
flag=1; can be written as flag=(x<0)?: 0:1;

Programming Exercises
 WAP to determine whether a given number is ODD or EVEN and print the
message NUMBER IS EVEN or NUMBER IS ODD
 Write a program to find the biggest of 3 numbers using conditional
operator/ternary operator?
 Write a program to find the roots of a quadratic equation?
 Program to find the average of students marks, when fail in any subject then
result is ‘FAIL’ otherwise print the grade as pass /first class/distinction
 Suppose the postal rates for mailing letters are as follows:
Rs.0.50 per 10 grams for the first 50 grams.
Rs.0.40 per 10 grams for the next 100 grams.
Rs.0.25 per 10 grams for the next 250 grams and
Rs25 per kilogram for the letters weighing more than 400 grams.
Write a program that prompts for the weight of a letter and prints the postage to
be paid.

ITERATIVE STATEMENTS
 Iterative statements are used to repeat the execution of a list of statements, depending on the
value of an integer expression. In this section, we will discuss all these statements.
While loop
Do-while loop
For loop
WHILE LOOP
• The while loop is used to repeat one or more statements while a particular condition is true.
• In the while loop, the condition is tested before any of the statements in the statement block is executed.
• If the condition is true, only then the statements will be executed otherwise the control will jump to the
immediate statement outside the while loop block.
• We must constantly update the condition of the while loop.
while (condition)
{
statement_block;
}
statement x;
Statement x
Condition
Statement y
FALSE
Update the condition
expression
Statement Block
TRUE

DO WHILE LOOP
 The do-while loop is similar to the while loop. The only difference is that in a do-while loop, the
test condition is tested at the end of the loop.
 The body of the loop gets executed at least one time (even if the condition is false).
 The do while loop continues to execute whilst a condition is true. There is no choice whether to
execute the loop or not. Hence, entry in the loop is automatic there is only a choice to continue it
further or not.
 The major disadvantage of using a do while loop is that it always executes at least once, so even
if the user enters some invalid data, the loop will execute.
 Do-while loops are widely used to print a list of options for a menu driven program.
Statement x;
do
{
statement_block;
} while (condition);
statement y;
Statement x
Statement y
Statement Block
Update the condition expression
Condition
FALSE
TRUE

FOR LOOP
 For loop is used to repeat a task until a particular condition is true.
 The syntax of a for loop
for (initialization; condition; increment/decrement/update)
{
statement block;
}
Statement Y;
 When a for loop is used, the loop variable is initialized only once.
 With every iteration of the loop, the value of the loop variable is updated and the condition is
checked. If the condition is true, the statement block of the loop is executed else, the
statements comprising the statement block of the for loop are skipped and the control jumps to
the immediate statement following the for loop body.
 Updating the loop variable may include incrementing the loop variable, decrementing the loop
variable or setting it to some other value like, i +=2, where i is the loop variable.

PROGRAM FOR FOR-LOOP
Look at the code given below which print first n numbers using a for loop.
#include<stdio.h>
int main()
{
int i, n;
printf(“n Enter the value of n :”);
scanf(“%d”, &n);
for(i=0; i<= n; i++)
printf(“n %d”, i);
return 0;
}

Nested Loop
 In many cases we may use loop statement inside another looping
statement. This type of looping is called nested loop. In nested loop
the inner loop is executed first and then outer.

Jumps in Loops
 Sometimes, while executing a loop, it becomes
necessary to skip a part of the loop or to leave the loop
as soon as certain condition becomes true, that is called
jumping out of loop. C language allows jumping from
one statement to another within a loop as well as
jumping out of the loop.

Break Statement
 The break statement is a jump instruction and can be
used inside a switch construct, for loop, while loop and
do-while loop. The execution of break statement
causes immediate exit from the concern construct and
the control is transferred to the statement following the
loop. In the loop construct the execution of break
statement terminates loop and further execution of the
program is reserved with the statement following the
body of the loop.

CONTINUE STATEMENT
• continue statement is a jump statement. The continue
statement can be used only inside for loop, while loop and
do-while loop. Execution of these statement does not cause
an exit from the loop but it suspend the execution of the
loop for that iteration and transfer control back to the loop
for the next iteration.

GOTO STATEMENT
 The goto statement is used to transfer control to a specified label.
 Here label is an identifier that specifies the place where the branch is to be made. Label can be any valid variable name that is
followed by a colon (:).
 Note that label can be placed anywhere in the program either before or after the goto statement. Whenever the goto statement is
encountered the control is immediately transferred to the statements following the label.
 Goto statement breaks the normal sequential execution of the program.
 If the label is placed after the goto statement then it is called a forward jump and in case it is located before the goto statement, it
is said to be a backward jump.
 int num, sum=0;
 read: // label for go to statement
 printf("n Enter the number. Enter 999 to end : ");
 scanf("%d", &num);
 if (num != 999)
 {
 if(num < 0)
 goto read; // jump to label- read
 sum += num;
 goto read; // jump to label- read
 }
 printf("n Sum of the numbers entered by the user is = %d", sum);

Control flow stataements

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