c programming control flow statements

A statement is an instruction that causes an action to be performed when executed. The C statements end with ‘;’.

In C there are 6 types of statements are available.

Selection statement
Iteration statement
Jumping statements
Label statements
Expression statement
Block statement

Selection statements are statements that are executed depending on the condition. These include if, if-else and switch.
Iteration statement is executed repeatedly until a condition is satisfied. These include for, while, do-while.
Jumping statements is alter the flow of execution. These are break, continue and goto.
Label statements label statement is used to give the location of the target statement to which control must transfer. Ex: case, default, and label.
Expression statement the statements composed of valid expression are expression statements.
Block statement a block consists of a set of statements enclosed within flower braces. These are also called compound statements.

Decision control instructions in C

We have seen that a C program is a set of statements which are normally executed sequentially in the order in which they appear. By default, the instructions in a program are executed sequentially.

In serious programming situations, we want a set of instructions to be executed in one situation, and a different set of instructions to be executed in another situation. This kind of situation is dealt with in C programs using decision control instruction.

A decision control instruction can be implemented in C using

if statement
conditional operator statement
switch statement
goto statement

If statement

The if statement is a powerful decision-making statement and is used to control the flow of execution of the statement.The keyword if tells the compiler that what follows is a decision control instruction.

The condition following the keyword if is always enclosed within a pair of parentheses. If the condition, whatever it is true, then the statement is executed.If the condition is not true, then the statement is not executed.

As a general rule, we express a condition using C’s ‘relational and logical operators’. The if statement may be implemented in different forms depending on the complexity of conditions to be tested. The different forms are:

Simple if statement
if-else statement
Nested if-else statement
else if ladder

Simple if Statement

The below figure describes the general form of a simple if statement. The ‘statement-block’ may be a single statement or a group of statements. If the condition is true, the statement block will be executed otherwise the statement-block will be skipped and the execution will jump to the statement-x. Remember, when the condition is true both the statement-block and statement-x are executed in sequence.

[quote]Note: In simple, if statement, only one statement is there no need to open body.[/quote]

Simple if syntax:[quote]

if ( condition )
{

statement-block;
}

Statement-x;[/quote]

if-else Statement

The if statement by itself will execute a single statement or a group of statements when the if condition true. When the condition is false, can we execute another group of statements? This is the purpose of else statement. The if-else statement is an extension of the simple if statement. The below figure describes a general form of if else. If the condition is true, the true-block statement(s), immediately following the if statements are executed; otherwise, the false-block statement(s) are executed.

If-else syntax

if ( condition )
{ 
   TRUE-Block…statement (s);
}
else
{
  FALSE-Block……statement (s);
}

Nested if-else Statement

When a series of decisions are involved, we may have to use more than one if-else statement in nested form. If the condition-1 is false, the statement-3 will be executed; otherwise, it continues to perform the second condition. If the condition-2 is true, the statement-1 will be evaluated; otherwise, the statement-2 will be evaluated and then the control jumps to statement-4.

[quote]Note: Else is always matching with the most recent unpaired if.[/quote]

if ( condition-1 )
{
     if( condition-2 )
     {
        statement1;
     }
     else
     {
          statement2;
      }
}
else
{
     statement3;
}
statement4;

Different Forms of if – else

if ( condition)

statement1;

if ( condition ) {

statement1;

………….n;

}

if ( condition)

statement1;

else

statement2;

if ( condition) {statement1;

……..n;

}

else {

statement1;

………n;

}

Conditional operator

The conditional operators ? and : are sometimes called ternary operators since they take three arguments. Their general form is,
expression 1 ? expression 2 : expression 3;

What this expression says is: “if expression 1 is true ( that is if its value is nonzero), then the value returned will be expression 2, otherwise the value returned will be expression 3”.

[quote]Note:

The number of question marks and colon should be equal.
Every colon should match with just before question marks.

[/quote]

Switch statement

The control statement that allows us to make a decision from the number of choices is called a switch.

Rules for switch statement:

The switch expression must be an integer type.
case labels must be constants or constants expression.
case labels must be unique.
case labels must end with a colon.
The break statement transfers the control out of the switch statement.
The break statement is optional. That is, two or more case labels may belong to the same statement.
The default label is optional. If present, it will be executed when the expression does not find a matching case label.
There can be at most one default label.
The default may be placed anywhere but usually placed at the end.
It is permitted to next switch statement.

[quote]Note:

Using case and default must be within switch body.
Using break is optional you can use in switch or loop body.

[/quote]

The integer expression following the keyword switch is any C expression that will yield an integer value or characters. It could be an integer constant or an expression that evaluates to an integer. The keyword case is followed by an integer or a character constant. Each of these values should be unique. Block-1, block-2.. are statement lists and may contain zero or more statements. There is no need to put braces around these blocks. Note that case labels must end with a colon (:).

What happens when we executed a program containing a switch? First, the integer expression following the keyword switch is evaluated, and compared against the values constant1, constan2,… if a matching case is found then the block of statements that follows the case is executed up to break(optional if it is there) or all statements from matching case including default, and transferring the control to the statement-x following the switch.

The default is an optional case. It will be executed if the value of the integer expression does not match with any of the case values. If not present, no action takes place if all matches fail and the control goes to the statement-x.

switch ( integer expression )
{
     case constant1:
       block-1;
       break;
     case constant2:
       block-2;
       break;
     ………….
     default:
      statement;
      break;
}
statement-x;

goto statement

The goto statement to branch unconditionally from one point to another point in the program. The goto requires a label in order to identify the place where the branch is to be made.

A label is any valid variable name and must be followed by a colon. The label is placed immediately before the statement where the control is to be transferred. The general form of goto and label statements is shown below:

Note that a goto breaks the normal sequential execution of the program. If the lable: is before the statement goto label; a loop will be formed and some statements will be executed repeatedly. Such a jump is known as a backward jump. On the other hand, if the label: is placed after the goto label; some statements will be skipped and jump is known as a forward jump.

goto label;
  ………….
  ………….
label:
  statement;label:
  statement;
.…………
………….
goto label;

Decision making and looping

In all programs written so far, execution begins at the start of the main( ) function and proceeds line by line until the end of the program. But if you want to print something repeatedly, say printing something on screen five times, you can include 5 print statements. But if you want to print 100 times it cannot be done like this. To do repetitive tasks, we use loops.

In C there are three types of loops. Such as,

while loop.
do-while loop.
for loop.

Looping Process includes four steps. They are As follows,

Setting And initializing variables (counter)
Evaluate the test EXPRESSION
Execute The body Of loop if the test expression is TRUE
Increment or decrement the counter variable.

while Loop

The while statement will be executed repeatedly as long as the Expression Remains true. Its syntax is as follows,

The body of the loop will be executed repeatedly as long as the condition remains true. When the while is reached, the computer evaluates condition. If it is found to be false, the body of the loop will not be executed, and the loop ends. Otherwise, the body of the loop will be executed, and then the condition is checked again and so on until the condition becomes false.

The syntax for while:

while(condition)
{    
  ..statement1;
   body of loop;
}

/* Ex1: calculating sum of first 5 Natural numbers */ 
#include <stdio.h> 
void main() 
{ 
    int i,sum=0;sum=0; 
    i=1; /* initialization */ 
    while( i<=5) /* condition */ 
    { 
        sum=sum+i; 
        i=i+1; /* incrementing */ 
    } 
    printf("sum=%d",sum); 
}

The code given above finds the sum of the first 5 Natural numbers. The execution of this while statement is don as follows,

Variables i and sum are initialized.
Condition (i<=5) is evaluated. The result is true i.e. hence body of while loop is executed.
The sum is evaluated and i is incremented by 1.
Again expression (i<=5) is evaluated. The result is true so body of loop is executed again. This process is continued until the condition becomes false.

do-while Loop

It is similar to while loop except that it always executes at least once. The test of expression for repeating is done after each time body of the loop is executed. The syntax for do-while is as follows,

Syntax for do-while:

do
{     
   ….statement1;
   body of loop;
}while(condition);

/* Ex2: calculating sum of first 5 Natural numbers */ 
#include <stdio.h> 
void main() 
{ 
    int i,n,sum; 
    printf("ENTER A NUMBER"); 
    scanf("%d",&n); sum=0; 
    i=1; /* initialization */ 
    do{ 
        sum=sum+i; 
        i=i+1; /* incrementing */ 
    } while( i<=n); /* condition */ 
    printf("\nsum=%d",sum); 
}

[quote]Output:
ENTER A NUMBER 5
sum=15[/quote]

The below example is used to find sum numbers from 1 to given input numbers this do-while loop is executed as given below,

Variables sum and i are initialized, and n value will enter at runtime (n=5).
Body of the loop is executed and sum, i values are computed.
Expression (i<=n) is evaluated. It is true to the body of the loop is executed again.
The loop is executed repeatedly until the condition “i<=n” (n=5) becomes false (it happens when i=6).

for Loop

For statement is another type of looping statement. the difference between for and while, the do-while statement is that, in while statements that the number of passes is not known in advance whereas in for- statement the number of passes is known in advance. The general syntax to for loop is

Syntax of for loop:

for( initialization; condition ; iteration )
{
      ….statement1;
     body of loop;
}

This loop contained three parts; these are

initialization
condition
iteration

It is possible to omit all the three expressions in the for a statement but it is necessary to have two semicolons. If assignment expression and unary expression are omitted, then another way should be present to initialize and modify the initial value. If the conditional expression is omitted then it is assumed that it has a static value 1 witch it true.

This static value makes the looping structure to be executed infinitely until terminated by using a break or return statement.

When for loop is executed for the first time the value will initialize. Then it checks the condition if the condition is true and the body of the loop is executed for the first time.
Upon reaching the closing brace of for, control is sent back to iteration part of the loop. Again it checks the condition, then the condition is true and the body of the loop is executed otherwise loop is terminated and execution continues with the next statement that immediately following the loop.

/* Ex1: calculating sum of first 5 Natural numbers */ 
#include <stdio.h> 
void main() 
{ 
    int i,n,sum; 
    printf("ENTER A NUMBER"); 
    scanf("%d",&n); 
    sum=0; 
    for( i=1 ; i<=n ; i++ ) 
    { 
        sum=sum+i; 
    } 
    printf("\nsum=%d",sum); 
}

[quote]Output:
ENTER A NUMBER 5
sum=15[/quote]

The above loop where i=1 is an expression that specifies the initial value and that controls the looping action. i<=5 is a condition, that is tested at each pass. The program is executed until this condition remains true. i++ is a unary expression, that is either increment or decrement to change the initial value. The conditional expression is evaluated and tested at the beginning while unary expression is evaluated at the end of each pass.

Nesting the loops

Nesting of the loop, that is one loop statement placing into another loop.

while ( condition )
{
       while loop body;
       for( initialization; condition ; iteration )
       {
             for loop body;
        }
}

break and continue

break and continue are an unconditional statement which is used within a program to alter the flow of control.

break: ‘break’ is a keyword used to terminate the loop or exit from a switch statement. It is written as,
statements; break;

It can be used with for, while, do-while and switch statement.

/* Ex1: Take char intput from user choice and prints */ 
#include <stdio.h> 
void main() 
{ 
    char choice; 
    scanf("%c",&choice); 
    switch(choice) 
    { 
        case 'y': 
            printf("yes"); 
            break; 
        case 'n': 
            printf("no"); 
            break; 
    } 
}

Here break statement is used in order to transfer control out of switch statement.

#include <stdio.h> 
void main() 
{ 
    int i; i=1; 
    while( i<5 ) 
    { 
        if( i%2==0) 
        break; 
        i++; 
    } 
}

when i=2, i%2==0
Hence, the break statement is executed and the loop terminates.

continue: ‘Continue’ is a keyword used to Skip the same specific statement in the loop body. It must be used with while, for and do-while.

#include <stdio.h> 
void main() 
{ 
    int i; 
    i=0; 
    while(i <= 100) 
    { 
        i+=2; 
        if( i>40 && i<60) 
            continue; 
        printf("%d ",i); 
    } 
}

In the above loop, when the value of ‘i’ is 42, continue statement is executed. So, i=i+2 is skipped and control is transferred to the beginning of while loop.

c programming control flow statements