Control Structure

 

A program is usually not limited to a linear sequence of instructions. During its process it may bifurcate, repeat code or take decisions. For that purpose, C provides control structures:

A block of instructions is a group of instructions separated by semicolons (;) but grouped in a block delimited by curly bracket signs shown below. If only one statement is present

Selection Statement/Branching Statement

1. if statement -  It executes an instruction or block of instructions only if a condition is fulfilled. Its form is

    if (condition){

          statement;

    }

where condition is the expression that is being evaluated. If this condition is true, statement is executed. If in is false, statement is ignored (not executed) and the program continues on the next instruction after the conditional structure.

Note: 1. If statement - It does the selection of none/one block out of given single block.

Note:  If only one instruction is present under selection or iterative statement then Block is Optional.

If  we want more  than  one  instruction  to be included under any selection or iterative statement  then, block { } is compulsory. It is always good programming practice to include a block whether selection iterative statement contains one or more statements. 

2. if-else statement -

Syntax:

if (condition)
{

    /*statement(s) will execute if the condition is true */

}
else

{
   /* statement(s) will execute if the condition is false */

}

If the condition evaluates to true, then if block of code will be executed, otherwise else block of code will be executed.

Note:

if-else statement: It does the selection of only one block from the given two blocks.

Flow Diagram :

For example

if (x==100)

{

  printf("x is 100");

}

else

{

  printf(“x is not 100”);

}

prints out on the screen x is 100 if indeed x is worth 100, but if is not and only if not-it prints out x is not 100.

3. if-else ladder statement- 

if-else ladder allows to select only one block (choice) from the given N Blocks (choices). If conditional true then statements present in the block1  will be executed. If the condition 1 is false then condition 2 is checked.  If condition is true then statements present in the block2 will be executed. Similarly remaining conditions will be checked and respective block will be executed. If condition 1, condition2 up to condition N are fake then last else block (Block N + 1) will be executed.

Note:

if-else Ladder: It does the selection of only one block from the given N-Blocks.

Nested if-else statement-

It is always legal in C programming to nest if-else statements, which means you can use one if or else if statement inside another if or else if statement(s).

Syntax:

The syntax for a nested if statement is as follows:

You can nest else if else in the similar way as you have nested if statement.

Code

#include <stdio.h>
void main()
{
	int a = 100;
	int b=200;
	if(a==100) /* Check condition */
	{
	 if(b==200)    /* if condition is true then check the following */
	 {
	   printf("Value of a is 100 and b is 200\n"); /*if condition is true then print the following */
 	 }
    }
    printf("Exact value of a is: %d\n", a );
    printf("Exact value of b is %d\n", b),
    getch();
}

When the above code is compiled and executed, it produces the following result:

Value of a is 100 and b is 200

Exact value of a is : 100

Exact value of b is : 200

4. Switch case

Note: It does the selection of one or more blocks from the given N-blocks,

It works in the following way:

switch evaluates expression and checks if it is equivalent to constant1,  if it is , it executes block of instructions  until it finds the break keyword, then the program will jump to the end of the switch selective structure, if expression was not equal to constant 1 it will check if expression is equivalent to constant2, if it is, it will execute block of instructions 2 until it finds the break keyword. Finally, if the value of expression has not matched any of the previously specified constants, (You may specify as many case sentences as values you want to check), the program will execute the instructions included in the default: section, if this one exists, since it is optional. 

Notice the inclusion of the break instructions at the end of each block. This is necessary because if, for example, we did not include it after block of instructions 1 the program would not jump to the end of the switch selective block (}) and it would continue executing the rest of the blocks of instructions until the first appearance of the break instruction or the end of the switch selective block.

Iterative/ Multi-pass Statements - Loops are used to execute the same set of statements n number of times.

1. The for loop -  Here is syntax and flow of control in for loop:

Note : If fixed  number of times execution is already known in advance, then we use for loop.

It works the following way:

1. The initialization step is executed first, and only once. This step allows you to declare and initialize any loop control variables. You are not required to put a statement here, as long as a semicolon appears 

2. Next, the condition is evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop does not execute and flow of control jumps to the next statement just after for loop.

3. After the body of for loop executes, the flow of control jumps back up to the increment statement. The statement allows you to update any loop control variables. This statement can be left blank, as long as a semicolon appears after the condition.

4. The condition is now evaluated again. If it is true, the loop executes and the process repeats itself (body of loop, then increment step, and then again condition). After the condition becomes false, for loop terminates Here is an example of countdown using a for loop.

// countdown using a for loop 

Code

#include <stdio.h>
main ()
{
 for (int n-10; n>0; n--)
 {
  printf("%d",n);  
 }
 printf("FIRE!");
}

Output:

10, 9, 8, 7, 6, 5, 4, 3, 2, 1, FIRE!

The initialization and increase fields are optional. They can be avoided but not the semicolon signs among them. For example we could write: for (;n<10;)

if  we want to specify no initialization and no increase, or for (;n<10; n++) if we want to include an increase field but not an initialization.

Optionally, using the comma operator (,) we can specify more than one instruction in any of the fields included  in a for loop, like in initialization, for example. The comma operator (,) is an instruction separator, it serves to separate more than one instruction where only one instruction is generally expected. For example, suppose that we wanted to initialize more than one variable in our loop:

for (n=0, i=100; n!=i ; n++, i --)

{

            //whatever here...

}

This loop will execute 50 times if neither n nor i are modified within the loop

n starts with 0 and i with 100, the condition is (n!=i) (that n be not equal to i). Because n is increased by one and i decreased by one, the loop's condition will become false after the 50th loop, when both n and i will be equal to 50.

2.  while loop - It repeats same set of statements while expression is true

Syntax -

Note:  When we don't know in advance, how many times loops will be executed then we have to use while loop.

For example, we are going to make a program to count down using a while loop

Code

#include <stdio.h> 
void main ()
{
 int n; 
 printf("Enter the start no"); 
 scanf("%d", &n); 
 while (n>0)
 {
  printf("%d", n); 
  --n;
 }
 printf ("FIRE!"); 
}

Output

Enter the starting number 8

8, 7, 6, 5, 4, 3, 2, 1, FIRE!

while loop begins if the value entered by the user fulfills the condition n>0 (that n be greater than 0) the block of instructions that follows will execute an indefinite number of times while the condition (n>0) remains true.

We must provide some method that forces condition to become false at some moment, otherwise the loop will continue looping forever. In this case we have included --n; therefore when n becomes 0, that is where our countdown ends.

3. The do-while loop

Syntax:

Its functionality is exactly the same as the while loop except that condition in the do-while is evaluated after the execution of statement instead of before, granting at least one execution of statement even if condition is never fulfilled.

For example, the following program echoes any number you enter until you enter 0.

Code

#include <stdio.h>
void main ()
{
 long int n;
 do
 {
  printf(“Enter no from 0 to end”);
  scanf(“%d”, &n);
  printf(“You entered:=%id\n”,n); 
 } while (n!=0)
}

The do-while loop is usually used when the condition that has to determine its end is determined within the loop statement, like in the previous case, where the user input within the block of instructions is what  determines the end of the loop. If you never enter the 0 value in the previous example the loop will never end.

Jump Statements -

1. The break instruction : 

Using break we can leave a loop even if the condition for its end is not fulfilled. It can be used to end an infinite loop, or to force it to end  before  its natural end. For example, we are going to stop the count down before it naturally finishes (an engine failure maybe):

Code

#include <stdio.h>
void main ()
{ 
 int i ;
 for (i=1; i<=10;i++)     
 {
  if (i%2==0)
  {
   printf(“%d “,i);
   break;
  }
 }
}

2. The continue instruction:

The continue instruction cause the program to skip the rest of the loop of the present iteration as if the end of the statement block would have been reached. Causing it to jump to the following iteration.

It causes the control to go directly to the test-condition and then continue the loop process. On encountering continue, cursor leave the current cycle of loop, and starts with the next cycle.

For example, we are going to skip the number 5 in our countdown:

Code

#include <stdio.h>
void main()
{
 for(int n=10;n>0;n--1)
 {
  if(n==5)
  continue;
  printf("%d",n);
 }
 printf("FIRE!");
}

Output:

10 9 8 7 6 4 3 2 1 FIRE!

3. The goto instruction 

It allows making an absolute jump to another point in the program. You should use this feature carefully since its execution ignores any type of nesting limitation. The destination point is identified by a label, which is then used as an argument for the goto instruction. A label is made of a valid identifier followed by a colon (:). This instruction does not have a concrete utility in structured or object oriented programming aside from those that low-level programming fans may find for it. For example, here is our countdown loop using goto:

Code

#include <stdio.h>
void main()
{
 int n=10;
 loop:
 printf("%d",n);
 n--;
 if(n>0) goto loop;
 printf("FIRE!");
}

Output:

10 9 8 7 6 5 4 3 2 1 FIRE!

Difference between for while and do while loop

Difference between the if-else and switch case

switch

• switch is used to select one or more cases choices out the given n cases/choices

• switch is usually more compact than lots of nested if else and therefore, more readable

• If you omit the break between two switch cases, you can fall through to the next case in many C-like languages. With if else you'd need a goto (which is not very nice to your readers………. if the language supports goto at all).

• switch only accepts primitive types as key and constants as cases. This means it can be optimized by the compiler using a jump table which is very fast.

• It is not really clear how to format switch correctly Semantically, the cases are labels for goto) which should be flush left. Things get worse when you have curly braces:

case constant

{ -----------------

-----------------

}

break;

Or should the braces go into lines of their own? Should the closing brace go behind the break Hoe unreadable would that be? etc.

if-else

simple if-else and if-else ladder selects only one choices out of the given n choices.

• if allows complex expressions in the condition while switch wants a constant

• You can't accidentally forget the break between ifs but you can forget the else (especially and paste)

• it accepts all data types.

Difference between break and continue