Control Structures in C language – {Module-4}

1. If then else – in C language

This control structure is used when the programmer has to make some decision while writing a block of code.

Syntax:

If(condition)                                       // This condition is true

{

Block of code;                    // Then this block of code will be executed

}

else                                                        // otherwise

{

Block of code:                    // This block of code will be executed

}

So, Let us start writing some simple program using if then else control structure

We will write a program for determining that whether the student is pass or fail by accepting the marks

Logic:   First, we will declare a variable of int data type. After declaring the value we will accept the value from the user and then store it in that variable. The using the if then else control structure we will determine whether the student is pass or fail. Let’s start writing this simple code.

int i;                       // variable declared

printf(“Enter the number”);       // “Enter the number” will be declared on the screen

scanf(“%d”, &i):              // value accepted and stored in variable i

if(i > 40)               // if the condition is true i.e. if the marks stored in i is greater than 40

{

                printf(“Pass”);                  // then pass will be printed on the screen

}

else                       //            If the above condition is not true i.e. if the marks stored in i is less than 40                                                         then the block of code in else section will be executed

{

                printf(“Fail”);

}

Nested if then else – in C language

If the programmer has to use more than one condition while writing a program the nested if then else control structure is used.

Syntax:

If(condition)       // If this condition is true

{

Block of code;    // Then this block of code will be executed

}

else        //otherwise

{

if(condition)                       // If this condition is true in the else section

{

Block of code;    // Then this block of code will be executed

}

else                        // Otherwise

{

Block of code:    // This block of code will be executed

}

}

Now, we will write the same program written above but with multiple conditions

Logic: The logic will be same but in the nested if then else the first condition will be if the marks is greater than 40 then the student is pass, second condition if the marks are in between 50 and 55 then second class and third condition will be if the marks are greater than 55 then higher second class else the student is fail.

int i;                       // variable declared

printf(“Enter the number”);       // “Enter the number” will be declared on the screen

scanf(“%d”, &i):              // value accepted and stored in variable i

if(i > 40)               // if the condition is true i.e. if the marks stored in i is greater than 40

{

                printf(“Pass”);                  // If the above condition is true then “Pass” will be printed on the                                                                              screen

}

else       // Otherwise the next if condition will be written and executed in this else section

{

                if ( 50 < i < 55)    if the marks are in between 50 and 55 then the block of code in this section                                                       will be executed

                {

                                printf(“second class”); // “second class” will be printed on the screen.

                }

                else       // The next if condition will be written and executed in this else section

                {

                                if( i > 55)              // If the marks are above 55 the higher second class will be displayed

                                {

                                                printf(“Higher second class”);

                                }

                                else // If any of the above mentioned condition is not true the this else section will be                                              executed

                                {

                                                printf(“Fail”);

                                }              // This else section is closed

                }              // previous else section is closed

} // The first else section in closed

The concept of if then else and nested if then else will be cleared to you. Its time for you to write your first program.

Problem Statement: Write a program to know whether the year given by the user is a leap year or not.

In this problem statement we will use modulus (%) operator. The modulus operator will return the remainder

For example: 20%5 will return 4 (i.e. the remainder)

Conditions for leap year

Store the year given by user in some variable say x

There are three conditions

1. if ( x%400 == 0) Then the year is leap year. “==” is the comparison operator. It compares the variable with the value.
2. if (x%100 == 0) Then the year is a leap year.
3. if (x%4 == 0) Then the year is leap is year.

else

not a leap year.

Hope this logic will be help you to develop the program.

If you have missed our Previous Modules then Read the Modules here by topic Wise:

1)   Basics of C language

2)   Data Types in C language

3)   Input and Output in C language

Now you have the knowledge about how to use if then else and nested if then else. For more problems on this control structure refer the book “C” by Dennis Ritchie.

Input and Output in C language {Module-3}

Any meaningful program written in any language has three sections:-

1. In the first section is the program will accept the input from the device like keyboard and store the data in memory.
2. In the second section the program will work with the data according to the required logic to generate some result and store the result in the memory.
3. In the third section the program will display the result on the display device like screen.

Accepting the data or displaying the data is not a simple process. There are several complications like the format of the data etc. For example if you want to accept a number like 107, it can be interpreted in many ways i.e. this 107 number can be considered as octal number, decimal number or hexadecimal number. The system cannot know how to regard the data. There are several other problems like keyboard and display with ASCII codes which have to be converted to hexadecimal etc. All this stuff is beyond the reach of normal programmers. Therefore most of the language including the ‘C’ language provide support in the form of inbuilt functions to simplify the process.

Scanf and Printf

In ‘C’ language we have “scanf” function for accepting the data from keyboard and “printf” function for displaying the data on screen. Both these function require “format specifiers” which will take care of the problem that what kind of data does the ‘C’ language is going to accept or what kind of data is it going to display. For example if you have declared a variable ‘i’ as ‘int’ then you can accept the decimal data from the keyboard through the following way.

int i;                       From this instruction you have declared a variable ‘i’ as integer data type and 2                                                  bytes of memory will be allocated in the memory.

scanf(“%d”, &i);

Now ‘scanf’ is the function which we have used to accept the data from the user. Here ‘%d’ is the format specifier which is used to let the system know that the value which the system is going to accept is a decimal value. The format specifier is put in “” and comma is given after that. Now the “&” is used to tell the system the address allocated to ‘i’ in the system so that the value which the system is going to accept will be stored in ‘i.

While writing a ‘C’ program we are free to specify multiple variable of different widths and data types in 1 “scanf” function.

Some more examples of “scanf” function:-

int i;

char j;

float k

scanf(“%d%c%f”, &i,&j,&k);

In the above example we have declared three variables i,j,k as decimal, character and float data type. And in the next instruction in the scanf function we have declared three format specifiers accordingly. After that we have given the address of i, j,k to store the data accordingly.

Remember that the data type must be matched with the associated format specifier.

“printf” is the function used for displaying the result on screen.

For example:  int i;

scanf(“%d”, & i);

printf(“%d”, i);

In the above example we declared a variable ‘i’ of int data type. Then we accepted the value from user and stored the value in ‘i’. After storing the value we displayed the value using ‘printf‘ function. The concept for using ‘printf‘ is same as ‘scanf‘. The only difference is that the ‘&’ sign is not used in the ‘printf‘ function. Some examples of ‘printf‘ is given below.

int i,j,k;

printf(“Enter the first number”);  Display “Enter the first number” on screen

scanf(“%d”, &i);                                Accept and store the value in i;

printf(“Enter the second number”);  Display “Enter the second number” on screen

scanf(“%d”, &j);                                               Accept and store the value in j;

printf(“Enter the third number”);    Display “Enter the third number” on screen

scanf(“%d”, &k);                              Accept and store the value in k;

printf(“The first number is %d”, i);   Display “The first number is (and the number you have stored in i)” on the place of %d the number will be displayed because the number you have stored in i is integer value.

printf(“The second number is %d”, j);   Display “The second number is (and the number you have stored in j)” on the place of %d the number will be displayed because the number you have stored in j is integer value.

printf(“The third number is %d”, k);   Display “The third number is (and the number you have stored in k)” on the place of %d the number will be displayed because the number you have stored in k is integer value.

if You have missed our Previous Modules

Read Here:

Module 1 – Basics of C Language

Module 2 – Data Types in C language

Data Types in C Language

Why Data Types are used in C language?

Suppose you are writing a program in C language and you need to store an integer value for your program, but how will C know that the value you are going to store for your program is an integer value. So, data types are used. Now if you are going to store an integer value you will declare a “int” as a data type. Now look at the example given below.

                int i;

Now from the above instruction you will see that a variable ‘i’ is declared and is given ‘int’ as a data type. Now what “C” will do is allocate 2 bytes of memory for ‘i’ in the storage area and you will be able to store an integer value in ‘i’.

The following are the commonly used data types used in C language:-

1. int: – This data type is used to store integer values. 2 bytes of memory is allocated for the storage. The range for int data type is from -32,768 to 32,767 i.e. you cannot store a value less than -32,768 and greater than 32,767. For higher precision ‘long int’ data type is used.
2. char :- This data type is used to store character values. 2 bytes of memory is allocated.
3. long int :- This data type is also used to store integer values but with higher precision than int. 4 bytes of memory is allocated i the system.
4. float :- This data type is used to store real numbers i.e. the fractional numbers. The memory allocate is 4 bytes.
5. double :- This data type is also used to store fractional numbers but the precision is higher. The memory allocated is 8 bytes

Note: – The memory allocation is in C under DOS i.e. Disk Operating System. The memory allocation in C under Linux will be different.

Remember: – The programmer should be aware of nature and range of data before writing any program. Accordingly, you will have to request the system to allocate storage for data following up standard data types supportive by ‘C’ under DOS (Disk Operating System).

C language is case sensitive. All standard function names, variable declaration are always in lower case.

The concept of Delimiter

A character has one ASCII associated. A name consists of ‘n’ such characters where ‘n’ is variable i.e. some name may have 10 characters while other may have something else. Therefore, the computer cannot automatically understand where the name ends. Now when you want to type a name in ‘C’ it gets stored in the form of strings. Now the question arises how ‘C’ will know that the string has ended. The end of the string is has an additional character equal to ’00’. This 00 is known as Delimiter. When you are finished typing a string you press enter, after pressing enter the system automatically put the delimiter that is ’00’ which tells ‘C’ that the string came to an end. For example if a string contains ‘HELLO’ it will actually contain 5 ASCII code followed by an additional byte containing ’00’ i.e. DELIMITER. This 00 is not the part of the string.

Format Specifiers

Why format specifiers are used?

When you are giving an input and displaying the result the concept of format specifiers are used. The system must know that which value is going to be stored or which value the system is going to display. For example, if you are going to give some decimal value or display decimal value ‘%d’ is used. Now don’t get confused with data types. Data types are used to tell the system that which value the user is going to use in the program and system will allocate the memory accordingly.

For example    int i;         Here ‘i’ is the variable and the data type is int. Now 2 bytes of memory will be                                                    allocated in the storage and decimal value will be stored.

Format Specifier

Scanf(“%d”, &i);              Here in this instruction the system will know that the value that                                                                                                the user is going to give is decimal value and that value will be                                                                                    stored in variable ‘i’

Here are some formats specifiers

1. %d :- This is for decimal numbers
2. %u :- unsigned decimal numbers
3. %x :- hexadecimal numbers
4. %f :- float numbers
5. %c :- Character equivalent

if You have Missed the Module 1 Read Here : Basics of C Language