C Programming: Constants, Variables, Keywords and Data Types

CONSTANTS, VARIABLES, KEYWORDS AND DATA TYPES

 

1. 'C's CHARACTER SET

The character set is the fundamental raw material of any language and they are used to represent information. Like natural languages, computer language will also have well defined character set, which is useful to build the programs.

The classical method of learning English language is learning first the alphabets or characters used in the language. Thus learn to combine these alphabets to plan for words, which in turn are combined to form sentences and sentences are combined to form paragraphs. Similarly the characters used to write 'C' program are basically of two types, namely

i) Source character set

ii) Execution character set

1. Source Character Set

These are used to construct the statements in the source program.

These are of four types;

2. Execution Character Set

These are employed at the time of execution. This set of characters are also called as non graphic characters because, these characters are invisible and cannot be printed or displayed directly. These characters will have effect only when the program is being executed. Execution characters set are always represented by a backslash (\) followed by a character. These are also called as 'escape sequences.'

 

2. C TOKENS

The tokens are usually referred as individual text and punctuation in a passage of text. The 'C' language program can contain the individual units called the C tokens and has the following types.

 

3. IDENTIFIERS AND KEYWORDS

In 'C' language every word is classified into either a keyword or an identifier.

Identifiers: Identifiers are names given to various program elements, such as variables, functions and arrays etc.

Rules for naming an identifier

• Identifiers consist of letters and digits in any order.

• The first character must be a character or may begin with (_).

• Both upper/lower cases allowed although uppercase character is not equivalent to corresponding lowercase character.

• The underscore '_' is considered as a letter.

• An identifier can be of any length while most of the 'C' compiler recognizes only the first 31 characters.

• No space and special symbols are allowed between the identifier.

Some valid Identifiers are

        STDNAME, SUB, TOT MARKS, TEMP, Y2K.

Some invalid Identifiers are

        Return, $stay, 1RECORD, STD NAME.

Keyword: There are certain reserved words called keywords that have standard and predefined meaning in 'C' language. Which cannot be changed and they are the basic building blocks for program statements. The 'C' keywords are listed below.

Table of ANSI 'C' keywords

auto        break        case        char        const        continue        default        do        double        enum        else        extern        for        goto        if        int        long        return        register        signed        short        static        sizeof        struct        switch        typedef        union        unsigned        void        volatile        while

All keywords must be written in lower case

 

4. DATA TYPES

Data type is the type of the data, that are going to access within the program. 'C' supports different data types, each data type may have predefined memory requirement and storage representation.

'C' supports the following 4 classes of data types

The bytes occupied by each of the primary data types are

The primary data types are divided into the following.

a) Integer type: Integers are the numbers with the supported range. Usually the integers occupy one word of storage typically 16 or 32 bits. The size of the integer depends upon the system.

Eg: For 16 bit, the size can be limited up to ‒32768 to +32767.

b) Character type: Characters are generally stored in 8 bits of the internal storage of computer and a single character can be defined as char data type.

c) Float type: The floating point numbers are generally stored in 32 bits with the 6 digits of precision. These numbers are defined by using the keywords float and double, whereas the double data type uses the 64 bits with 14 digits of precision.

d) Null data type: The void is the null data type in 'C' language. This is generally specified with the function which has no arguments.

1. Type Qualifiers

If we use a 16 bit word length, size of data type is limited to the specific range. So in order to provide some control over the range of numbers and storage space of the data type, some basic data types can be argumented by using the data type qualifiers, they are,

short, long, signed, unsigned

The declaration of long and unsigned integer permits us to increase the range of values. The precedence of signed declaration for an integer is essential as the default declaration assumes signed numbers.

Sizes and Ranges of data types with type qualifiers

 

5. VARIABLES, CONSTANTS AND DELIMETERS!

1. Variable

A Variable is an identifier that is used to represent some specified type of information within a designated portion of the program. A variable may take different values at different times during the execution. i.e. it is the named memory location.

2. Variable Declaration

After designing suitable variable names, we must declare them in a program, and this declaration tells the compiler what the variable name and type of the data that the variable will hold.

Syntax: data type v1, v2, v3......., vn;

Description: data type v1, v2, v3....,vn - is the type of the data are the list of variables

Example:

int code;

char gender;

float price;

char name[10];

Here the variables, code is type of integer, gender is type of character, price is type of float and name[10] is defined an array of character.

The data item must be assigned to the variable at some part of the program. The data item can then be accessed later in the program simply by referring to the variable name.

3. Initializing variables

Initialization of variables can be done using the assignment operator (=). The variables can be initialized while declaration itself.

Syntax: Variable = constant;

Description: i,f,c are the type of int, float, char data types

Example:

int; 

float f;

char c;

4. typedef and enumeration Constants

a) Type declaration: 'C' Language provides a feature to declare a variable of the type of user defined type declaration. This allows users to define an identifier that would represent an existing data type and this can later be used to declare variables.

The typedef is a keyword in the C programming languages. The purpose of typedef is to form complex types from more basic machine types and assign simpler names to such combinations. They are most often used when a standard declaration is cumbersome, potentially confusing, or likely to vary from one implementation to another.

Syntax: typedef data_type identifier;

Description:

typedef is the user defined type declaration,

data_type is the existing data type,

identifier is the identifier refers to the new name given to the data type.

Example

typedef int marks;

marks m1, m2, m3;

Here marks is the type of int and this can be later used to declare

b) Enumerated data type: The C language provides another user defined data type called enumerated data type.

Syntax: enum identifier {value 1, value 2,…. value n};

Description: identifier is the user defined enumerated data type. value 1, value 2, value n are the enumeration constants

Example:

enum day {mon, tue, wed,...., sun};

enum day w_st, w_end;

w_st = mon;

w_end = sun;

In C, an enumerated data type is a data type consisting of a set of named values called elements, members or enumerators of the type. The enumerator names are usually identifiers that behave as constants in the language. A variable that has been declared as having an enumerated type can be assigned any of the enumerators as a value.

The identifier follows with the keyword enum is used to declare the variable, that can have only one value from the enumeration constants.

NOTE: The compiler assigns integer values from 0 to all the enumeration constants automatically.

5. Scope of Variables

Scope of a variable implies the availability of variables within the program. Variables have two types of scopes described below.

i) Local Variables: The variables which are defined inside a function block or inside a compound statement of a function sub‒program are called local variables.

Example:

function()

{

      int i, j;

      /* body of the function*/

}

The integer variables i, j are defined inside the function block of function(). Hence ij are called local variables i.e., the availability of the variables i, j are local to that function only in which they are declared.

ii) Global/External Variables: The variables that are declared before the function main() are called the global/external variables. These are available for all the functions inside the program.

Example:

int a,b=2;

main()

{

        ……..

        fun();

}

fun()

{

    int sum;

    sum=a+b;

}

The integer variables a, b are global/external variables, as they are declared before the function main(). These global/external variables are later used in the function sub program fun().

 

6. CONSTANTS

The item whose values cannot be changed during the execution of program are called constants. 'C' constants can be classified as follows.

1. Numeric Constants

a) Integer Constants: An integer constant formed with the sequence of digits. There are three types of integer constants which forms different number system.

Decimal number - 0 to 9

Octal number - 0 to 7

Hexadecimal number - 0 to 9, A, B, C, D, E, F

Example:

Marks = 90;

Per = 75;

Discount = 15;

There are basically three types of integer namely,

Decimal - Eg: 10, 153, ‒321, 019, etc.

Octal - Eg: 037, 0, 052, 0541, etc.

Hexadecimal - Eg: 0x4, 0x8F, 0xBCF etc.

b) Real Constants: A real constant is made up of a sequence of numeric digits with presence of a decimal point. Real constants serve as a good purpose to represent quantities that vary continuously such as distance, heights, temperatures etc.

Example:

distance = 126.0;

height = 5.6;

speed = 3e11;

2. Character Constants

a) Single Character Constants: The character constant contains a single character enclosed within a pair of single inverted commas both pointing to the left.

Example: 's', 'M', '3', '-'

b) String Constants: A string constant is a sequence of characters enclosed in double quotes, the characters may be letters, numbers, special characters and blank spaces etc. At the end of string '\0' is automatically placed.

Example:

"HI"

"Muni"

"39.77"

c) Declaring a variable as constant: When the value of some of the variables may remain constant during the execution of the program. In such a situation, this can be done by using the keyword const.

Syntax: const datatype variable = constant

Description:

const is the keyword to declare constant

variable is the name of the variable

datatype is the type of the data

constant is the constant

Example: const int dob‒3977;

d) Declaring a variable as volatile: The volatile variables are those variables that are changed at any time by other external program or the same programe. ANSI standard defines qualifier as volatile.

Syntax: volatile datatype variable = constant

Description:

volatile is the keyword to declare volatile

variable is the name of the variable

datatype is the type of the data

constant is the constant

Example: volatile int year=2007;

The value of a variable declared as volatile can be modified by its own program as well If the value of a variable in the current program is to be maintained constant and desired not to be changed by any other external operation.

 

7. DELIMITERS

These are the symbols, which has some syntactic meaning and has got significance. These will not specify any operation. 'C' language delimiters list is given below.

 

8. STATEMENTS

Statements can be defined as a set of declaration, or sequence of action. Statement causes the computer to perform some action. All statements in 'C' language ends with semicolon except conditioned and control structured. The types of statements are as follows:

1. Assignment statements

Here, the assignment operator is used for assigning values to the variables.

Example:

2. Null statements

A statement without any characters and it has only semicolon is called null statement.

Example:

; (null statement)

3. Block of statements

Block contains several statements that are enclosed within a pair of braces {}. These can be of any expression, assignments and keywords etc.

Example:

4. Expression statements

These consists of expressions and can be arithmetic, relational or logical.

Example:

a=29;

b=a+77;

fun(a,b);