Lec2_1: tokens, keywords, constant......

What are Tokens?

 C language syntax specify rules for sequence of characters to be written in C language. In simple language it states how to form statements in a C language program - How should the line of code start, how it should end, where to use double quotes, where to use curly brackets etc.

The rule specify how the character sequence will be grouped together, to form tokens. A smallest individual unit in C program is known as C Token. Tokens are either keywords, identifier, constants, Variables or any symbol which has some meaning in C language. A C program can also be called as a collection of various tokens.

In the following program,

#include 
int main()
{
    printf("Hello,World");
    return 0;
}

if we take any one statement:

printf("Hello,World");

Then the tokens in this statement are→ printf, (, "Hello,World", ) and ;.

So C tokens are basically the building blocks of a C program.

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What are Keywords in C?

Keywords are preserved words that have special meaning in C language. The meaning of C language keywords has already been described to the C compiler. These meaning cannot be changed. Thus, keywords cannot be used as Variables names because that would try to change the existing meaning of the keyword, which is not allowed.(Don't worry if you do not know what variables are, you will soon understand.) There are total 32 keywords in C language. 

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

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What are Identifiers?

In C language identifiers are the names given to variables, constants, functions and user-define data. These identifier are defined against a set of rules.

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Rules for an Identifier

1. An Identifier can only have alphanumeric characters(a-z , A-Z , 0-9) and underscore(_).
2. The first character of an identifier can only contain alphabet(a-z , A-Z) or underscore (_).
3. Identifiers are also case sensitive in C. For example name and Name are two different identifiers in C.
4. Keywords are not allowed to be used as Identifiers.
5. No special characters, such as semicolon, period, whitespaces, slash or comma are permitted to be used in or as Identifier.

When we declare a variable or any function in C language program, to use it we must provide a name to it, which identified it throughout the program, for example:

int myvariable = 10;

Here myvariable is the name or identifier for the variable which stores the value 10 in it.

Variables in C Language

When we want to store any information(data) on our computer/laptop, we store it in the computer's memory space. Instead of remembering the complex address of that memory space where we have stored our data, our operating system provides us with an option to create folders, name them, so that it becomes easier for us to find it and access it.

Similarly, in C language, when we want to use some data value in our program, we can store it in a memory space and name the memory space so that it becomes easier to access it.

The naming of an address is known as variable. Variable is the name of memory location. Unlike constant, variables are changeable, we can change value of a variable during execution of a program. A programmer can choose a meaningful variable name. Example : average, height, age, total etc.

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Datatype of Variable

A variable in C language must be given a type, which defines what type of data the variable will hold.

It can be:

• char: Can hold/store a character in it.
• int: Used to hold an integer.
• float: Used to hold a float value.
• double: Used to hold a double value.
• void

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Rules to name a Variable

1. Variable name must not start with a digit.
2. Variable name can consist of alphabets, digits and special symbols like underscore _.
3. Blank or spaces are not allowed in variable name.
4. Keywords are not allowed as variable name.
5. Upper and lower case names are treated as different, as C is case-sensitive, so it is suggested to keep the variable names in lower case.

Declaration  and initialisation 

Declaring, Defining and Initializing a variable

Declaration of variables must be done before they are used in the program. Declaration does the following things.

1. It tells the compiler what the variable name is.
2. It specifies what type of data the variable will hold.
3. Until the variable is defined the compiler doesn't have to worry about allocating memory space to the variable.
4. Declaration is more like informing the compiler that there exist a variable with following datatype which is used in the program.
5. A variable is declared using the externkeyword, outside the main() function.

extern int a;
extern float b;
extern double c, d;

Defining a variable means the compiler has to now assign a storage to the variable because it will be used in the program. It is not necessary to declare a variable using extern keyword, if you want to use it in your program. You can directly define a variable inside the main()function and use it.

To define a function we must provide the datatype and the variable name. We can even define multiple variables of same datatype in a single line by using comma to separate them.

int a;
float b, c;

Initializing a variable means to provide it with a value. A variable can be initialized and defined in a single statement, like:

int a = 10;

Let's write a program in which we will use some variables.

#include <stdio.h>

// Variable declaration(optional)
extern int a, b;
extern int c;

int main () {

    /* variable definition: */
    int a, b;
 
    /* actual initialization */
    a = 7;
    b = 14;
    /* using addition operator */
    c = a + b;
    /* display the result */
    printf("Sum is : %d \n", c);
    
    return 0;
}

Sum is : 21

Difference between Variable and Identifier?

An Identifier is a name given to any variable, function, structure, pointer or any other entity in a programming language. While a variable, as we have just learned in this tutorial is a named memory location to store data which is used in the program.

Identifier	Variable
Identifier is the name given to a variable, function etc.	While, variable is used to name a memory location which stores data.
An identifier can be a variable, but not all indentifiers are variables.	All variable names are identifiers.
Example: // a variable int studytonight; // or, a function int studytonight() { .. }	Example: // int variable int a; // float variable float a;