• This converts the source code into tokens. The individual characters of the source file must be grouped into tokens.
  
  e.g. x = 150;  will be broken into  4 tokens:

x

=

150

;

Note that tokens are not just the characters that make up the line, but the parts that make up the entire line.  In the above example , 150 is recognized as a single token and not 3 tokens of 1, 5 and 0.

• Next, a symbol (identifier) table is generated which stores the attributes of each identifier including:
  • name,
  • datatype ,
  • Scope: where in the source code it was declared (Globa , within functions)

For our line of code above, the following table entry may be generated:

Identifier	Type	Value	Memory Location	Scope
x	int	150	0x1001	Global

Note that  in practice, symbol tables may store more metadata – we have simplified the concept for easy understanding.

Symbol Table Example With A Full Program

Code

#include <stdio.h>   float PI = 3.14159265359;  // Define PI as a variable   float calculateArea(float radius) {     return PI * radius * radius; }   int main() {     float radius;       printf(“Enter the radius of the circle: “);     scanf(“%f”, &radius);       float area = calculateArea(radius);         printf(“The area of the circle with radius %.2f is %.2f\n”, radius, area);       return 0; }

Symbol Table

The following simplified symbol table is generated:

Identifier	Type	Value	Memory Location	Scope
PI	float	3.14159265359	0x1001	Global
calculateArea	float()			Global
main	int()			Global
radius	float		0x1002	main
area	float		0x1003	main

Error Detection (Lexical Error Handling)

Errors in the tokens are detected.

 Common errors include detecting :

• invalid characters  not specified in the language , e.g
  
  int café = 42; // The ‘é’ character is not valid in C.
  
  
  
• invalid identifier declaration, e.g,
  int $var = 10;  // Invalid character ‘$’
  
  
• The misspelling of keywords eg
  innt x; //should be int:
  
  

Examples of lexical Errors In C

Example	Description/Explanation
int n%mber = 5;	Illegal character % in identifier.
floar x = 5.0;	Misspelling of keyword float as floar.
int 1stNumber = 1;	Identifiers cannot start with a digit.
#incldue <stdio.h>	Misspelling of directive #include.
double price = 45.6L;	Incorrect use of float literal suffix (L is for long int).
char* str = ‘Hello’;	Single quotes used instead of double quotes for string literal.

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