As it is known that Lexical Analysis is the first phase of compiler also known as scanner. It converts the input program into a sequence of Tokens.
A C program consists of various tokens and a token is either a keyword, an identifier, a constant, a string literal, or a symbol.
For Example:
1) Keywords: Examples- for, while, if etc. 2) Identifier Examples- Variable name, function name etc. 3) Operators: Examples- '+', '++', '-' etc. 4) Separators: Examples- ', ' ';' etc
Below is a C program to print all the keywords, literals, valid identifiers, invalid identifiers, integer number, real number in a given C program:
#include <stdbool.h> #include <stdio.h> #include <string.h> #include <stdlib.h> // Returns 'true' if the character is a DELIMITER. bool isDelimiter(char ch) { if (ch == ' ' || ch == '+' || ch == '-' || ch == '*' || ch == '/' || ch == ',' || ch == ';' || ch == '>' || ch == '<' || ch == '=' || ch == '(' || ch == ')' || ch == '[' || ch == ']' || ch == '{' || ch == '}') return (true); return (false); } // Returns 'true' if the character is an OPERATOR. bool isOperator(char ch) { if (ch == '+' || ch == '-' || ch == '*' || ch == '/' || ch == '>' || ch == '<' || ch == '=') return (true); return (false); } // Returns 'true' if the string is a VALID IDENTIFIER. bool validIdentifier(char* str) { if (str[0] == '0' || str[0] == '1' || str[0] == '2' || str[0] == '3' || str[0] == '4' || str[0] == '5' || str[0] == '6' || str[0] == '7' || str[0] == '8' || str[0] == '9' || isDelimiter(str[0]) == true) return (false); return (true); } // Returns 'true' if the string is a KEYWORD. bool isKeyword(char* str) { if (!strcmp(str, "if") || !strcmp(str, "else") || !strcmp(str, "while") || !strcmp(str, "do") || !strcmp(str, "break") || !strcmp(str, "continue") || !strcmp(str, "int") || !strcmp(str, "double") || !strcmp(str, "float") || !strcmp(str, "return") || !strcmp(str, "char") || !strcmp(str, "case") || !strcmp(str, "char") || !strcmp(str, "sizeof") || !strcmp(str, "long") || !strcmp(str, "short") || !strcmp(str, "typedef") || !strcmp(str, "switch") || !strcmp(str, "unsigned") || !strcmp(str, "void") || !strcmp(str, "static") || !strcmp(str, "struct") || !strcmp(str, "goto")) return (true); return (false); } // Returns 'true' if the string is an INTEGER. bool isInteger(char* str) { int i, len = strlen(str); if (len == 0) return (false); for (i = 0; i < len; i++) { if (str[i] != '0' && str[i] != '1' && str[i] != '2' && str[i] != '3' && str[i] != '4' && str[i] != '5' && str[i] != '6' && str[i] != '7' && str[i] != '8' && str[i] != '9' || (str[i] == '-' && i > 0)) return (false); } return (true); } // Returns 'true' if the string is a REAL NUMBER. bool isRealNumber(char* str) { int i, len = strlen(str); bool hasDecimal = false; if (len == 0) return (false); for (i = 0; i < len; i++) { if (str[i] != '0' && str[i] != '1' && str[i] != '2' && str[i] != '3' && str[i] != '4' && str[i] != '5' && str[i] != '6' && str[i] != '7' && str[i] != '8' && str[i] != '9' && str[i] != '.' || (str[i] == '-' && i > 0)) return (false); if (str[i] == '.') hasDecimal = true; } return (hasDecimal); } // Extracts the SUBSTRING. char* subString(char* str, int left, int right) { int i; char* subStr = (char*)malloc( sizeof(char) * (right - left + 2)); for (i = left; i <= right; i++) subStr[i - left] = str[i]; subStr[right - left + 1] = ' '; return (subStr); } // Parsing the input STRING. void parse(char* str) { int left = 0, right = 0; int len = strlen(str); while (right <= len && left <= right) { if (isDelimiter(str[right]) == false) right++; if (isDelimiter(str[right]) == true && left == right) { if (isOperator(str[right]) == true) printf("'%c' IS AN OPERATOR
", str[right]); right++; left = right; } else if (isDelimiter(str[right]) == true && left != right || (right == len && left != right)) { char* subStr = subString(str, left, right - 1); if (isKeyword(subStr) == true) printf("'%s' IS A KEYWORD
", subStr); else if (isInteger(subStr) == true) printf("'%s' IS AN INTEGER
", subStr); else if (isRealNumber(subStr) == true) printf("'%s' IS A REAL NUMBER
", subStr); else if (validIdentifier(subStr) == true && isDelimiter(str[right - 1]) == false) printf("'%s' IS A VALID IDENTIFIER
", subStr); else if (validIdentifier(subStr) == false && isDelimiter(str[right - 1]) == false) printf("'%s' IS NOT A VALID IDENTIFIER
", subStr); left = right; } } return; } // DRIVER FUNCTION int main() { // maximum legth of string is 100 here char str[100] = "int a = b + 1c; "; parse(str); // calling the parse function return (0); } |
Output:
'int' IS A KEYWORD 'a' IS A VALID IDENTIFIER '=' IS AN OPERATOR 'b' IS A VALID IDENTIFIER '+' IS AN OPERATOR '1c' IS NOT A VALID IDENTIFIER
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