Notes

#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 OPERATORn", 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 KEYWORDn", subStr);

else if (isInteger(subStr) == true)
printf("'%s' IS AN INTEGERn", subStr);

else if (isRealNumber(subStr) == true)
printf("'%s' IS A REAL NUMBERn", subStr);

else if (validIdentifier(subStr) == true
&& isDelimiter(str[right - 1]) == false)
printf("'%s' IS A VALID IDENTIFIERn", subStr);

else if (validIdentifier(subStr) == false
&& isDelimiter(str[right - 1]) == false)
printf("'%s' IS NOT A VALID IDENTIFIERn", 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);
}