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C program to detect tokens in a C program
  • Difficulty Level : Medium
  • Last Updated : 26 Aug, 2017

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] = '\0';
    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\n", 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\n", subStr);
  
            else if (isInteger(subStr) == true)
                printf("'%s' IS AN INTEGER\n", subStr);
  
            else if (isRealNumber(subStr) == true)
                printf("'%s' IS A REAL NUMBER\n", subStr);
  
            else if (validIdentifier(subStr) == true
                     && isDelimiter(str[right - 1]) == false)
                printf("'%s' IS A VALID IDENTIFIER\n", subStr);
  
            else if (validIdentifier(subStr) == false
                     && isDelimiter(str[right - 1]) == false)
                printf("'%s' IS NOT A VALID IDENTIFIER\n", 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

This article is contributed by MAZHAR IMAM KHAN. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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