Lex program to check whether given number is armstrong number or not

Problem: Lex program to check whether given number is armstrong number or not.

Explanation:
Lex is a computer program that generates lexical analyzers and was written by Mike Lesk and Eric Schmidt. Lex reads an input stream specifying the lexical analyzer and outputs source code implementing the lexer in the C programming language.

Description:
An Armstrong number is a number that is the sum of its own digits each raised to the power of the number of digits. For example, 153 is an Armstrong number,

(1^3) + (5^3) + (3^3) = 153 

Examples:

Input: 153 
Output: 153 is a Armstrong number

Input: 250
Output: 250 is not a Armstrong number 

Implementation:

/* Lex program to check whether given
    - number is armstrong number or not */
%
{
/* Definition section */
#include <math.h>
#include <string.h>
    void check(char*);
    %
}

/* Rule Section */
% %
        [0 - 9]
    + check(yytext);
% %

int main()
{
    /* yyin as pointer of File type */
    extern FILE* yyin;
    yyin = fopen("num", "r");

    // The function that starts the analysis
    yylex();

    return 0;
}
void check(char* a)
{
    int len = strlen(a), i, num = 0;
    for (i = 0; i < len; i++)
        num = num * 10 + (a[i] - '0');

    int x = 0, y = 0, temp = num;
    while (num > 0) {
        y = pow((num % 10), len);
        x = x + y;
        num = num / 10;
    }

    if (x == temp)
        printf("%d is armstrong number \n", temp);
    else
        printf("%d is not armstrong number\n", temp);
}

Output: