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Lex program to check perfect numbers

  • Difficulty Level : Expert
  • Last Updated : 30 Apr, 2019
Geek Week

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: Perfect number, a positive integer that is equal to the sum of its proper divisors, for example: 6 = 1+2+3.

Examples:

Input: 3
Output: 3 is not perfect number

Input: 6
Output: 6 is perfect number 

Implementation:




/*Lex program to check perfect numbers*/
  
%
{
#include <string.h>
    void check(char*);
    %
}
  
/*Rule Section*/
% %
        [0 - 9]
    + check(yytext);
% % int main()
{
    // the input stream pointer
    extern FILE* yyin;
  
    // open a file handle to a particular file
    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, temp = num;
    for (i = 1; i < num; i++) {
        if (num % i == 0)
            x = x + i;
    }
    if (x == temp)
        printf("%d is perfect number \n", num);
  
    else
        printf("%d is not perfect number \n", num);
}

Output:

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