Write a program that reads a string of characters and interprets it as a positive integer in the decimal base. For the conversion, use the functions of <ctype.h> and the alphabetic precedence of the characters from '0' to '9'. Save the result in a variable of type long. The conversion stops when the first character does not represent a decimal digit. Use an OK logical indicator that specifies whether the string correctly represents an integer and positive value.

#include <stdio.h>
#include <ctype.h>
#include <conio.h>
void main()
{
	/* Declarations */
	char CH[100]; /* numerical string to convert */
	long N; /* numerical result */
	int I;  /* counter */
	int OK; /* logical flag indicating whether the */
			/* string was converted successfully */

	/* Reading */
	printf("Enter a positive integer number : ");
	gets(CH);
	/* Conversion of the string */
	OK = 1;
	N = 0;
	for (I = 0; OK && CH[I]; I++)
		if (isdigit(CH[I]))
			N = N * 10 + (CH[I] - '0');
		else
			OK = 0;

	/* dislay */
	if (OK)
		printf("Numerical Value : %ld\n", N);
	else
		printf("\a\"%s\" does not represent a correct "
			"positive integer number.\n", CH);
	getch();
}