#include <stdio.h>

/**
 * ctoi(char)
 *   Convert a digit character to the correspnding integer.
 */
int 
ctoi(char ch)
{
  return ch - '0';
} // ctoi

/**
 * atoi(str)
 *   "Parse" a string to the underlying integer.
 */
int
atoi (char a[])
{
  int i = 0; 		// Counter variable for string
  int result = 0;	// What I've computed so far
  // For a two digit number, we can use
  //   10 * ctoi(a[0]) + ctoi(a[1])
  // Generalize:
  //   As long as characters remain,
  while (a[i] != '\0')
    {
      // multiply the current result by 10
      // add the converted character
      result = result * 10 + ctoi (a[i]);
      // Move on to the next character
      i = i + 1;
    } // while
  // And we're done
  return result;
} // atoi

int 
main ()
{
  printf ("%d\n", atoi("5317"));
  printf ("%d\n", atoi("235"));
  printf ("%d\n", atoi("1"));
  printf ("%d\n", atoi("two"));

  return 0;
} // main