/* preval.c, class example 1 March 2012
  Lin Jensen
  see assembly program preval.a

  There is a problem in C, that when a pointer is passed to a function,
  a COPY of the address is passed, if this copy is incremented, that doesn't affect
  the original. The solution will be to store a "working pointer" in memory, pass the address
  of this pointer, which is now of type char** (a pointer to the pointer to the (current) start
  of the string.

Algorithm:

preval()
   if numeric character
      return binary value of it
   if '+'
      return preval() + preval()
   if '*' 
      return preval() * preval()
   else 
      try again (ignore space, for example)
*/
#include <stdio.h>
/*-----------------------------------------------------------------------------*/
int preval(char ** place) {
  char* e = *place;		// e points to the next char to be considered
  char	ch;
  while (ch = *e++)		// get next character, and "consume"
  {				// this is a loop, to detect end of string, and skip blanks
    *place = e;			// update the stored pointer
    if ('0' <= ch && ch <= '9')
	return ch & 0xF;	// binary value of digit
    if (ch =='+')
	return preval(place) + preval(place);	// add -- note *place will be changed
    if (ch =='*')
	return preval(place) * preval(place);	// multiply
						// now, you can implement subtract, divide
  }
  return -999;			// EOS too soon
}  
/*-----------------------------------------------------------------------------*/

int main() {
  puts("Enter prefix arith expression, single digits and + * allowed");
  char expression[100];
  while (*(gets(expression))) {	// get an expression, until empty line (first char is '\0')
    char *head = expression;		// start of string stored
    int result = preval (&head);			// pass ADDRESS of this pointer
    printf("value= %d\n>",result);
  }
  return 0;
}