Converting from Infix to Postfix and evaluating Postfix notation

问题

I'm writing a program that reads an Infix notation, converts it to Postfix and then evaluate that Postfix. Here's my program:

#include<stdio.h> 
#include <ctype.h>
#define SIZE 50            /* Size of Stack */

char s[SIZE];
int top = -1; /* Global declarations */

push(char elem) { /* Function for PUSH operation */
 s[++top] = elem;
}

char pop() { /* Function for POP operation */
 return (s[top--]);
}

int pr(char elem) { /* Function for precedence */
 switch (elem) {
 case '#':
  return 0;
 case '(':
   return 1;
 case '+':
 case '-':
  return 2;
 case '*':
 case '/':
  return 3;
 }
}
pushit(int ele){                       /* Function for PUSH operation */
 s[++top]=ele;
}

int popit(){                      /* Function for POP operation */
 return(s[top--]);
}

 main() { /* Main Program */
  char infx[50], pofx[50], ch, elem;
 int i = 0, k = 0, op1, op2,ele;
 printf("\n\nRead the Infix Expression   ");
 scanf("%s", infx);
 push('#');
 while ((ch = infx[i++]) != '\0') {
  if (ch == '(')
   push(ch);
  else if (isalnum(ch))
   pofx[k++] = ch;
  else if (ch == ')') {
   while (s[top] != '(')
    pofx[k++] = pop();
   elem = pop(); /* Remove ( */
  } else { /* Operator */
   while (pr(s[top]) >= pr(ch))
    pofx[k++] = pop();
   push(ch);
  }
 }
  while (s[top] != '#') /* Pop from stack till empty */
  pofx[k++] = pop();
 pofx[k] = '\0'; /* Make pofx as valid string */
 printf("\n\nGiven Infix Expn: %s  Postfix Expn: %s\n", infx, pofx);

 while( (ch=pofx[i++]) != '\0')
 {
  if(isdigit(ch)) pushit(ch-'0'); /* Push the operand */
  else
  {        /* Operator,pop two  operands */
   op2=popit();
   op1=popit();
   switch(ch)
   {
   case '+':pushit(op1+op2);break;
   case '-':pushit(op1-op2);break;
   case '*':pushit(op1*op2);break;
   case '/':pushit(op1/op2);break;
   }
  }
 }
 printf("\n Given Postfix Expn: %s\n",pofx);
 printf("\n Result after Evaluation: %d\n",s[top]);
}

The program converts my Infix to a Postfix notation correctly. However, for the evaluation part, it always returns 0 as a result.

Also, when converting from Infix to Postfix , I would like to print the result in each step, how can I do that?

回答1:

One problem is your are storing values in s as a char with storage of 1 byte per element and then attempt to push integers into s with:

pushit (int ele) {      /* Function for PUSH operation */
    s[++top] = ele;
}

After mixing int/char in s, you attempt to read:

op2=popit();
op1=popit();

which attempts to create an int from popit(). popit() is simply a 1 byte char. So op1 and op2 are not getting the values you want:

int popit(){                      /* Function for POP operation */
return(s[top--]);
}

You need to look at how your are storing integers if you expect to get integers back. Lastly, look at your warnings. At a minimum, build with the -Wall option. It reveals:

popit.c:8:1: warning: return type defaults to ‘int’
popit.c:32:1: warning: return type defaults to ‘int’
popit.c:41:1: warning: return type defaults to ‘int’

Which may be what you intended. However, your code should build without warning to help insure it is doing what you think it is doing.

回答2:

In line no. 9 Type:

char push(char elem)

& Line no. 32 Type:

int pushit(int ele)

来源：https://stackoverflow.com/questions/24579424/converting-from-infix-to-postfix-and-evaluating-postfix-notation

标签

stack

postfix-notation

infix-notation