Evaluation of a given prefix expression containing single digits number as operands

prefix_evaluation.c

#include<stdio.h>
#include<string.h>
int top=-1; char a[20];
void push(char x)
{
    a[++top]=x;
}
char pop()
{
  return(a[top--]);
}
int evaluate(char s[])
{
  int i=0,j=0; char ch,c,b; int sum=0,x=0;
  while(s[i]!='\0')
  {    ch=s[i];
      if((ch=='+')||(ch=='-')||(ch=='/')||(ch=='*'))
      {
       push(ch);
      }
      else
     {   x=ch-'0';
          while((top!=-1)&&(a[top]=='@'))
         {
          c=pop(); b=pop();
         if(b=='+')
          sum+=x;
          else if(b=='-')
          sum-=x;
          else if(b=='/')
          sum/=x;
          else if(b=='*')
          sum*=x;
          x=sum;
          }
         sum=x; push('@');
       }
    i++;
  }
  return(sum);
}
main()
{   char s[20]; printf("Enter a prefix expression:  ");
    gets(s);
    int sum=evaluate(s);
    printf("Answer : %d",sum);
}

/////////////////////
//OUTPUT
//Enter a prefix expression:  */123
//Answer : 0