Infix to Prefix Conversion and Evaluation in C

Description

 This program is used to Convert Infix Expression (eg. a+b) to Prefix Expression (eg. +ab) and Evaluation of the Expression 

Note : This program works only for one digit operands.

#include<stdio.h>
void prefix();
char stack[50];
int s[50];
char pre[50];
int p = -1;
int top = -1;
int rtop = -1;
int xtop=-1;      
 
xpush(int elem)
{     
 s[++xtop]=elem;                
}

int xpop()
{                      
 return(s[xtop--]);
}

void push(char x)
{
 stack[++top] = x;
}
 
char pop()
{
    if(top == -1)
        return -1;
    else
     return stack[top--];
}
 
int priority(char x)
{
    if(x == '(')
        return 0;
    if(x == '+' || x == '-')
        return 1;
    if(x == '*' || x == '/')
        return 2;
}

main()
{
 int choice;
 do
 {
  printf("\n\n----MENU----\n1.INFIX - PREFIX\n2.EXIT\n");
  printf("Enter your choice\n");
  scanf("%d",&choice);
  switch(choice)
  {
   case 1 : prefix();break;
   case 2 : printf("~~~~~ Thank You ~~~~~\n");break;
   default: printf("INVALID INPUT !!!!\n");
  }
 }while(choice!=2);
}
void prefix()
{
 xtop=-1;rtop=-1;p=-1;
 int i=0,j=0;
 char exp[50],rexp[50];
    char *e,*r, x;
    printf("Enter the expression :: ");
    scanf("%s",exp);
    while(exp[i]!='\0')
    i++;
    i--;
    while(i>=0)
    {
     if(exp[i]==')')
     {
      rexp[j++]='(';
      i--;
  }
  else if(exp[i]=='(')
  {
   rexp[j++]=')';
   i--;
  }
  else
  rexp[j++]=exp[i--];
    
 }
 rexp[j]='\0'; 
 r=rexp;
 while(*r != '\0')
 {
  if(isalnum(*r))
        {
            pre[++p]=*r;
        }
        else if(*r=='(')
        {
         push(*r);
  }
  else if(*r==')')
  {
   while((x=pop())!='(')
   pre[++p]=x;
  }
  else
  {
   while(priority(stack[top])>=priority(*r))
   pre[++p]=pop();
   push(*r);
  }
  r++;
 }
 while(top != -1)
    {
        pre[++p]=pop();
    }
    int print=p;
    printf("\nPrefix expression :: ");
    while(print!=-1)
    printf("%c",pre[print--]);
    
///////////evaluation//////////////
 i=0;
 j=0;
    int op1,op2;
    char ch;
    while(i!=(p+1))
    {
     i++;
     ch=pre[j++];
     if(isdigit(ch))
     xpush(ch-'0');
     else
     {
      op1=xpop();
      op2=xpop();
      switch(ch)
      {
       case '+':xpush(op1+op2);break;
      case '-':xpush(op1-op2);break;
      case '*':xpush(op1*op2);break;
      case '/':xpush(op1/op2);break;
   }
  }
 }
 printf("\n\nResult after Evaluation: %d\n",s[xtop]);
}

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OUTPUT