Question

C++ code:

Problem 3. Convert the following infix expression to a prefix expression by Stack operation. A + B* C + (D^E) * F/G/H + I

Evaluate the value of prefix expression when A=5, B=10, C=3, D=12, E=3, F=5, G=8, H=4, I=100

Answer 1

/* Source code*/

#include <bits/stdc++.h>
using namespace std;

bool isOperator(char c)
{
   return (!isalpha(c) && !isdigit(c));
}

int charPriority(char C)
{
   if (C == '-' || C == '+')
       return 1;
   else if (C == '*' || C == '/')
       return 2;
   else if (C == '^')
       return 3;
   return 0;
}

string infixToPostfix(string infix)
{
   infix = '(' + infix + ')';
   int l = infix.size();
   stack<char> char_stack;
   string output;

   for (int i = 0; i < l; i++) {

       // If the scanned character is an
       // operand, add it to output.
       if (isalpha(infix[i]) || isdigit(infix[i]))
           output += infix[i];

       // If the scanned character is an
       // ‘(‘, push it to the stack.
       else if (infix[i] == '(')
           char_stack.push('(');

       // If the scanned character is an
       // ‘)’, pop and output from the stack
       // until an ‘(‘ is encountered.
       else if (infix[i] == ')') {

           while (char_stack.top() != '(') {
               output += char_stack.top();
               char_stack.pop();
           }

           // Remove '(' from the stack
           char_stack.pop();
       }

       // Operator found
       else {

           if (isOperator(char_stack.top())) {
               while (charPriority(infix[i]) <= charPriority(char_stack.top())) {
                   output += char_stack.top();
                   char_stack.pop();
               }

               // Push current Operator on stack
               char_stack.push(infix[i]);
           }
       }
   }
   return output;
}

string infixToPrefix(string infix)
{
   /* Reverse String
   * Replace ( with ) and vice versa
   * Get Postfix
   * Reverse Postfix * */
   int l = infix.size();

   // Reverse infix
   reverse(infix.begin(), infix.end());

   // Replace ( with ) and vice versa
   for (int i = 0; i < l; i++) {

       if (infix[i] == '(') {
           infix[i] = ')';
           i++;
       }
       else if (infix[i] == ')') {
           infix[i] = '(';
           i++;
       }
   }

   string prefix = infixToPostfix(infix);

   // Reverse postfix
   reverse(prefix.begin(), prefix.end());

   return prefix;
}

/* These functions are for evaluate of prefix expressions*/

bool isOperand(char c)
{
// If the character is a digit then it must
// be an operand
return isdigit(c);
}

double evaluatePrefix(string exprsn)
{
stack<double> Stack;

for (int j = exprsn.size() - 1; j >= 0; j--) {

// Push operand to Stack
// To convert exprsn[j] to digit subtract
// '0' from exprsn[j].
if (isOperand(exprsn[j]))
Stack.push(exprsn[j] - '0');

else {

// Operator encountered
// Pop two elements from Stack
double o1 = Stack.top();
Stack.pop();
double o2 = Stack.top();
Stack.pop();

// Use switch case to operate on o1
// and o2 and perform o1 O o2.
switch (exprsn[j]) {
case '+':
Stack.push(o1 + o2);
break;
case '-':
Stack.push(o1 - o2);
break;
case '*':
Stack.push(o1 * o2);
break;
case '/':
Stack.push(o1 / o2);
break;
}
}
}

return Stack.top();
}
// Driver code
int main()
{
   string s = ("A+B*C+(D^E)*F/G/H+I");
   int A=5, B=10, C=3, D=12, E=3, F=5, G=8, H=4, I=100;
   string exprsn = infixToPrefix(s);
   cout << "INFIX to Prefix notation : " << exprsn << std::endl;
   string changedString;
   //now change charactor values with respective value
   for (int j = 0; j < exprsn.size(); j++) {
   switch(exprsn[j]){
case 'A': changedString.append("(5)"); break;
case 'B': changedString.append("10"); break;
case 'C' : changedString.append("3"); break;
case 'D': changedString.append("12"); break;
case 'E': changedString.append("3"); break;
case 'F': changedString.append("5"); break;
case 'G': changedString.append("8"); break;
case 'H': changedString.append("4"); break;
case 'I': changedString.append("100"); break;
default:
string s;
s.push_back(exprsn[j]);
changedString.append(s);
   }
   }
   cout<<"Changed string with the value is " << changedString<<endl;
   cout << "Evaluation of prefix expression with A=5, B=10, C=3, D=12, E=3, F=5, G=8, H=4, I=100" << evaluatePrefix(changedString) << endl;
   return 0;
}

/Output*/