Question

In C++, write a program that reads a postfix expression from an input file. Then, the program should evaluate the postfix format and display the results.

Use these concepts for the program:

1) Struct Node 2) Enqueue and Dequeue 3) Push and Pop 4) DO NOT USE classes 5) Possibly Stack

Precondition:

• The expression will be read from a file (input.txt) that contains a single line. There will be no spaces between the operands and the operators.
• The following operators are allowed: + - * /
  • The normal rules of precedence will apply.
• No error checking is necessary.
• The numbers will all be greater than 0 and less than 10 (positive, single digit numbers)

Example:

12+

Result is 3

Answer 1

C++ Code:

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

struct Stack
{
    int top;
    int size;
    int* vec;
};

int isEmpty(struct Stack* stack)
{
    if(stack->top == -1)
    {
        return 1;
    }
    else
        return 0;
}

char pop(struct Stack* stack)
{
    if (!isEmpty(stack))
    {
        stack->top--;
        return stack->vec[stack->top+1] ;
    }
    return '$';
}

void push(struct Stack* stack, char operand)
{
    stack->top+=1;
    stack->vec[stack->top] = operand;
}

int Postfix(string line)
{
    struct Stack* stack = (struct Stack*) malloc(sizeof(struct Stack));
    stack->size = line.length();
    stack->top = -1;
    stack->vec = (int*) malloc(stack->size * sizeof(int));
    for (int i = 0; i<line.length(); i++)      // Scan all characters one by one
    {
        if (line[i]>='0' && line[i] <='9')    // if numbers push them in the stack
            push(stack, line[i] - '0');

        else                                 // else it is an operator, so pop two operands and evalutae them and push result to stack
        {
            int operand1 = pop(stack);
            int operand2 = pop(stack);
            if(line[i]=='+')
            {
                push(stack, operand2 + operand1);
            }
            else if(line[i]=='-')
            {
                push(stack, operand2 - operand1);
            }
            else if(line[i]=='*')
            {
                push(stack, operand2 * operand1);
            }
            else
            {
                push(stack, operand2 / operand1);
            }
        }
    }
    return pop(stack);
}

int main()
{
    string line;
    ifstream myfile ("input.txt");
    if (myfile.is_open())
    {
        while ( getline (myfile,line) )
        {
            //cout<<line<<endl;
        }
        myfile.close();
    }
    cout<<"Result is "<< Postfix(line);
    return 0;
}

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Happy Learning.