Database System Concepts
7th Edition
ISBN: 9780078022159
Author: Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher: McGraw-Hill Education
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#include<iostream>
#include<cmath>
#include<stack>
#include<string>
#include<fstream>
#include<sstream>
using namespace std;
bool isNumber(string line);
void evolutePostfixExpr(string,ostream&);
int operation(int , int , string);
int main()
{
ifstream inFile;
ofstream outFile;
string filename;
string expression;
int choice;
cout << "Enter the file name: ";
cin >> filename;
inFile.open(filename);
cout << "\n1. Ouput on console." << endl;
cout << "2. Ouput on file." << endl<<"Your choice: ";
cin >> choice;
if (choice == 2)
{
cout << "\nEnter the output file name: ";
cin >> filename;
outFile.open(filename);
cout << "\nWrite to " << filename << endl;
}
while (!inFile.eof())
{
getline(inFile, expression);
if (choice == 2)
evolutePostfixExpr(expression,outFile);
else
evolutePostfixExpr(expression, cout);
}
return 1;
}
void evolutePostfixExpr(string expr,ostream &out)
{
stack<int> stk;
string value,errorMsg;
bool invalid = false;
stringstream ss(expr);
int ans = 0;
int a, b;
while (ss >> value)
{
if (isNumber(value))
stk.push(stoi(value));
else if (value == "+" || value == "-" || value == "*" || value == "/")
{
if (stk.empty() || stk.size() == 1)
{
invalid = true;
errorMsg = "To many operators";
break;
}
a = stk.top();
stk.pop();
b = stk.top();
stk.pop();
ans = operation(a, b, value);
stk.push(ans);
}
else
{
invalid = true;
errorMsg = "Illegal Operation";
break;
}
}
if (invalid)
{
out.width(30); out << left << expr;
out << errorMsg << endl;
return;
}
if (stk.size() > 1)
{
invalid = true;
errorMsg = "To few operators.";
}
if (invalid)
{
out.width(30); out << left << expr;
out << errorMsg << endl;
return;
}
else
{
out.width(30); out << left << expr;
out << errorMsg << stk.top()<<endl;
}
}
bool isNumber(string s) {
if (s.size() == 0) return false;
for (int i = 0; i<s.size(); i++) {
if ((s[i] >= '0' && s[i] <= '9') == false) {
return false;
}
}
return true;
}
int operation(int a, int b, string op) {
//Perform operation
if (op == "+")
return b + a;
else if (op == "-")
return b - a;
else if (op == "*")
return b * a;
else if (op == "/")
return b / a;
return 0;
}
Transcribed Image Text:Use the C++ Standard Template Library's stack class to write a program for processing a file of postfix
expressions.
Recall that in a postfix expression, a binary operator comes after its two operands. Thus the infix
expression (w +x) * (v-2) becomes wx+ys-* in postfix.
A postfix expression can be efficiently evaluated using a stack as follows.
1. Read the expression from the left to the right.
2. When an operand is encountered, push it on the stack.
3. When an operator is encountered, pop the top two elements from the stack, perform the
operation, and then push the result.
4. There should be exactly one element on the stack at the end and this element is the value of the
еxpression.
For example, if the expression 20 5 - 3 * is entered, the evaluation should proceed as follows.
Symbol read
Actions taken
20
Push 20
5
Push 5
Pop 5, Pop 20, Push 20 - 5
Push 3
3
Pop 3, Pop 15, Push 15 * 3
The result of 45 should then be output.
Your program should ask the user for the name of an input file. The input file will contain postfix
expressions one per line. The program should open the file and evaluate the postfix expressions in the
file. Your program should give the user the option of writing the results to an output file or to the screen.
Your program should also check the entered postfix expression for validity. A postfix expression is
invalid if there is more than one element on the stack at the end of the evaluation or if ever there are not|
enough operands on the stack when an operation is performed.
E.g. If the input file contained:
The output would be:
20 5 - 3 *
45
100 50 60 + +
210
3 45 +
3 4 + +
3 4 ?
Too few operators.
Too many operators.
Illegal operation
Limit your postfix expression to the arithmetic operations of +, -, *, and / on integer values.
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