convert infix to postfix and then solve the equation - c++

Question

So, I'm having issues with this program. Can anyone tell me what I'm doing wrong here? The program is supposed to take a simple infix notation mathematical expression (e.g. "5 - 2 + 1") and then convert it to (e.g. "5 2 - 1 +") and then solve it which would be 4. It converts just fine but as soon as it gets into the evaluating part, it doesn't show anything on the command prompt. Can I get some help? Thanks!
#include <iostream>
#include <stack>
#include <string>
#include <sstream>
#include <vector>
using namespace std;
int priority(string item) //prioritizing the operators
{
if(item == "(" || item == ")")
{
return 0;
}
else if(item == "+" || item == "-")
{
return 1;
}
else //if(item == "/" || item == "*") <-- guess didnt need to define this one
{
return 2;
}
}
void welcome()//welcome text
{
cout << "Welcome to this program!" << endl;
cout << "Please enter your equation" << endl;
}
int main()
{
welcome(); // welcome text
stack <string> myStack; // initializing the stack.
char line[256];
cin.getline( line, 256); // this and the proceeding line get the input.
string exp = line;
string item;
string postFix;
istringstream iss(exp);
iss >> item;
while(iss)
{
if(item != "+" && item != "-" && item != "/" && item != "*" && item != "(" && item != ")") //If the char is a number
{
cout << item;
postFix = postFix + item;
}
else if(myStack.size() == 0) // If the stack is empty
{
myStack.push(item);
}
else if( item == "+" || item == "-" || item == "/" || item == "*") //If the char is an operator
{
if(priority(myStack.top()) < priority(item)) // the item on the stack is greater priority than the array item
{
myStack.push(item);
}
else
{
while(myStack.size() > 0 && priority(myStack.top()) >= priority(item)) //while the stack contains something, and the item on
{
cout << myStack.top();
postFix = postFix + item;
myStack.pop();
}
myStack.push(item);
}
}
else if(item == "(") // left peren
{
myStack.push(item);
}
else if(item == ")") // right peren
{
while(myStack.top() != "(")
{
cout << myStack.top();
postFix = postFix + item;
myStack.pop();
}
myStack.pop();
}
iss >> item;
}
while (myStack.size() > 0 ) //When nothing is left to evaluate
{
cout << myStack.top();
postFix = postFix + myStack.top();
myStack.pop();
}
cout << endl;
//PART 2
int x1;
int x2;
int x3;
stack<int> thirdStack;
string exp2 = postFix;
string item2;
istringstream iss2(exp2);
iss2 >> item2;
while(iss2)
if(item2 != "+" && item2 != "-" && item2 != "/" && item2 != "*") //if its a number
{
int n;
n = atoi(item2.c_str());
thirdStack.push(n);
}
else if( item2 == "+" || item2 == "-" || item2 == "/" || item2 == "*") //if its an operator
{
x1 = thirdStack.top();
thirdStack.pop();
x2 = thirdStack.top();
thirdStack.pop();
if(item2 == "+")
{
x3 = x1 + x2;
thirdStack.push(x3);
}
else if(item2 == "-")
{
x3 = x1 - x2;
thirdStack.push(x3);
}
else if(item2 == "/")
{
x3 = x1 * x2;
thirdStack.push(x3);
}
else if(item2 == "*")
{
x3 = x1 / x2;
thirdStack.push(x3);
}
}
}
cout << "The conversion into infix notation is" + thirdStack.top() << endl;
}

Answer 1

There are a number of problems with this code.
In part 1, while your code appears to write out the correct postfix conversion, the postFix string it builds up isn't the same thing.
For example, in some places you have code like this:
cout << myStack.top();
postFix = postFix + item;
You're writing out myStack.top() but adding item to your postFix result. It should be this:
cout << myStack.top();
postFix = postFix + myStack.top();
In addition, you ought to be including spaces between each item that you add to the postFix string. So your result should be 5 2 - 1 + rather than 52-1+. Otherwise when trying to interpret that expression, the first item is going to be interpreted as 52.
Then in part 2, you're missing a call to iss2 >> item2; at the end of the while loop. You're basically just interpreting the first item over and over again, so the code will end up in an infinite loop.
And in your calculations, you have the operand order incorrect. This doesn't matter for addition and multiplication, but it does for subtraction and division. For example, the substraction calculation should be x3 = x2 - x1; rather than x3 = x1 - x2;.
Finally, when streaming out the result you've got a plus when you should have a stream operator. It should look like this:
cout << "The conversion into infix notation is" << thirdStack.top() << endl;