了解后缀表达式评估代码

Question

I am trying to understand this piece of code. What it does it postfix expression evaluation. I am having problems understanding the code. I will be very thankful if anyone can help me out.

#include <iostream>
#include <cstring>
#include <stack>
using namespace std;
int main()
{


    //suppose a contains the string..
    //n is the length of string...
    char a[10]="A+B/C";
    int n = strlen(a)
    stack<int>s;
    for (int i=0;i<n;i++)
    {
        if (a[i]=='+')
        {
            s.push(s.pop()+s.pop());
            int temp1 = s.top();
            s.pop();
            int temp2 = s.top();
            s.pop();
            s.push(temp1 * temp2);
        }
        if (a[i]=='*')
            s.push(s.pop() * s.pop());

        if ((a[i]>='0') && (a[i]<='9'))
            s.push(0);
        while ((a[i]>='0') && (a[i]<='9'))
            s.push(10*s.pop()+(a[i++]-'0'));
    }

    cout<<s.pop()<<endl;

    return 0;
}

Thanks in advance.

Answer 1

This site looks to be a good resource on what's happening here, but the expression should use numbers, not letters.

Say you have the infix expression 1+2*3-4*5. The corresponding postfix would be 123*+45*-. First, you start by scanning the string from left to right. The first three numbers are operands so they will be stored on the stack in the order 1 (bottom), 2 (middle), 3 (top). Next, there is a * operator. To deal with this, pop the first two operands off of the stack and multiply them (the first one popped is the right operand and the second is the left). This will evaluate to 2*3=6, and 6 will be stored on the stack, making it 1, 6.

Next, there is a + operator. 1 and 6 are popped and added and 7 is stored on the stack. After this, 4 and 5 are pushed on the stack as well (7, 4, 5). The next character is another * operator, so it evaluates 4*5=20 and pushes 20 onto the stack (7, 20).

Finally, there is a - operator. 7 and 20 are popped and evaluated as 7-20=(-13). This is pushed onto the stack and is ready to be popped as your final answer.

Hope that helps clear up any confusion (assuming I read your question correctly).