Finished assignment2

forPezy/as2/assignment2_reading_pezy_version.cpp

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+#include <cassert>
+#include <fstream>
+#include <string>
+#include <iostream>
+
+using std::cout; using std::endl; using std::cerr;
+
+struct StackNode {
+    int val;
+    StackNode *next;
+    StackNode(int x) : val(x), next(nullptr) {}
+};
+
+class Stack {
+public:
+    ~Stack() 
+    {
+        for (StackNode *pDel=node; node; delete pDel)
+            node = node->next;
+    }
+    
+    bool empty() { return node == nullptr; }
+    void push(int val)
+    {
+        StackNode *new_node = new StackNode(val);
+        new_node->next = node;
+        node = new_node;
+    }
+    int pop()
+    {
+        StackNode *top = node;
+        assert(top);
+        int ret = top->val;
+        node = top->next;
+        delete top;
+        return ret;
+    }
+private:
+    StackNode *node{nullptr};
+};
+
+int main(int argc, char **argv)
+{
+    if (argc != 2) { cerr << "cannot read file" << endl; exit(0); }
+    std::ifstream ifs(argv[1]);
+    Stack stk;
+    for (std::string exp; getline(ifs, exp); ) {
+        if (isdigit(exp[0])) {
+            cout << "reading number " << exp << endl; 
+            stk.push(stoi(exp));
+        }
+        else if (exp[0] == '+' || exp[0] == '-' || exp[0] == '*' || exp[0] == '/') {
+            cout << "reading operator " << exp[0] << endl;
+            int op1{0}, op2{0};
+            if (!stk.empty()) op2 = stk.pop();
+            else { cerr << "too many operators" << endl; exit(0); }
+            if (!stk.empty()) op1 = stk.pop();
+            else { cerr << "too many operators" << endl; exit(0); }
+            switch (exp[0]) {
+            case '+' : stk.push(op1 + op2); break;
+            case '-' : stk.push(op1 - op2); break;   
+            case '*' : stk.push(op1 * op2); break;
+            case '/' : stk.push(op1 / op2); break;
+            }
+        }
+    }
+    int ret = stk.pop();
+    if (stk.empty()) cout << "The result is " << ret << endl;
+    else cerr << "too many numbers" << endl;
+}
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