Add parsing of simple arithmetic expressions (shunting yard algorithm)

Author: yassin64b

String/ParsingArithmetic/ShuntingYard.cpp

@@ -0,0 +1,77 @@
+#include <iostream>
+#include <map>
+#include <string>
+#include <stack>
+#include <queue>
+#include <sstream>
+#include <cassert>
+
+using namespace std;
+
+// precedences
+map<char, int> prec{{'+', 1}, {'-', 1}, {'*', 2}, {'/', 2}};
+
+// handles +, -, *, / and parentheses
+// only handles single digit, positive numbers for simplicity
+int shuntyard(stringstream& ss) {
+    char c;
+    queue<char> output;
+    stack<char> ops;
+    while (ss >> c) {
+        if (isdigit(c)) {
+            output.push(c);
+        } else if (c == '(') {
+            ops.push(c);
+        } else if (c == ')') {
+            while (ops.top() != '(') {
+                output.push(ops.top());
+                ops.pop();
+            }
+            ops.pop();
+        } else {
+            assert(c == '+' || c == '-' || c == '*' || c == '/');
+            while (!ops.empty() && (ops.top() != '(' && prec[ops.top()] >= prec[c])) {
+                output.push(ops.top());
+                ops.pop();
+            }
+            ops.push(c);
+        }
+    }
+    while (!ops.empty()) {
+        assert(ops.top() != '(' && ops.top() != ')');
+        output.push(ops.top());
+        ops.pop();
+    }
+    
+    stack<int> st;
+    while (!output.empty()) {
+        c = output.front();
+        output.pop();
+        if (isdigit(c)) {
+            st.push(c - '0');
+        } else {
+            int x, y;
+            y = st.top(); st.pop();
+            x = st.top(); st.pop();
+            if (c == '+') {
+                st.push(x + y);
+            } else if (c == '-') {
+                st.push(x - y);
+            } else if (c == '/') {
+                st.push(x / y);
+            } else {
+                st.push(x * y);
+            }
+        }
+    }
+    return st.top();
+}
+
+int main() {
+    string input;
+    getline(cin, input);
+    stringstream ss(input);
+    int val = shuntyard(ss);
+    cout << input << " = " << val << endl;
+    return 0;
+}