Create ExpEvaluation

ExpEvaluation

@@ -0,0 +1,144 @@
+// CPP program to evaluate a given 
+// expression where tokens are 
+// separated by space. 
+#include <bits/stdc++.h> 
+using namespace std; 
+
+// Function to find precedence of 
+// operators. 
+int precedence(char op){ 
+	if(op == '+'||op == '-') 
+	return 1; 
+	if(op == '*'||op == '/') 
+	return 2; 
+	return 0; 
+} 
+
+// Function to perform arithmetic operations. 
+int applyOp(int a, int b, char op){ 
+	switch(op){ 
+		case '+': return a + b; 
+		case '-': return a - b; 
+		case '*': return a * b; 
+		case '/': return a / b; 
+	} 
+} 
+
+// Function that returns value of 
+// expression after evaluation. 
+int evaluate(string tokens){ 
+	int i; 
+
+	// stack to store integer values. 
+	stack <int> values; 
+
+	// stack to store operators. 
+	stack <char> ops; 
+
+	for(i = 0; i < tokens.length(); i++){ 
+
+		// Current token is a whitespace, 
+		// skip it. 
+		if(tokens[i] == ' ') 
+			continue; 
+
+		// Current token is an opening 
+		// brace, push it to 'ops' 
+		else if(tokens[i] == '('){ 
+			ops.push(tokens[i]); 
+		} 
+
+		// Current token is a number, push 
+		// it to stack for numbers. 
+		else if(isdigit(tokens[i])){ 
+			int val = 0; 
+
+			// There may be more than one 
+			// digits in number. 
+			while(i < tokens.length() && 
+						isdigit(tokens[i])) 
+			{ 
+				val = (val*10) + (tokens[i]-'0'); 
+				i++; 
+			} 
+
+			values.push(val); 
+		} 
+
+		// Closing brace encountered, solve 
+		// entire brace. 
+		else if(tokens[i] == ')') 
+		{ 
+			while(!ops.empty() && ops.top() != '(') 
+			{ 
+				int val2 = values.top(); 
+				values.pop(); 
+
+				int val1 = values.top(); 
+				values.pop(); 
+
+				char op = ops.top(); 
+				ops.pop(); 
+
+				values.push(applyOp(val1, val2, op)); 
+			} 
+
+			// pop opening brace. 
+			ops.pop(); 
+		} 
+
+		// Current token is an operator. 
+		else
+		{ 
+			// While top of 'ops' has same or greater 
+			// precedence to current token, which 
+			// is an operator. Apply operator on top 
+			// of 'ops' to top two elements in values stack. 
+			while(!ops.empty() && precedence(ops.top()) 
+								>= precedence(tokens[i])){ 
+				int val2 = values.top(); 
+				values.pop(); 
+
+				int val1 = values.top(); 
+				values.pop(); 
+
+				char op = ops.top(); 
+				ops.pop(); 
+
+				values.push(applyOp(val1, val2, op)); 
+			} 
+
+			// Push current token to 'ops'. 
+			ops.push(tokens[i]); 
+		} 
+	} 
+
+	// Entire expression has been parsed at this 
+	// point, apply remaining ops to remaining 
+	// values. 
+	while(!ops.empty()){ 
+		int val2 = values.top(); 
+		values.pop(); 
+
+		int val1 = values.top(); 
+		values.pop(); 
+
+		char op = ops.top(); 
+		ops.pop(); 
+
+		values.push(applyOp(val1, val2, op)); 
+	} 
+
+	// Top of 'values' contains result, return it. 
+	return values.top(); 
+} 
+
+int main() { 
+	cout << evaluate("10 + 2 * 6") << "\n"; 
+	cout << evaluate("100 * 2 + 12") << "\n"; 
+	cout << evaluate("100 * ( 2 + 12 )") << "\n"; 
+	cout << evaluate("100 * ( 2 + 12 ) / 14"); 
+	return 0; 
+} 
+
+// This code is contributed by Nikhil jindal.