Create postFixEvaluation

postFixEvaluation

@@ -0,0 +1,96 @@
+// C++ program to evaluate value of a postfix expression
+#include <iostream>
+#include <string.h>
+
+using namespace std;
+
+// Stack type
+struct Stack
+{
+	int top;
+	unsigned capacity;
+	int* array;
+};
+
+// Stack Operations
+struct Stack* createStack( unsigned capacity )
+{
+	struct Stack* stack = (struct Stack*) malloc(sizeof(struct Stack));
+
+	if (!stack) return NULL;
+
+	stack->top = -1;
+	stack->capacity = capacity;
+	stack->array = (int*) malloc(stack->capacity * sizeof(int));
+
+	if (!stack->array) return NULL;
+
+	return stack;
+}
+
+int isEmpty(struct Stack* stack)
+{
+	return stack->top == -1 ;
+}
+
+char peek(struct Stack* stack)
+{
+	return stack->array[stack->top];
+}
+
+char pop(struct Stack* stack)
+{
+	if (!isEmpty(stack))
+		return stack->array[stack->top--] ;
+	return '$';
+}
+
+void push(struct Stack* stack, char op)
+{
+	stack->array[++stack->top] = op;
+}
+
+
+// The main function that returns value of a given postfix expression
+int evaluatePostfix(char* exp)
+{
+	// Create a stack of capacity equal to expression size
+	struct Stack* stack = createStack(strlen(exp));
+	int i;
+
+	// See if stack was created successfully
+	if (!stack) return -1;
+
+	// Scan all characters one by one
+	for (i = 0; exp[i]; ++i)
+	{
+		// If the scanned character is an operand (number here),
+		// push it to the stack.
+		if (isdigit(exp[i]))
+			push(stack, exp[i] - '0');
+
+		// If the scanned character is an operator, pop two
+		// elements from stack apply the operator
+		else
+		{
+			int val1 = pop(stack);
+			int val2 = pop(stack);
+			switch (exp[i])
+			{
+			case '+': push(stack, val2 + val1); break;
+			case '-': push(stack, val2 - val1); break;
+			case '*': push(stack, val2 * val1); break;
+			case '/': push(stack, val2/val1); break;
+			}
+		}
+	}
+	return pop(stack);
+}
+
+// Driver program to test above functions
+int main()
+{
+	char exp[] = "231*+9-";
+	cout<<"postfix evaluation: "<< evaluatePostfix(exp);
+	return 0;
+}