Merge 1e03cd2 into 30ca4ae

algorithms/stack/evaluate_postfix.py

@@ -0,0 +1,32 @@
+"""
+Given a postfix expression, a function eval_postfix 
+takes a string of postfix expression and evaluates it.
+Note that numbers and operators are seperated by whitespace.
+
+For example:
+eval_postfix('5 14 - 3 /') should return -3.0.
+eval_postfix('-1.3 3 + 2 *') should return 3.4.
+
+"""
+
+def eval_postfix(expression):
+    stack = []
+    split_exp = expression.split()
+    for item in split_exp:
+        try:
+            stack.append(float(item))
+        except ValueError:
+            val1 = stack.pop()
+            val2 = stack.pop()
+            if item == '+':
+                stack.append(val2 + val1)
+            elif item == '-':
+                stack.append(val2 - val1)
+            elif item == '*':
+                stack.append(val2 * val1)
+            elif item == '/':
+                stack.append(val2 / val1)
+    return stack.pop()
+
+print(eval_postfix('5 14 - 3 /'))
+print(eval_postfix('-1.3 3 + 2 *'))