Add a program to evaluate a string in prefix notation (Polish Notation) (#1675)

* Create infix_evaluation.py

* fix doctests

* Rename infix_evaluation.py to prefix_evaluation.py

* Add prefix_evaluation.py to directory

DIRECTORY.md

@@ -127,6 +127,7 @@
     * [Stock Span Problem](https://github.com/TheAlgorithms/Python/blob/master/data_structures/stacks/stock_span_problem.py)
   * Trie
     * [Trie](https://github.com/TheAlgorithms/Python/blob/master/data_structures/trie/trie.py)
+    * [Prefix Evaluation](https://github.com/TheAlgorithms/Python/blob/master/data_structures/prefix_evaluation.py)
 
 ## Digital Image Processing
   * [Change Contrast](https://github.com/TheAlgorithms/Python/blob/master/digital_image_processing/change_contrast.py)

data_structures/stacks/prefix_evaluation.py

@@ -0,0 +1,60 @@
+"""
+Python3 program to evaluate a prefix expression.
+"""
+
+calc = {
+    "+": lambda x, y: x + y,
+    "-": lambda x, y: x - y,
+    "*": lambda x, y: x * y,
+    "/": lambda x, y: x / y,
+}
+
+
+def is_operand(c):
+    """
+    Return True if the given char c is an operand, e.g. it is a number
+
+    >>> is_operand("1")
+    True
+    >>> is_operand("+")
+    False
+    """
+    return c.isdigit()
+
+
+def evaluate(expression):
+    """
+    Evaluate a given expression in prefix notation.
+    Asserts that the given expression is valid.
+
+    >>> evaluate("+ 9 * 2 6")
+    21
+    >>> evaluate("/ * 10 2 + 4 1 ")
+    4.0
+    """
+    stack = []
+
+    # iterate over the string in reverse order
+    for c in expression.split()[::-1]:
+
+        # push operand to stack
+        if is_operand(c):
+            stack.append(int(c))
+
+        else:
+            # pop values from stack can calculate the result
+            # push the result onto the stack again
+            o1 = stack.pop()
+            o2 = stack.pop()
+            stack.append(calc[c](o1, o2))
+
+    return stack.pop()
+
+
+# Driver code
+if __name__ == "__main__":
+    test_expression = "+ 9 * 2 6"
+    print(evaluate(test_expression))
+
+    test_expression = "/ * 10 2 + 4 1 "
+    print(evaluate(test_expression))