From b61d395c2c0aebd21a14a918fa776da092d2adda Mon Sep 17 00:00:00 2001
From: Leon Morten Richter <31622033+M0r13n@users.noreply.github.com>
Date: Sat, 11 Jan 2020 16:21:41 +0100
Subject: [PATCH 1/4] Create infix_evaluation.py

---
 data_structures/stacks/infix_evaluation.py | 60 ++++++++++++++++++++++
 1 file changed, 60 insertions(+)
 create mode 100644 data_structures/stacks/infix_evaluation.py

diff --git a/data_structures/stacks/infix_evaluation.py b/data_structures/stacks/infix_evaluation.py
new file mode 100644
index 000000000000..529f5a696e2e
--- /dev/null
+++ b/data_structures/stacks/infix_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 2 ")
+    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))

From bc20692a52adf8b455f22c1fc097922b401ba0de Mon Sep 17 00:00:00 2001
From: Leon Morten Richter <31622033+M0r13n@users.noreply.github.com>
Date: Sat, 11 Jan 2020 16:26:41 +0100
Subject: [PATCH 2/4] fix doctests

---
 data_structures/stacks/infix_evaluation.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/data_structures/stacks/infix_evaluation.py b/data_structures/stacks/infix_evaluation.py
index 529f5a696e2e..00df2c1e63b0 100644
--- a/data_structures/stacks/infix_evaluation.py
+++ b/data_structures/stacks/infix_evaluation.py
@@ -27,9 +27,9 @@ def evaluate(expression):
     Evaluate a given expression in prefix notation.
     Asserts that the given expression is valid.
 
-     >>> evaluate("+ 9 * 2 6")
+    >>> evaluate("+ 9 * 2 6")
     21
-    >>> evaluate("/ * 10 2 + 4 2 ")
+    >>> evaluate("/ * 10 2 + 4 1 ")
     4.0
     """
     stack = []

From cd1f7f198f9a513751ab65d510c66985a7629e16 Mon Sep 17 00:00:00 2001
From: Leon Morten Richter <31622033+M0r13n@users.noreply.github.com>
Date: Sat, 11 Jan 2020 16:29:37 +0100
Subject: [PATCH 3/4] Rename infix_evaluation.py to prefix_evaluation.py

---
 .../stacks/{infix_evaluation.py => prefix_evaluation.py}          | 0
 1 file changed, 0 insertions(+), 0 deletions(-)
 rename data_structures/stacks/{infix_evaluation.py => prefix_evaluation.py} (100%)

diff --git a/data_structures/stacks/infix_evaluation.py b/data_structures/stacks/prefix_evaluation.py
similarity index 100%
rename from data_structures/stacks/infix_evaluation.py
rename to data_structures/stacks/prefix_evaluation.py

From a338bd9f1fc91ce32122c339d0f179ab72a1f9b8 Mon Sep 17 00:00:00 2001
From: Leon Morten Richter <31622033+M0r13n@users.noreply.github.com>
Date: Sat, 11 Jan 2020 16:33:25 +0100
Subject: [PATCH 4/4] Add prefix_evaluation.py to directory

---
 DIRECTORY.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/DIRECTORY.md b/DIRECTORY.md
index 73ecbc36fdc4..1116e8539536 100644
--- a/DIRECTORY.md
+++ b/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)