Reto mouredev#28 - python

Retos/Reto #28 - EXPRESIÓN MATEMÁTICA [Media]/python/nlarrea.py

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+"""
+ * Crea una función que reciba una expresión matemática (String)
+ * y compruebe si es correcta. Retornará True o False.
+ * - Para que una expresión matemática sea correcta debe poseer
+ *   un número, una operación y otro número separados por espacios.
+ *   Tantos números y operaciones como queramos.
+ * - Números positivos, negativos, enteros o decimales.
+ * - Operaciones soportadas: + - * / % 
+ *
+ * Ejemplos:
+ * "5 + 6 / 7 - 4" -> True
+ * "5 a 6" -> False
+"""
+
+import re
+
+def check_expression(expression):
+    OPERATORS = ["+", "-", "*", "/", "%"]
+    expression_list = re.split("\s+", expression.strip())
+
+
+    # if no 3 elements -> NOK
+
+    if len(expression_list) < 3:
+        print("\nThere must be at least 3 elements in the expression.")
+        return False
+
+
+    # if no operators at all -> NOK
+
+    if not any([element for element in expression_list if element in OPERATORS]):
+        print("\nThere are no operators in this expression.")
+        return False
+
+
+    # if expression starts or ends with operator -> NOK
+    # * expression can start with "-" or "+"
+
+    if (
+        expression_list[0] in OPERATORS and expression_list[0] != "-" and expression_list[0] != "+" or
+        expression_list[len(expression_list) - 1] in OPERATORS
+    ):
+        print(
+            "\nExpressions can not start with operators (except "-" or "+").\nExpressions can not end with operators."
+        )
+        return False
+
+
+    # check operators between numbers
+
+    last_element = ""
+
+    for element in expression_list:
+        # if two operators together (no space between them) -> NOK
+        if len(element) > 1 and not re.match(r"^\d+$", element):
+            print("\nThis expression has more than one operator without spacing.")
+            return False
+
+        # if no operators between numbers -> NOK
+        if re.match(r"^\d+$", last_element) and re.match(r"^\d+$", element):
+            print("\nMissing operator between numbers.")
+            return False
+
+        # if two operators between numbers -> NOK
+        if (
+            last_element in OPERATORS and element in OPERATORS and
+            element != "-"    # allowed for the number sign
+        ):
+            print("\nMissing number between operators.")
+            return False
+
+        # if not allowed operator -> NOK
+        if re.match(r"(?!-|\+|\/|%|\*)[\W]", element):
+            print("\nThere is a not allowed operator in this expression.")
+            return False
+
+        last_element = element
+
+
+    # if no errors -> OK
+
+    return True
+
+
+print(check_expression("5 + 6 / 7 - 4"))      # True
+print(check_expression("a + b / c - d"))      # True -> letters can be considered numbers
+print(check_expression("5 / - 6"))            # True -> "-" operator allowed after another operator (number sign)
+
+print(check_expression("5 a 6"))              # False -> no operators
+print(check_expression("+ 5 - 6 8"))          # False -> missing operator between numbers
+print(check_expression("+ 5 - 6 / % 8"))      # False -> missing number between operators
+print(check_expression("5 & - 6"))            # False -> not allowed operator