Merge pull request #123 from ChristopherChudzicki/no-vec-triple-mult

disallow multiplication of 3+ vectors w/o parens

mitxgraders/helpers/calc/calc.py

@@ -958,6 +958,12 @@ def eval_product(self, parse_result):
         Traceback (most recent call last):
         CalcError: Unexpected symbol + in eval_product
         """
+        num_vectors = sum([isinstance(operand, MathArray) and operand.ndim == 1
+                           for operand in parse_result])
+        if num_vectors >= 3:
+            raise CalcError("Multiplying three or more vectors is ambiguous. "
+                            "Please place parentheses around vector multiplications.")
+
         result = parse_result[0]
         data = parse_result[1:]
         while data:

tests/helpers/calc/test_calc_arrays.py

@@ -3,7 +3,7 @@
 import numpy as np
 from mitxgraders import evaluator, MathArray
 from mitxgraders.helpers.calc.math_array import equal_as_arrays
-from mitxgraders.helpers.calc.exceptions import UnableToParse
+from mitxgraders.helpers.calc.exceptions import UnableToParse, CalcError
 
 def test_array_input():
     """Test that vectors/matrices can be inputted into calc.py"""
@@ -54,3 +54,25 @@ def test_math_arrays():
     expr = '(z*[[1, 5], [4, -2]]^n + 10*A/x)*v'
     result = evaluator(expr, variables, max_array_dim=2)[0]
     assert equal_as_arrays(result, (z*A**n + 10*A/x)*v)
+
+def test_triple_vector_product_raises_error():
+    # Since vector products are interpretted as dot products, they are
+    # ambiguous, and we disallow them.
+
+    variables = {
+    'i': MathArray([1, 0]),
+    'j': MathArray([0, 1]),
+    }
+
+    assert equal_as_arrays(
+        evaluator("(i*i)*j", variables)[0],
+        variables['j']
+    )
+
+    match = ("Multiplying three or more vectors is ambiguous. "
+             "Please place parentheses around vector multiplications.")
+    with raises(CalcError, match=match):
+        evaluator("i*i*j", variables)[0]
+
+    with raises(CalcError, match=match):
+        evaluator("i*2*i*3*j", variables)[0]