Fixed an error in symmath_check that occurred when

the expected result = 0.  symmath_check() was dividing
a quantity by the expected result, leading to a NaN value
that caused the answer to be marked incorrect.

Added test_symmath_check.py with basic tests for
the symmath_check() function.

lms/lib/symmath/symmath_check.py

@@ -149,6 +149,12 @@ def make_error_message(msg):
     msg = '<div class="capa_alert">%s</div>' % msg
     return msg
 
+def is_within_tolerance(expected, actual, tolerance):
+    if expected == 0:
+        return (abs(actual) < tolerance)
+    else:
+        return (abs(abs(actual - expected) / expected) < tolerance)
+
 #-----------------------------------------------------------------------------
 # Check function interface, which takes pmathml input
 #
@@ -217,14 +223,14 @@ def symmath_check(expect, ans, dynamath=None, options=None, debug=None, xml=None
     # do a numerical comparison if both expected and answer are numbers
     if (hasattr(fexpect, 'is_number') and fexpect.is_number 
         and hasattr(fans, 'is_number') and fans.is_number):
-        if abs(abs(fans - fexpect) / fexpect) < threshold:
+        if is_within_tolerance(fexpect, fans, threshold):
             return {'ok': True, 'msg': msg}
         else:
             msg += '<p>You entered: %s</p>' % to_latex(fans)
             return {'ok': False, 'msg': msg}
 
     if do_numerical:		# numerical answer expected - force numerical comparison
-        if abs(abs(fans - fexpect) / fexpect) < threshold:
+        if is_within_tolerance(fexpect, fans, threshold):
             return {'ok': True, 'msg': msg}
         else:
             msg += '<p>You entered: %s (note that a numerical answer is expected)</p>' % to_latex(fans)

lms/lib/symmath/test_symmath_check.py

+from unittest import TestCase
+from symmath_check import symmath_check
+
+class SymmathCheckTest(TestCase):
+    def test_symmath_check_integers(self):
+        number_list = [i for i in range(-100, 100)]
+        self._symmath_check_numbers(number_list)
+
+    def test_symmath_check_floats(self):
+        number_list = [i + 0.01 for i in range(-100, 100)]
+        self._symmath_check_numbers(number_list)
+
+    def _symmath_check_numbers(self, number_list):
+
+        for n in number_list:
+
+            # expect = ans, so should say the answer is correct
+            expect = n
+            ans = n
+            result = symmath_check(str(expect), str(ans))
+            self.assertTrue('ok' in result and result['ok'],
+                            "%f should == %f" % (expect, ans))
+
+            # Change expect so that it != ans
+            expect += 0.1
+            result = symmath_check(str(expect), str(ans))
+            self.assertTrue('ok' in result and not result['ok'],
+                            "%f should != %f" % (expect, ans))