bug: raise error if interpolant x values decreasing (#5061)

* bug: raise error if interpolant x values decreasing

* add changelog

* Update src/pybamm/expression_tree/interpolant.py

Co-authored-by: Agriya Khetarpal <74401230+agriyakhetarpal@users.noreply.github.com>

* bug: fix test which was using interpolant incorrectly

---------

Co-authored-by: Agriya Khetarpal <74401230+agriyakhetarpal@users.noreply.github.com>
Co-authored-by: Eric G. Kratz <kratman@users.noreply.github.com>

Author: 3 people

CHANGELOG.md

@@ -7,6 +7,7 @@
 ## Bug fixes
 
 - Converts sensitivities to numpy objects, fixing bug in `DiscreteTimeSum` sensitivity calculation ([#5037](https://github.com/pybamm-team/PyBaMM/pull/5037))
+- Raises error if `pybamm.Interpolant` given 1D x values that are not strictly increasing ([#5061](https://github.com/pybamm-team/PyBaMM/pull/5061))
 
 ## Breaking changes
 

src/pybamm/expression_tree/interpolant.py

@@ -107,6 +107,9 @@ def __init__(
                     "len(x1) should equal y=shape[0], "
                     f"but x1.shape={x1.shape} and y.shape={y.shape}"
                 )
+            # if 1d then x should be monotonically increasing
+            if np.any(x1[:-1] > x1[1:]):
+                raise ValueError("x should be monotonically increasing")
         # children should be a list not a symbol or a number
         if isinstance(children, pybamm.Symbol | numbers.Number):
             children = [children]

tests/unit/test_expression_tree/test_interpolant.py

@@ -76,6 +76,11 @@ def test_interpolation(self):
                 interp.evaluate(y=np.array([2]))[:, 0], np.array([np.nan])
             )
 
+    def test_interpolation_non_increasing(self):
+        x = np.flip(np.linspace(0, 1, 200))
+        with pytest.raises(ValueError, match="x should be monotonically increasing"):
+            pybamm.Interpolant(x, 2 * x, 0.5)
+
     def test_interpolation_float(self):
         x = np.linspace(0, 1, 200)
         interp = pybamm.Interpolant(x, 2 * x, 0.5)

tests/unit/test_expression_tree/test_unary_operators.py

@@ -773,7 +773,7 @@ def test_discrete_time_sum(self):
             pybamm.DiscreteTimeSum(2 * y)
 
         # check that raises error if two data are present
-        data2 = pybamm.DiscreteTimeData(values, times, "test2")
+        data2 = pybamm.DiscreteTimeData(times, values, "test2")
         with pytest.raises(pybamm.ModelError, match="only have one DiscreteTimeData"):
             pybamm.DiscreteTimeSum(data + data2)