Added ICDF for the discrete uniform distribution.

Author: gokuld

pymc/distributions/discrete.py

@@ -1058,6 +1058,15 @@ def logcdf(value, lower, upper):
             msg="lower <= upper",
         )
 
+    def icdf(value, lower, upper):
+        res = pt.ceil(value * (upper - lower + 1)).astype("int64") + lower - 1
+        res = check_icdf_value(res, value)
+        return check_icdf_parameters(
+            res,
+            lower <= upper,
+            msg="lower <= upper",
+        )
+
 
 class Categorical(Discrete):
     R"""

tests/distributions/test_discrete.py

@@ -29,7 +29,7 @@
 import pymc as pm
 
 from pymc.distributions.discrete import Geometric, _OrderedLogistic, _OrderedProbit
-from pymc.logprob.abstract import logcdf
+from pymc.logprob.abstract import icdf, logcdf
 from pymc.logprob.joint_logprob import logp
 from pymc.logprob.utils import ParameterValueError
 from pymc.pytensorf import floatX
@@ -118,13 +118,21 @@ def test_discrete_unif(self):
             Domain([-10, 0, 10], "int64"),
             {"lower": -Rplusdunif, "upper": Rplusdunif},
         )
+        check_icdf(
+            pm.DiscreteUniform,
+            {"lower": -Rplusdunif, "upper": Rplusdunif},
+            lambda q, lower, upper: st.randint.ppf(q=q, low=lower, high=upper + 1),
+            skip_paramdomain_outside_edge_test=True,
+        )
         # Custom logp / logcdf check for invalid parameters
         invalid_dist = pm.DiscreteUniform.dist(lower=1, upper=0)
         with pytensor.config.change_flags(mode=Mode("py")):
             with pytest.raises(ParameterValueError):
                 logp(invalid_dist, 0.5).eval()
             with pytest.raises(ParameterValueError):
                 logcdf(invalid_dist, 2).eval()
+            with pytest.raises(ParameterValueError):
+                icdf(invalid_dist, np.array(1)).eval()
 
     def test_geometric(self):
         check_logp(