fix: use HiGHS to parse CBC sol correctly (#442)

* fix: use HiGHS to parse CBC sol correctly

* Update release notes

---------

Co-authored-by: Fabian Hofmann <fab.hof@gmx.de>

Author: siddharth-krishna

doc/release_notes.rst

@@ -6,6 +6,8 @@ Upcoming Version
 
 **Bug Fixes**
 
+* Fix the parsing of solutions returned by the CBC solver when solving from a file to not
+  assume that variables start with `x`.
 * Fix the retrieval of solutions from the SCIP solver, and do not turn off presolve.
 
 Version 0.5.2

linopy/solvers.py

@@ -453,6 +453,11 @@ def solve_problem_from_file(
             status = Status(SolverStatus.warning, TerminationCondition.unknown)
         status.legacy_status = data
 
+        # Use HiGHS to parse the problem file and find the set of variable names, needed to parse solution
+        h = highspy.Highs()
+        h.readModel(path_to_string(problem_fn))
+        variables = {v.name for v in h.getVariables()}
+
         def get_solver_solution() -> Solution:
             objective = float(data[len("Optimal - objective value ") :])
 
@@ -467,7 +472,7 @@ def get_solver_solution() -> Solution:
                 usecols=[1, 2, 3],
                 index_col=0,
             )
-            variables_b = df.index.str[0] == "x"
+            variables_b = df.index.isin(variables)
 
             sol = df[variables_b][2]
             dual = df[~variables_b][3]