Re-add ExpressionState.get_name and fix to_sympy

2 files changed, 21 insertions, 1 deletions

diff --git a/polymatrix/expression/to.py b/polymatrix/expression/to.py
index c7f9af4..7f9bac7 100644
--- a/polymatrix/expression/to.py
+++ b/polymatrix/expression/to.py
@@ -54,7 +54,7 @@ def to_sympy(
             sympy_poly_terms = []
             for monomial, coeff in poly.terms():
                 sympy_monomial = math.prod(
-                        sympy.Symbol(state.get_symbol(variable.index)) ** variable.power
+                        sympy.Symbol(state.get_name(variable.index)) ** variable.power
                         for variable in monomial)
 
                 if math.isclose(coeff, 1.):
diff --git a/polymatrix/expressionstate.py b/polymatrix/expressionstate.py
index 562f07a..c4d94ad 100644
--- a/polymatrix/expressionstate.py
+++ b/polymatrix/expressionstate.py
@@ -179,6 +179,26 @@ class ExpressionState(StateCacheMixin):
 
         raise KeyError(f"There is no symbol named {name}")
 
+    def get_name(self, index: int) -> str:
+        """
+        Get the name of a variable given its index. 
+
+        What is the difference between the name and the symbol? Suppose M is a
+        nonzero matrix, M is the symbol, but the matrix entry in the leftmost
+        corner has name M_1. If the symbol represent a scalar, the symbol and
+        the name concide.
+        """
+        for symbol, (start, end, _) in self.indices.items():
+            if start <= index < end:
+                # Variable is not scalar
+                if end - start > 1:
+                    return f"{symbol}_{index - start}"
+
+                return symbol
+
+        raise IndexError(f"There is no variable with index {index}.")
+        raise KeyError(f"There is no symbol named {name}")
+
     # -- Old API ---
 
     @property