Add the matrix_subs_expr function.

diff --git a/mjo/symbolic.py b/mjo/symbolic.py
index e2645a30ed7fe33ceb8fb787d51bfaf87305f1ad..ec3fc99e1e451b95b33eed22790b3dc089f617d1 100644 (file)
--- a/mjo/symbolic.py
+++ b/mjo/symbolic.py
@@ -44,3 +44,45 @@ def safe_simplify(expr):
     expr = expr.simplify_factorial()
     expr = expr.simplify_log()
     return expr
+
+
+def matrix_subs_expr(m, *equations):
+    """
+    Symbolic matrices have a `subs()` method, but no `subs_expr()`.
+    This makes it diffucult to substitute in a list of solutions obtained
+    with `solve()`.
+
+    INPUT:
+
+      - ``m`` -- A symbolic matrix.
+
+      - ``equations`` - One or more symbolic equations, presumably for
+        the entries of `m`.
+
+    OUTPUT:
+
+    The result of substituting each equation into `m`, one after another.
+
+    EXAMPLES::
+
+    sage: w,x,y,z = SR.var('w,x,y,z')
+    sage: A = matrix(SR, [[w,x],[y,z]])
+    sage: matrix_subs_expr(A, w == 1, x == 2, y == 3, z == 4)
+    [1 2]
+    [3 4]
+
+    """
+    from sage.symbolic.expression import is_SymbolicEquation
+
+    if not m.base_ring() == SR:
+        raise TypeError, 'the matrix "m" must be symbolic'
+
+    if isinstance(equations[0], dict):
+        eq_dict = equations[0]
+        equations = [ x == eq_dict[x] for x in eq_dict.keys() ]
+
+    if not all([is_SymbolicEquation(eq) for eq in equations]):
+            raise TypeError, "each expression must be an equation"
+
+    d = dict([(eq.lhs(), eq.rhs()) for eq in equations])
+    return m.subs(d)