Add tridiagonal() and its tests.

diff --git a/tests/tridiagonal_tests.m b/tests/tridiagonal_tests.m
new file mode 100644 (file)
index 0000000..b93e36b
--- /dev/null
+++ b/tests/tridiagonal_tests.m
@@ -0,0 +1,26 @@
+A = tridiagonal(0, -1, 2, -1);
+expected = [];
+unit_test_equals("poisson tridiagonal of size zero is correct", ...
+                expected, ...
+                A);
+
+A = tridiagonal(1, -1, 2, -1);
+expected = [2];
+unit_test_equals("poisson tridiagonal of size one is correct", ...
+                expected, ...
+                A);
+
+A = tridiagonal(2, -1, 2, -1);
+expected = [2, -1; ...
+           -1, 2];
+unit_test_equals("poisson tridiagonal of size two is correct", ...
+                expected, ...
+                A);
+
+A = tridiagonal(3, -1, 2, -1);
+expected = [2, -1, 0; ...
+           -1, 2, -1;
+           0, -1, 2];
+unit_test_equals("poisson tridiagonal of size three is correct", ...
+                expected, ...
+                A);

diff --git a/tridiagonal.m b/tridiagonal.m
new file mode 100644 (file)
index 0000000..ba99b63
--- /dev/null
+++ b/tridiagonal.m
@@ -0,0 +1,25 @@
+function A = tridiagonal(integerN, x, y, z)
+  %
+  % Construct the integerN x integerN (square) tridiagonal matrix
+  % diag(x,y,z).
+  %
+  % INPUT:
+  %
+  %   - ``integerN`` -- The dimensions of the resulting matrix.
+  %
+  %   - ``x`` -- The value on the subdiagonal.
+  %
+  %   - ``y`` -- The value on the diagonal.
+  %
+  %   - ``z`` -- The value on the superdiagonal.
+  %
+  if (integerN < 0)
+    A = NA;
+    return;
+  end
+
+  sub_diag = repmat(x, 1, integerN-1);
+  on_diag = repmat(y, 1, integerN);
+  super_diag = repmat(z, 1, integerN-1);
+  A = diag(on_diag) + diag(super_diag, 1) + diag(sub_diag, -1);
+end