X-Git-Url: http://gitweb.michael.orlitzky.com/?p=octave.git;a=blobdiff_plain;f=envelope.m;fp=envelope.m;h=e54f8185f49a1452846411ee654888f01f3cd1c4;hp=0000000000000000000000000000000000000000;hb=8ca30867e869231f85848dc14e15f11e6d9c732b;hpb=4c1c0fdc1eab7fbe1e322ff651e9c98357a8ce15

diff --git a/envelope.m b/envelope.m
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--- /dev/null
+++ b/envelope.m
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+function envelope = envelope(A)
+  ## Compute the envelope of the matrix ``A``. The envelope of a matrix
+  ## is defined as the set of indices,
+  ##
+  ##   E = { (i,j) : i < j, A(k,j) != 0 for some k <= i }
+  ##
+  if (!issymmetric(A) && !is_upper_triangular(A))
+    ## The envelope of a matrix is only defined for U-T or symmetric
+    ## matrices.
+    envelope = {NA};
+    return;
+  end
+
+  ## Start with an empty result, and append to it as we find
+  ## satisfactory indices.
+  envelope = {};
+
+  for j = [ 1 : columns(A) ]
+    ## Everything below the first non-zero element in a column will be
+    ## part of the envelope. Since we're moving from top to bottom, we
+    ## can simply set a flag indicating that we've found the first
+    ## non-zero element. Thereafter, everything we encounter should be
+    ## added to the envelope.
+    found_nonzero = false;
+
+    for i = [ 1 : j-1 ]
+      if (A(i,j) != 0)
+        found_nonzero = true;
+      end
+
+      if (found_nonzero)
+	envelope{end+1} = [i,j];
+      end
+    end
+
+  end
+
+end