combinations_with_replacement

Author: shailesh1729

library/+spx/+discrete/combinations_with_replacement.m

@@ -0,0 +1,29 @@
+function X = combinations_with_replacement(x, k)
+    [m, n] = size(x);
+    % It should be a row vector
+    if m > 1
+        x = x.';
+    end
+    [m, n] = size(x);
+    if m > 1
+        error('Input must be a row vector');
+    end
+    % Number of output rows
+    rows = nchoosek(n+k-1, k);
+    X = zeros(rows, k);
+    % row index for storing a combination
+    cur_row = 1;
+    chosen = zeros(1, k);
+    function iterate(index, r, s, e)
+        if (index > r)
+            X(cur_row, :) = x(chosen);
+            cur_row = cur_row + 1;
+            return;
+        end
+        for i=s:e
+            chosen(index) = i;
+            iterate(index + 1, r, i, e);
+        end
+    end
+    iterate(1, k, 1, n);
+end