prelude: add ltranspose

prelude.tcl

@@ -57,6 +57,26 @@ proc transpose strings {
     return $out
 }
 
+# transpose a list of lists
+#
+#     % ltranspose {{a b c} {1 2 3}}
+#     {a 1} {b 2} {c 3}
+#     % ltranspose {{a 1} {b 2} {c 3}}
+#     {a b c} {1 2 3}
+#
+proc ltranspose lists {
+    set out {}
+    set C [llen [lindex $lists 0]]
+    for {set i 0} {$i < $C} {incr i} {
+        set column {}
+        foreach row $lists {
+            lappend column [lindex $row $i]
+        }
+        lappend out $column
+    }
+    return $out
+}
+
 # extracts one or more indexes from a strided list
 # e.g.
 #       % set l {0 1 2   a b c   x y z}