prelude: add lextract

prelude.tcl

@@ -56,3 +56,25 @@ proc transpose strings {
     }
     return $out
 }
+
+# extracts one or more indexes from a strided list
+# e.g.
+#       % set l {0 1 2   a b c   x y z}
+#       % lextract $l 3 0
+#       0 a x
+#       % lextract $l 3 {1 2}
+#       1 2 b c y z
+#
+# equivalent to [lmap {a b c} $lst {list $b $c}] except that
+# you don't have to name the list elements
+proc lextract {lst stride index} {
+    set i 0
+    set out {}
+    if {$stride <= 0} { error "stride must be positive: $stride" }
+    for {set i 0} {$i < [llength $lst]} {incr i $stride} {
+        foreach j $index {
+            lappend out [lindex $lst $i+$j]
+        }
+    }
+    return $out
+}