MapIndex returns the index in a list instead of as shown in my revised example. I can work with it (or work around it), but what's the design concept? It seems like an unnecessary complication, but I'm sure there is a reason I don't see.

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I tagged this with a design-pattern tag. I'm not sure if that's appropriate. Editors please remove the tag if it's not.

  • 2
    $\begingroup$ I guess this is due to the fact that once you provide a levelspec, it becomes necessary, consider: MapIndexed[f, {{a, b}, {c, d, e}}, {2}] $\endgroup$ Commented Sep 17, 2013 at 18:30
  • $\begingroup$ In addition to what others said, one can remove this {} using First@ like this: MapIndexed[f[#1, First@#2] &, {a, b, c, d}] gives {f[a, 1], f[b, 2], f[c, 3], f[d, 4]} $\endgroup$
    – Nasser
    Commented Sep 17, 2013 at 18:33
  • $\begingroup$ @PinguinDirk I just looked at your example and see how it works. Level specifications are tricky: e.g. 2 vs {2}. I know how it works, but I try to stay on deck one. $\endgroup$ Commented Sep 17, 2013 at 18:44
  • $\begingroup$ Today's interest in expression diffs reminded me about my expression diff code, where there is an application of MapIndexed mapping on possibly deeply nested expression - there I used the second argument essentially as a unique Id for a part in an expression. $\endgroup$ Commented Sep 18, 2013 at 10:38

1 Answer 1


The "unnecessary" complication is needed for those cases where you specify deeper levels than the first:

MapIndexed[f, {{a}, {b}}, {2}]
(* {{f[a, {1, 1}]}, {f[b, {2, 1}]}} *)

The following code produces what you want:

myMapIndexed[f_, l_] := Inner[f, l, Range[Length[l]], List];
myMapIndexed[f, {a, b, c, d}]
(* {f[a, 1], f[b, 2], f[c, 3], f[d, 4]} *)
  • $\begingroup$ I get it. I haven't had an application with a nested structure come up in anything I've done. Thanks. $\endgroup$ Commented Sep 17, 2013 at 18:36

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