The very last example in the documentation for MapAt is:

MapAt applies f repeatedly if a part is mentioned repeatedly:

In[1]:= MapAt[f, {a, b, c, d}, {{1}, {3}, {1}}]

Out[1]= {f[f[a]], b, f[c], d}

So f is applied twice to the first element.

I have a problem in which f takes an additional arguments which should be different in the first and second invocations. Any ideas on how to do this?

If a MapAtIndexed function existed, I could do something like this:

d = {{1, "x"}, {3, "w"}, {1, "q"}};

MapAtIndexed[f[#, Extract[d, #2]] &, {a, b, c, d}, List /@ First /@ d]

And the output would be:

{f[f[a, "x"], "q"], b, f[c, "w"], d}
list = {a, b, c, d};
app = {{1, "x"}, {3, "w"}, {1, "q"}};
Fold[MapAt[f[#, u] & /. u -> #2[[2]], #1, #2[[1]]] &, list, app]

(* {f[f[a, "x"], "q"], b, f[c, "w"], d}*)

Or perhaps:

r = list
(r[[#1]] = f[r[[#1]], #2]) & @@@ app; r
(* {f[f[a, "x"], "q"], b, f[c, "w"], d} *)
  • $\begingroup$ @gus muchas de nadas :) $\endgroup$ Mar 2 '15 at 12:22

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