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The list of compilable functions (given by Compile`CompilerFunctions[] // Sort) shows one of the more interesting- and core-sounding functions: Compile`SetIterate. It does not appear in the Mathematica Documentation Center. Moreover, except for references to merely the list of compilable functions, a search for Compile`SetIterate on the Mathematica Stack Exchange comes up with nothing and a Google search for Compile`SetIterate reveals nothing I can see relating to Mathematica.

What does Compile`SetIterate do?

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1 Answer 1

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It sets an iterator:

cfun = Compile[{{n, _Integer}},
  Table[If[x == 2, Compile`SetIterate[x, 4]]; x, {x, n}]
  ]

Note that 2 and 3 are skipped and the entries beyond the x = n one are junk:

cfun[5]
(*  {1, 4, 5, -5, 0}  *)

cfun[8]
(*  {1, 4, 5, 6, 7, 8, 5244412656, 5244438248}  *)

You need to truncate the table to avoid the uninitialized entries. [A table of size n is preallocated, and essentially a for-loop intializes the table entries. Here Compile`SetIterate short-circuits the for-loop and the initialization.]

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  • $\begingroup$ I would think that decreasing the iterate would cause the loop to overrun the end of the table, but no error is given and you get a table of size n. E.g. Compile[{{n, _Integer}}, Block[{go = True}, Table[If[go && x == 4, go = False; Compile`SetIterate[x, 2]]; x, {x, n}] ] ][8] $\endgroup$
    – Michael E2
    Sep 16, 2021 at 17:12

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