I have part of a code that looks like this:

horpatt = (* some code giving a List of n elements *);
verpatt = (* some code giving a List of n elements *);
tabpatt = Riffle[horpatt, verpatt] (* some ReplaceAll code *);

It is run several times with different values of the first two variables. Sometimes it produces this warning:

The third argument {2,-1,2} should be a positive integer or a list with three integers

which doesn't seem to be documented anywhere on the Internet. I try to see what might be wrong with the first two variables with the following code:

tabpatt = Check[
  Riffle[horpatt, verpatt] (* some code *),
  riffletrack = {horpatt, verpatt}; Abort[]

riffletrack gives me exact values of horpatt and verpatt that causes the warning. But when I try those values, the warning doesn't actually occur.

horpatt = {{"S", "Q", "U", "I", "Z"}, {"V", "U", "L", "G", "O"}, {Null, "O", Null, "G", "O"}, {"S", "T", "E", "E", "N"}, {Null, "H", Null, "D", "S"}};
verpatt = {{"S", "V", Null, "S", Null}, {"Q", "U", "O", "T", "H"}, {"U", "L", Null, "E", Null}, {"I", "G", "G", "E", "D"}, {"Z", "O", "O", "N", "S"}};
tabpatt = Riffle[horpatt, verpatt] /. Null -> _

Even more strangely,

Riffle[horpatt, verpatt, {2, -1, 2}] /. Null -> _

doesn't produce a warning, but a desired result that is identical to both of the following:

Riffle[horpatt, verpatt] /. Null -> _
Riffle[horpatt, verpatt, {2, 10, 2}] /. Null -> _

I'm not even sure what questions to ask. Does Mathematica automatically insert {2, -1, 2} as a third argument into Riffle? The warning seems to suggest that such argument doesn't work but why does it work? Why is there the warning in the first place? Should I always insert a proper third argument (akin to {2, 10, 2} above) to avoid this seemingly random warning? Is this a bug?

I'm using Mathematica for Mac.


I have actually put {2, 10, 2} as a third argument into my code:

tabpatt = Riffle[horpatt, verpatt, {2, 10, 2}] (* some ReplaceAll code *);

After many iterations, Mathematica eventually gives several instances of:

The third argument {2,10,2} should be a positive integer or a list with three integers

I'm pretty sure there's a bug in there somewhere.

  • $\begingroup$ Actually the syntax you have discovered isn't completely undocumented: the 5th example under the "Scope" section suggests that such generalization could work: "Cyclically riffle in x and y: Riffle[{1, 2, 3, 4, 5, 6, 7, 8, 9}, {x, y}, {5, -1, 2}]." $\endgroup$ May 10, 2016 at 7:17
  • $\begingroup$ The issue is not readily reproducible; it appears at random. The code just before 'Even more strangely' had given me the issue (I caught it with Check); running it again didn't. $\endgroup$
    – Taiki
    May 10, 2016 at 13:31
  • $\begingroup$ I suggest you to contact the tech support on this issue. $\endgroup$ May 18, 2016 at 2:37


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