How to write this code as a pure function?

Question

The code has three places with same variable ls. So I think it would be possible to write it as a pure function.

ls = {{1, a}, {2, b}, {3, c}};
MapThread[Insert, {ls, Table[0, {Length[ls]}], Table[2, {Length[ls]}]}]

My attempt is:

MapThread[Insert, {#, Table[0, {Length[#]}], Table[2, {Length[#]}]} & ls]

which is not correct.

This gives some answer: MapThread[Insert, {#, Table[0, {Length[#]}], Table[2, {Length[#]}]}] &[ls] — Andrew, Jan 07 '16 at 07:15
@Andrew can you explain why this works? why put operator there — an offer can't refuse, Jan 07 '16 at 07:19
& @ls instead of & ls would also work. Writing & ls is like writing f ls, which in Mathematica syntax means f multiplied by ls - and you can't multiply an anonymous function with a list. — C. E., Jan 07 '16 at 07:34
@Pickett What about Length@(#&ls), does this contradict with what you are saying? — an offer can't refuse, Jan 07 '16 at 07:39
@buzhidao take a look at what # & ls returns and why Length is "correct". I'voting to close this question as a duplicate of common pitfals. But you may also by interested in Function Pure Function entries in documentation. — Kuba, Jan 07 '16 at 07:43
@buzhidao If you interpret me literally I guess; you can multiply an anonymous function by a list, but it doesn't make sense to do so. What you end up with is a list of values multiplied by anonymous functions, but what good is that. Use Column @@ FullForm[# & ls] to see what I mean, those expressions that you generate by multiplying ls with # & don't mean anything. — C. E., Jan 07 '16 at 07:43

score 2 · Accepted Answer · answered Jan 07 '16 at 11:30

2

f = Thread@Riffle[Transpose@#, 0, 2] & ;

or ciao's solution

f = Map[Riffle[#, 0] &]

gives

f[ls]

{{1, 0, a}, {2, 0, b}, {3, 0, c}}

answered Jan 07 '16 at 11:30

rhermans

1 Answers1