remove all boxes with one element

Question

How do I remove all boxes with one element? E.g. I would like to turn this:

┌─────┬───┬─┬─────┐
│1 1 1│2 2│3│2 2 2│
└─────┴───┴─┴─────┘

Into this:

┌─────┬───┬─────┐
│1 1 1│2 2│2 2 2│
└─────┴───┴─────┘

Answer 1

Suppose your value is in a:

   ]a =. 1 1 1; 2 2; 3; 2 2 2
┌─────┬───┬─┬─────┐
│1 1 1│2 2│3│2 2 2│
└─────┴───┴─┴─────┘

The trick here is to get the length of each box:

   #@> a
3 2 1 3

If the length > 1, you get a "mask" you can pass to copy.

   1 < #@> a
1 1 0 1

   a #~ 1 < #@> a
┌─────┬───┬─────┐
│1 1 1│2 2│2 2 2│
└─────┴───┴─────┘

13: will tell us how to make this tacit:

   13 : 'y #~ 1<#@>y'
] #~ 1 < #@>

As it happens, needing to put id in the last position of a fork is exactly what a hook does, so we can even simplify one more step:

   remsingles =. #~ 1 < #@>
   remsingles a
┌─────┬───┬─────┐
│1 1 1│2 2│2 2 2│
└─────┴───┴─────┘