Sorting words in a string by their length in J

Question

I have the following string: s=:'when can we dance' I can find the length of each word with this: # each ;:s I can sort the lengths in ascending/descending order:(/:~) # each ;:s which gives me boxed output. But how do I get the words printed?

Answer 1

You're already using /:~ which is the Reflex of dyadic Sort Up; /:~ y is equivalent to y /: y. This is a convenient shortcut, but it this case what you're skipping past is the solution you're looking for. /: (without Reflex) permutes its x according to the permutation that would result in a sorted y. That "permutation that would result in a sorted y" is incidentally what monadic /: (Grade Up) is for.

   ]w=:;:'when can we dance'
+----+---+--+-----+
|when|can|we|dance|
+----+---+--+-----+
   /: w
1 3 2 0
   (/: w) { w
+---+-----+--+----+
|can|dance|we|when|
+---+-----+--+----+
   w /: w
+---+-----+--+----+
|can|dance|we|when|
+---+-----+--+----+

So to sort w by something else, like the lengths of its items, just provide that something else as Sort Up's y:

   w /: # each w
+--+---+----+-----+
|we|can|when|dance|
+--+---+----+-----+
   (/: # each) w     NB. a hook
+--+---+----+-----+
|we|can|when|dance|
+--+---+----+-----+

;: gives you the J words in a string, which might not be what you want:

   ;: 'when can''t we dance'
|open quote
|       ;:'when can''t we dance'

One of the ;. Cut family or stdlib's splitstring or regexes might help.

   13 : ';: ''when can we dance'''  NB. 13 : can be a mnemonic
(<;._1 ' when can we dance')"_

   <;._1 ' ','when can''t we dance'
+----+-----+--+-----+
|when|can't|we|dance|
+----+-----+--+-----+
   ' ' splitstring 'when can''t we dance'
+----+-----+--+-----+
|when|can't|we|dance|
+----+-----+--+-----+

   s=:'when can''t   we dance'  NB. excess space
   <;._1 ' ',s  NB. empty boxes in result
+----+-----+++--+-----+
|when|can't|||we|dance|
+----+-----+++--+-----+
   require 'regex'
   '[\w'']+' rxall s
+----+-----+--+-----+
|when|can't|we|dance|
+----+-----+--+-----+

To get these word-lists back into a string,

   w
+----+---+--+-----+
|when|can|we|dance|
+----+---+--+-----+
   ;: inv w
when can we dance
   ' ' joinstring w
when can we dance