How would be a functional approach to shifting certain array elements?

Question

I have a Scala app with a list of items with checkboxes so the user select some, and click a button to shift them one position up (left). I decided to write a function to sh

Answer 1

A solution in J:

   ('abcdefghijklmnopqrstuvwxyz';'ABCDEFGHIJKLMNOPQRSTUVWXYZ') (4 : '(y#~y e. >1{x)([: I. '' ''= ])} }._1&|.&.((1,~y e. >0{x)&#)y,'' ''') 'abcDEfghI'
abDEcfgIh

Let’s break this into named pieces for easier comprehension. The final string “abDEcfgIh” is the result of applying a function to the string “abcDEfghI” which is the right argument to the function. The pair of alphabets constitute the left argument to the function (which is the part beginning “(4 :…”). So, instead of the 2-element vector of boxed strings, we could name each one individually:

   'lc uc'=. 'abcdefghijklmnopqrstuvwxyz';'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

Now that we have two variables “lc” and “uc” for the lower-case and upper-case alphabets, let’s examine the body of the function in detail. Taking a logically coherent chunk from the right end, since this would be evaluated first, we could name this like so:

   rmUCshift=: 4 : 0
   }._1&|.&.((1,~y e. >0{x)&#)y,' '
)

This defines “rmUCshift” as something that requires a right and left argument (the “4 :” specifies this) with the body beginning on the next line and continuing to the bare closing paren. The “4 : 0” form, followed by the body, is a variant of the “4 :‘body’” form shown initially. This verb rmUCshift can be invoked independently like this:

   (lc;'') rmUCshift 'abcDEfghI'  NB. Remove upper-case, shift, then insert
ab  cfg h                         NB. spaces where the upper-case would now be.

The invocation is indented three spaces and the output immediately follows the it. The left argument (lc;'') is a two-element vector with the empty array specified as the second element because it’s not used in this piece of the code – we could have used any value after the semicolon but the two single quotes are easy to type.

The next pieces to name are these (definitions followed by examples):

  ixSpaces=: [:I.' '=]
  ixSpaces 'ab  cfg h'
2 3 7
   onlyUC=: 4 : 'y#~y e.>1{x'
   ('';uc) onlyUC 'abcDEfghI'
DEI

Combining these named pieces together gives us this:

   (lc;uc) (4 : '(x onlyUC y)(ixSpaces x rmUCshift y)}x rmUCshift y') 'abcDEfghI'
abDEcfgIh

However, the repetition of “x rmUCshift y” is unnecessary and can be simplified to give us this:

   (lc;uc) (4 : '(x onlyUC y) ((ixSpaces ]) } ]) x rmUCshift y') 'abcDEfghI'
abDEcfgIh