How to just skip an element when building new list? - j

Question

data =: '"MARY","PATRICIA","LINDA","BARBARA","ELIZABETH"`
];._1 data
MARY
,
PATRICIA
,
LINDA
,
BARBARA
,
ELIZABETH
(one more blank line here)
So I want just skip every second line:
]`(0&#);._1 data
MARY
PATRICIA
LINDA
BARBARA
ELIZABETH
(one more blank line here)
Doesn't work as I thought.
And I encounter this situation quite often. For example when using code like this:
]`(Do nothing. Just skip. Just SKIP!!!)#.(some condition)

Answer 1

You could do it this way, by appending a ,',' (important that this is a list and not an atom) after removing the last LF and then using Cut ;. again this time based on the end value.
( ];._2) #: ((,','),~ }:) #: (];._1) data
MARY
PATRICIA
LINDA
BARBARA
ELIZABETH
The blank lines are not really blanks but an artifact of the shape.
$(];._2) #: ((,','),~ }:) #: (];._1) data
5 1 9
Insert Append ,/ cleans this up
,/ #: (];._2) #: ((,','),~ }:) #: (];._1) data
MARY
PATRICIA
LINDA
BARBARA
ELIZABETH
$,/ #: (];._2) #: ((,','),~ }:) #: (];._1) data
5 9
This doesn't really solve your "do nothing" issue with skipping, but there is irony in wanting to do nothing and at the same time wanting to SKIP! Skipping would be doing something, wouldn't it? That might suggest another approach such as Copy # may be better than Agenda #. but one would have to know the specific case.

Answer 2

if you want to skip every second element of a list, you can use (_2 {.\ ])
In other words: break the list up into pairs and get the first element of each pair.

Answer 3

A couple of options to filter out every 2nd element of a list:
mylist=: ];._1 data
(#~ 2 -.#| i.##) mylist NB. retain even indexed items
MARY
PATRICIA
LINDA
BARBARA
ELIZABETH
(_2 {.\ ]) mylist NB. get first item of pairs
MARY
PATRICIA
LINDA
BARBARA
ELIZABETH
A couple of options to parse this data:
> '","' splitstring }. }: data
MARY
PATRICIA
LINDA
BARBARA
ELIZABETH
require 'csv'
> (',' ; '"') fixdsv data,LF
MARY
PATRICIA
LINDA
BARBARA
ELIZABETH