Append a Value to Every Sublist

Question

Given the following list of lists:

l:(1 2 3 4;7 2 4 1;1 3 5 1 8;0 8;8 2 4)

Join the number 9 to the end of each sublist.

Expected result:

1 2 3 4 9
7 2 4 1 9
1 3 5 1 8 9
0 8 9
8 2 4 9

Solution

l,\:9

Alternative Solution

l,'9

Alternative Solution

@[l;(::);,;9]

Explanation

The each-left iterator:

\:

pairs the fixed right argument 9 against every sublist of l in turn, joining it to the end of each one independently.

Because 9 is a single atom rather than a list, each-both (,') produces the same result here — with an atomic right argument there's nothing for ' to line up against per item, so it behaves the same as \:. This stops being true the moment the right argument is itself a list with one value per sublist.

The amend form, @[l;(::);,;9], takes a different route: the index (::) selects every item of l, and amend applies , with 9 to each selected item, giving the same result. It is useful to know because it generalises to amending only some sublists — for example @[l;0 2;,;9] appends 9 to the first and third sublists only.