file order after nextflow groupTuple

Question

Below is the code from nextflow site (https://www.nextflow.io/docs/edge/operator.html#grouptuple).

For the output, the first is A B C, and the second is C, A.

Is there a way to make the output like this

[1, [A, B, C]] [2, [A, C]]

That is, the stuff inside [] is in some order.

Channel
    .of(
        [1, 'A'],
        [1, 'B'],
        [2, 'C'],
        [3, 'B'],
        [1, 'C'],
        [2, 'A'],
        [3, 'D']
    )
    .groupTuple()
    .view()

[1, [A, B, C]]
[2, [C, A]]
[3, [B, D]]

Answer 1

You can change the sorting criteria for the grouped items using the sort keyword. To perform a natural sort, just use sort: true. For example:

Channel.of(
    [1, 'A'],
    [1, 'B'],
    [2, 'C'],
    [3, 'B'],
    [1, 'C'],
    [2, 'A'],
    [3, 'D']
)
.groupTuple(sort: true)
.view()

Results:

$ nextflow run main.nf 
N E X T F L O W  ~  version 22.04.0
Launching `main.nf` [special_darwin] DSL2 - revision: 2e641ccfd9
[1, [A, B, C]]
[2, [A, C]]
[3, [B, D]]

Otherwise, a custom sorting criteria can be specified using a closure.