Get first N elements from a channel

Nextflow Snippets
1

You may want to consume the first N elements from a channel and don’t know how to do it. Even though there is an operator for that (take, check details here), I think this is a nice opportunity to try to reproduce its behavior by combining other operators. It’s also important to mention that the first channel operator (details here) has a different purpose.

Here, we’ll use the buffer channel operator (details here) that splits a channel into subsets of a specific size and then we will use the first channel operator to consume this first subset which contains the first N elements from the channel.

Channel
  .of(1..100)
  .buffer(size: 5)
  .first()
  .flatten()
  .view()

Or

Channel
  .of('a'..'z')
  .buffer(size: 5)
  .first()
  .flatten()
  .view()

Notice that the buffer operator will create subsets of size 5, where each subset is a single element. Then, the first operator will consume the first element, which is this subset with 5 items. The flatten operator will turn this single element with five items into a channel with five elements. The view operator will print the channel to the screen :smiley:

In case you’re curious, you can find below the output for the last snippet:

N E X T F L O W  ~  version 23.09.2-edge
Launching `marcel.nf` [romantic_waddington] DSL2 - revision: c2d9ba8d7b
a
b
c
d
e
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