Terraform: Ability to get N elemets from a list

[dene14]

Hi there,

is there any ability to get a sublist from a list? After data sources release (damn, they're so sweet) that become really desired functional.

For e.g. there is "aws_availability_zones" data source and we can easily allocate multi-zoned environment with interpolation, however in some cases there is 5 AZ available whereas only 2 required for particular project. Is there any way to do that or can we expect such feature soon?

[apparentlymart]

Hi @dene14!

There isn't a direct way to do this right now, but I have some workarounds for you:

If you specifically want the first two in the list, and you know there are at least two (which for AWS Availability Zones seems like a reasonable assumption) then you can just list them out manually:

[
  "${data.aws_availability_zones.available.names[0]",
  "${data.aws_availability_zones.available.names[1]",
]

We also have the random_shuffle resource, whose initial use-case was to pick any two AZs from the available set:

resource "random_shuffle" "az" {
  input = ["${data.aws_availability_zones.available.names}"]
  result_count = 2
}

If you use this one, it's important to understand the constraints and caveats relating to the random provider.

With all of that said, a function for slicing a sub-list out of a list seems like a very reasonable request, so I'll leave this issue here as a request for that. :grinning:

[wr0ngway]

👍 a slice interpolation function would do it for me
slice(begin_idx[, end_idx]) where an index can be negative to indicate counting back from the the end of the list, and end_index is optional to indicate slicing to the end of the list.

[ewbankkit]

One way I've found of working around the lack of such functionality is to use the replace() function on a string representation of the list. For example, to remove the last ID from a list of subnet IDs:

"${split(",", replace(join(",", aws_subnet.foo.*.id), "/,subnet-[[:xdigit:]]+$/", ""))}"

• Create a string representation of the list using join()
• Manipulate using replace() and the power of regular expressions
• List-ify the result using split()

BTW, not recommending this as in any way maintainable in the long term 😄.

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