# How can I assign a default channel to another channel if it’s empty, while retaining its original content when it’s not empty, considering the default value is also a channel? **URL:** \ **Category:** Ask for help\ **Tags:** nextflow, platform\ **Created:** [November 17, 2024, 7:33am UTC](https://community.seqera.io/t/how-can-i-assign-a-default-channel-to-another-channel-if-it-s-empty-while-retaining-its-original-content-when-it-s-not-empty-considering-the-default-value-is-also-a-channel/1435 "2024-11-17T07:33:32Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Kanna\_Dhasan](https://dub1.discourse-cdn.com/flex013/user_avatar/community.seqera.io/kanna_dhasan/32/1217_2.png) [@Kanna\_Dhasan](https://community.seqera.io/u/Kanna_Dhasan)\ **Post date:** [November 17, 2024, 7:33am UTC](https://community.seqera.io/t/how-can-i-assign-a-default-channel-to-another-channel-if-it-s-empty-while-retaining-its-original-content-when-it-s-not-empty-considering-the-default-value-is-also-a-channel/1435/1 "2024-11-17T07:33:32Z")
Hi all, I’m working on a larger workflow and have encountered a challenge. Here’s the scenario: I have a channel v1, which may sometimes be empty. In that case, I want to assign another channel sample (holding [SNP-2024-FT]) to v1. However, if v1 is not empty, it should retain its original value. Here’s the code snippet I tried: ```auto groovy params.sampleId = "SNP-2024-FT" workflow { input_ch = Channel.of( tuple(params.sampleId) ) sample = input_ch.map { it -> [it[0]] } sample.view() v1 = Channel.empty().ifEmpty(true).map { value -> if (value == true) { sample } else { value } } v1.view() } ``` The issue is that this gives the error:` DataflowBroadcast around DataflowStream[?]`. It seems related to the fact that sample is itself a channel, not a static value, and the `ifEmpty(true)` only works with direct static values. Additionally, consider that Channel.empty() is an empty output channel from a process in this context, so the behavior of assigning it can be tricky. When I set a direct value like `ifEmpty("some value")`, it works, but since sample is an output channel, it’s always a channel rather than a direct value. Does anyone know how to work around this or handle such cases? Any advice or alternative approaches would be greatly appreciated! Thanks in advance! ---
**Author:** ![mribeirodantas](https://dub1.discourse-cdn.com/flex013/user_avatar/community.seqera.io/mribeirodantas/32/235_2.png) [@mribeirodantas](https://community.seqera.io/u/mribeirodantas)\ **Post date:** [November 18, 2024, 2:26am UTC](https://community.seqera.io/t/how-can-i-assign-a-default-channel-to-another-channel-if-it-s-empty-while-retaining-its-original-content-when-it-s-not-empty-considering-the-default-value-is-also-a-channel/1435/2 "2024-11-18T02:26:28Z")
Hi @Kanna_Dhasan, I believe the snippet below will be helpful for you. ```Groovy Channel .of(params.sample_id) .flatMap() .ifEmpty('Hello') .view() ``` Output ![Screenshot 2024-11-17 at 23.26.15](https://europe1.discourse-cdn.com/flex013/uploads/seqera/original/2X/6/60e40b7f143291290a5bb696ec695774dff1d67f.jpeg) ---
**Author:** ![mahesh.binzerpanchal](https://dub1.discourse-cdn.com/flex013/user_avatar/community.seqera.io/mahesh.binzerpanchal/32/253_2.png) [@mahesh.binzerpanchal](https://community.seqera.io/u/mahesh.binzerpanchal)\ **Post date:** [November 18, 2024, 10:26am UTC](https://community.seqera.io/t/how-can-i-assign-a-default-channel-to-another-channel-if-it-s-empty-while-retaining-its-original-content-when-it-s-not-empty-considering-the-default-value-is-also-a-channel/1435/3 "2024-11-18T10:26:17Z")
Here’s one way to solve the problem of if one channel is empty, use another one. ```nextflow params.fill = false workflow { ch_something = params.fill ? Channel.of(1..3) : Channel.empty() ch_backfill = Channel.of(4..6).toList() ch_something.collect().concat(ch_backfill).first().flatMap().view() } ``` This uses the property that `collect` only emits a list if there’s something in the channel. Use `concat` to make sure the channel contents stay in order. and then use `first()` to take the first list from the concatenated channels. If `ch_something` is empty, there will be nothing, and the list from `ch_backfill` is taken. ---
**Author:** ![Kanna\_Dhasan](https://dub1.discourse-cdn.com/flex013/user_avatar/community.seqera.io/kanna_dhasan/32/1217_2.png) [@Kanna\_Dhasan](https://community.seqera.io/u/Kanna_Dhasan)\ **Post date:** [November 19, 2024, 11:37am UTC](https://community.seqera.io/t/how-can-i-assign-a-default-channel-to-another-channel-if-it-s-empty-while-retaining-its-original-content-when-it-s-not-empty-considering-the-default-value-is-also-a-channel/1435/4 "2024-11-19T11:37:10Z")
Thank you so much for your response! Just to clarify, `ifEmpty` only accepts direct static values and not channels. However, in my case, if the channel is empty, I want to assign another channel. Here, `sample` is a value channel holding a single value ---
**Author:** ![Kanna\_Dhasan](https://dub1.discourse-cdn.com/flex013/user_avatar/community.seqera.io/kanna_dhasan/32/1217_2.png) [@Kanna\_Dhasan](https://community.seqera.io/u/Kanna_Dhasan)\ **Post date:** [November 19, 2024, 11:38am UTC](https://community.seqera.io/t/how-can-i-assign-a-default-channel-to-another-channel-if-it-s-empty-while-retaining-its-original-content-when-it-s-not-empty-considering-the-default-value-is-also-a-channel/1435/5 "2024-11-19T11:38:50Z")
@mahesh.binzerpanchal Thank you so much for this workaround! It works exactly as I needed and solves my problem perfectly. I really appreciate your help! 🙂 ---
**Author:** ![system](https://dub1.discourse-cdn.com/flex013/user_avatar/community.seqera.io/system/32/2402_2.png) [@system](https://community.seqera.io/u/system)\ **Post date:** [November 26, 2024, 11:39am UTC](https://community.seqera.io/t/how-can-i-assign-a-default-channel-to-another-channel-if-it-s-empty-while-retaining-its-original-content-when-it-s-not-empty-considering-the-default-value-is-also-a-channel/1435/6 "2024-11-26T11:39:16Z")
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