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I have seen the following two SO questions here and here and both do not clearly explain the purpose of the two reserved symbols .BY and .EACHI.

I understand the symbols .N, .SD and .SDcols very well and use them in my code all the time but can't get my head around the real use of .BY and EACHI. I have scanned all the data table documentation on CRAN but these two symbols are missing in most and at best give a superficial explanation without concrete examples.

I went through this excellent tutorial on data.tables on CRAN here and again it misses to explain the symbols .BY and .EACHI. Can someone please explain these two symbols with real use case. Preferably take the same airlines example code given in the datatable vignette here?

Lazarus Thurston
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    See several SO links in this open issue: [Add examples using .BY to documentation](https://github.com/Rdatatable/data.table/issues/1363) – Henrik Sep 25 '18 at 09:46
  • I have studied the examples posted on the links and have got a good understanding of .BY after trying it on my own data. Soon I will post a comprehensive answer with what I feel will make it clearer to anyone else trying to use `.BY`. I have deleted my short answer and will soon post a detailed one. Thanks @Henrik – Lazarus Thurston Sep 25 '18 at 18:01

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