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After long searching, I noticed that the SortBy[list,f] function has an annoying side effect: If the function f yields the same value for consecutive elements of the list, those are still sorted using the canonical ordering of the elements. This is very annoying - I want to sort only on the first element of a list but leave the order of list elements unchanged if the first element is not changing. How can this be achieved?

e.g.:

    y1 = {{1., 2.}, {1., 1.}, {3., 2.}}
    SortBy[y1, #[[1]] &]

Gives:

    {{1.,1.},{1.,2.},{3.,2.}}

But I would like to obtain:

    {{1.,2.},{1.,1.},{3.,2.}}
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  • $\begingroup$ I can verify this, MMA 12.3 Windows $\endgroup$
    – Daniel Huber
    Jun 4, 2021 at 15:46
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    $\begingroup$ Or, even better, stackoverflow.com/q/3304632, which was referenced in the question cited by @LeonidShifrin $\endgroup$
    – bbgodfrey
    Jun 4, 2021 at 16:48

1 Answer 1

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SortBy[y1, {#[[1]] &}]

{{1., 2.}, {1., 1.}, {3., 2.}}
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  • $\begingroup$ Nice - thanks! Too bad that this is not better described in Help. $\endgroup$
    – hippo3773
    Jun 5, 2021 at 15:59
  • $\begingroup$ Or you can use SortBy[y1, {#[[1]] &, Nothing}] or SortBy[y1, {#[[1]] &, #[[1]] &}] if you need to have multiple explicit functions in the second argument. $\endgroup$
    – István Zachar
    Nov 24, 2022 at 13:54

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