Undesired side effect of SortBy

Question

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.}}

Or, even better, https://stackoverflow.com/q/3304632, which was referenced in the question cited by @LeonidShifrin — bbgodfrey, Jun 04 '21 at 16:48

score 9 · Answer 1 · answered Jun 04 '21 at 15:15

9

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

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

answered Jun 04 '21 at 15:15

kglr

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Nice - thanks! Too bad that this is not better described in Help. – hippo3773 Jun 05 '21 at 15:59
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. – István Zachar Nov 24 '22 at 13:54

Undesired side effect of SortBy

1 Answers1