3
$\begingroup$

I have a long table in this form:

A = {{a, 1}, {c, 2}, {b, 5}, {c, 1}, {a, 14}, {b, 7}, {b, 10}, {c, 6}, {a, 4}}

and I want to order it by the second column, but first all with an "a" in the first element should be ordered. And then by order of appearance. It should look like this:

{{a, 1},{a, 4},{a, 14},{c, 1},{c, 2},{c, 6},{b, 5},{b, 7},{b, 10}}

SortBy[A, #[[2]] &] would sort the second column, ignoring the first.

Any ideas? Maybe a for-loop?

Improve this question
$\endgroup$
4
  • 2
    $\begingroup$ So, you don't want SortBy[A, {#[[1]] &, #[[2]] &}]? $\endgroup$ – J. M.'s ennui♦ Mar 31 '17 at 12:50
  • 3
    $\begingroup$ Why don't you use just Sort[A] ? $\endgroup$ – Artes Mar 31 '17 at 12:52
  • 2
    $\begingroup$ If you want it as in the OP, just try Sort /@ GatherBy[A, First]. $\endgroup$ – Artes Mar 31 '17 at 12:59
  • $\begingroup$ Thank you! Sort /@ GatherBy[A, First] was what I wanted. The other two answers would also sort the first element alphabetically. I wanted it by order of appearance. $\endgroup$ – drcyberz Mar 31 '17 at 13:34
6
$\begingroup$

From Artes' comment:

A = {{a, 1}, {c, 2}, {b, 5}, {c, 1}, {a, 14}, {b, 7}, {b, 10}, {c, 6}, {a, 4}};
Sort /@ GatherBy[A, First]

{{{a, 1}, {a, 4}, {a, 14}}, {{c, 1}, {c, 2}, {c, 6}}, {{b, 5}, {b, 7}, {b, 10}}}

Improve this answer
$\endgroup$
2
  • 1
    $\begingroup$ To get the specified output you need to include a Flatten: Flatten[Sort /@ GatherBy[A, First], 1] $\endgroup$ – Bob Hanlon Mar 31 '17 at 16:45
  • $\begingroup$ Or Catenate instead of Flatten. $\endgroup$ – corey979 Mar 31 '17 at 16:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.