4
$\begingroup$

Suppose I have a list as the following:

list={{1,9},{7,0},{8,6}....}

How can I reverse the element in such way that the output would be the following:

{{9,1},{0,7},{6,8}....}

I tried Map[Reverse,{list}] but it didn't work.

$\endgroup$
3
  • 1
    $\begingroup$ Either list[[All, {2,1}]], or more generally list[[All, Reverse[Range[Length[First[list]]]]]] $\endgroup$
    – Coolwater
    Feb 3, 2016 at 20:12
  • 3
    $\begingroup$ Take away the {}'s: Map[Reverse, list] or Reverse /@ map. $\endgroup$
    – march
    Feb 3, 2016 at 20:38
  • 3
    $\begingroup$ Closely related Why is multidimensional Reverse slow? $\endgroup$
    – Kuba
    Feb 3, 2016 at 20:45

2 Answers 2

10
$\begingroup$

Reverse takes a level specification so:

Reverse[list, 2]
$\endgroup$
0
$\begingroup$

Another approach would be to use a simple pattern match and replacement rule, such as:

list = {{1, 9}, {7, 0}, {8, 6}};
list /. {a_?NumberQ, b_?NumberQ} :> {b, a}
(*  {{9, 1}, {0, 7}, {6, 8}}. *)

For this specific example, you could drop the ?NumberQs from the pattern to be matched. But, without them, the approach would not produce the intended outcome if the length of the outer list were 2. For that case, the outcome would be as follows:

{{1, 9}, {7, 0}} /. {a_, b_} :> {b, a}
(*. {{7, 0}, {1, 9}}. *)
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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