How to extract the first and last element

Question

It is a elemental question maybe,but I am confusing still.If I have a 1-dimension list list

a = Range[5]
(* {1, 2, 3, 4, 5} *)

So I can do this to get the first and last element:

a[[{1, -1}]]
(* {1, 5} *)

But when I have a matrix mat,why I cannot do this with same method?

mat = Partition[Range[12], 4];
mat[[{{1, 1}, {-1, -1}}]]

This expression will put some error information.

You can use mat[[1,1]] and mat[[-1,-1]]. Or to use same syntax as 1D example, can Flatten the matrix first. — Nasser, Dec 27, 2016 at 4:28
@march Good point out,just little pitty,we cannot asign it by same syntax Extract[mat,{{1,1},{-1,-1}}]={c,d}.This is my purpose actually. — yode, Dec 27, 2016 at 4:28
I find using [[x,y]] much easier and more common with other languages (Matlab, etc...) than having to call functions to do it — Nasser, Dec 27, 2016 at 4:31

user6546 · Accepted Answer · 2016-12-27 04:33:13Z

3

Does the following help:

mat = Partition[Range[12], 4];
{#[[1, 1]], #[[-1, -1]]} & @ mat

Or, alternatively,

{First@First@#, Last@Last@#} & @ mat

answered Dec 27, 2016 at 4:33

user6546

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$\begingroup$ suspect you are looking for something different than the following. mat[[1, 1]] = a; mat[[-1, -1]] = b; mat // TableForm $\endgroup$
– user6546
Dec 27, 2016 at 4:42
$\begingroup$ @Thanks, :)If we just extract,we can use {First@#, Last@#} &@Flatten[mat] or Through[{First,Last}[Flatten[mat]]] $\endgroup$
– yode
Dec 27, 2016 at 5:14

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