Hi everyone!

I would like to "concatenate" many columns with missing values in one. Below is an example of my subset:

A B C D

1 . . .
. 2 . .
. . 3 .
. . . 4


What I would like is to "merge" from variables A to D in order to have a variable Z :

Z

1
2
3
4

Any help would be greatly appreciated, thank you.