{$["j";x in V]'[x;"_"]} / Case

Community Support
1

https://code.kx.com/q/learn/reading/strings/#c_ns_n_nts 

{$[“j”;x in V]'[x;“_”]} / Case

May I ask how does each both work here please? 

2

As the comment says, this is case, https://code.kx.com/q/ref/maps/#case