If conditions $ vs ?

[Denakaran]

Sometimes i wonder how the $ differs with ? since most of the times both of them  returning the same results. Not sure when to use $ and ?.  Help me to understand this guys!

q)$[1b;`true;`false]
`true
q)?[1b;`true;`false]
`true
q)t:([]name:enlist `hi)
q)t
name
----
hi
q)$[(count t)>0;`values;`novalues]
`values
q)?[(count t)>0;`values;`novalues]
`values

[BaiChen · Accepted solution]

? can be used in vector-based condition . For example:

?[101b;`a;`b]
`a`b`a
q))

$ is used in single condition :
$[1b;`a;`b]
`a
q))

---

[Denakaran]

Thanks, that helps! 

[sujoy]

Apart from vector-based condition mentioned above, ? Will always run the fail condition unlike $.

$[1;2;’err]

?[1;2;’err]

[Denakaran]

Thanks, I tried both of them and seeing the difference between them

q)$[1;2;'err]
2
q)?[1;2;'err]
'err
[0] ?[1;2;'err]