The q operators and functions are a combination of optimized c functions and wrappers on top of the k language.
k preceded q by about a decade. These days you are supposed to write q code so k is not documented anymore, unfortunately parse trees and debugging require some knowledge of k.
So how do we translate between the 2 languages?
The .q namespace contains all the bits of q which are are wrappers of k and hence gives us a good starting point.
If we type lj we can see its definition, .q.lj is the fully qualified name.
q)lj
k){$[$[99h=@y;(98h=@!y)&98h=@. y;()~y];.Q.ft[,\:[;y];x];'"type"]}
In order to understand this we need to know what @ and ! mean in k.
So lets take .q namespace, and filter out all the functions, this gives us a good starting point for translating q into k.(I have manually rearranged the results to group similar things together)
\c 2000 200
q)where[1_not type'[.q]in -10 100 106 110h]#.q
neg | -:
not | ~:
hdel | ~:
null | ^:
string | $:
mmu | $
reciprocal| %:
ltime | %:
floor | _:
There are 5 categories. We already excluded lambdas, aliases of internals like -15!, projections like ceiling(neg floor neg@), named adverb'ed operators like sums, and the k operators.
Both get and value map to the same k operator which is why they are used interchangeably when people write code. Many of the operators are overloaded like ! which does both inv and key depending on the input.
In k we use a trailing : to indicate that the operator is being used in its monadic form. The : is only required when the statement is ambiguous, the alternative to using : is to use @ or () eg:
