Typical q code operates on all of the elements of a container at once:
q)container: 1 2 3 q)100 * container 100 200 300 q)
Sometimes we are interested in only a subset of elements from a container:
q)container: til 10 q)container 0 1 2 3 4 5 6 7 8 9 q)container[where 0 = container mod 2] 0 2 4 6 8 q)
Sometimes, however, you need to update particular elements of a structure while leaving the remaining elements unchanged. That’s what functional apply and amend are for; they transform specific elements of a container without touching the others. The variations are distinguished by 3 choices:
- container or name
The first argument is either the value of a container or the name of a global variable referring to a container. In the former case, a new object is returned; in the latter, the global variable is modified and its name is returned.
- monadic or dyadic function
If the transformation requires additional information beyond that contained in each element itself, that is accomplished by using a dyadic function and supplying the additional information in a fourth argument to the operator.
For a detailed discussion, please see the following faqs:
Lastly, note that there is another pair of overloads for @ and . – each with three arguments – called protected execution, which are invoked when the first argument to @ or . is a function or projection; protected execution is discussed in another faq.