Replace elements in list not matching a sub-string in KDB Q

user_6822876 0
Viewed 756

How would I iterate through a list in KDB Q and replace elements which do not match a certain sub-string criteria?

Logic Example Pseudocode:

list.stream()
    .forEach(x -> {
        if (x matches substring) :
            newList.add(x)
        else :
            newList.add("")
     })

Current List:

S: ("Lint"; "Stack"; "Linode"; "Overflow";"Linux")

The sub-string to match is "Li" here. As such, strings "Stack" and "Overflow" are replaced by empty Strings as they do not contain the sub-string.

Resultant List:

S: ("Lint"; ""; "Linode"; "";"Linux")
kdb
2 Answers

A couple of ways you could go about it. First iterating over each element using conditional evaluation like below:

q){$[x like "Li*";x;" "]}each S
"Lint"
" "
"Linode"
" "
"Linux"

Or using an at apply like below:

q)@[S;where not S like\: "Li*";:;" "]
"Lint"
" "
"Linode"
" "
"Linux"
user_89124980

Vector conditional would also work:

q)?[S like "Li*";S;count[S]#enlist ""]
"Lint"
""
"Linode"
""
"Linux"
user_38956970
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