kdb+q

Return value from list of list when criteria met in KDB

I have a list of list which has 4 columns. Now I want to iterate over each list in list of list to find the first list which has say value ABC for first column and value XYZ for 3rd col. If this criteria satisfy I want to get out of the loop and return what was in 4th col. E.g of how list would look like below.

`AAA 23 `BBB  value
`AAA 23 `CCC  value
`ABC 23 `XYZ  value
`BBB 23 `CCC  value
......
.....

I tried to do something like below, but this does not seem to work.

myfunc:{if [x[0] =`ABC and x[2]=`XYZ ; val:x[3] ; 'break ] } 

@[myfunc;x; if [x[0] =`ABC and x[2]=`XYZ; :x[3]]] each ListOfList

Note that i do not want to traverse whole of list of list as it has millions of list in it, and is interested in fetching value of first list where criteria is matched.

Solution

  • If all the list within the list has at least 4 entries then it might be more efficient to perform a vector comparison rather than using an iterator.

    See: https://code.kx.com/q4m3/3_Lists/#310-elided-indices

    list[;3]where(list[;0]=`ABC)&list[;2]=`XYZ / to get all the matching entries
list[;3]first where(list[;0]=`ABC)&list[;2]=`XYZ / to get only the first match

    If you want to use a while loop:

    {while[not[`ABC`XYZ~x[0:0 2]]&count x;x:1_x];x[0;3]}list

    In your solutions, there are a few syntax errors.