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Challenge 3 The Runner-Up
Thanks again to everyone who takes the time to interact / comment on these challenges, it really does benefit my knowledge of the q language!
My next challenge is a simpler one, but I know theres lots of different ways to solve it.
Given the participants’ score sheet for your University Sports Day, you are required to find the runner-up score. You are given n scores. Store them in a list and find the score of the runner-up.
Here’s my approach:
q)list1: (4;7;9;7;2;8;9) q)list1: asc list1 q)list1 `s# 2 4 7 7 8 9 9 q)list2:list1 except max list1 q)runner_up:max list2 q)runner_up 8
Also a solution in python:
n = 5
arr = [2,3,6,6,5]
outmax = min(arr)
for i in range(len(arr)): if outmax < arr[i] and arr[i] != max(arr): outmax = arr[i]
print(outmax)
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4 Replies
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Indexing into the descending sorted list of distinct scores works pretty well ok
q)f:{desc[distinct x]1} q)l:4 7 9 7 2 8 9 q)f[l] 8EDIT: type and scale can matter here when looking for the best option. A version of the original suggestion outperforms mine in a lot of cases
q)f1:{max x except max x} // original suggested solution q)f2:{desc[distinct x]1}q)l:4 7 9 7 2 8 9
q)t:10000 f1 l
9
q)t:10000 f2 l
16q)longs:100000?100 //scale up
q)t:100 f1 longs
38
q)t:100 f2 longs //lots of duplicates gives the “distinct” solution an advantage
19q)floats:100000?100f // change type to float
q)t:100 f1 floats
392
q)t:100 f2 floats //advantage disappears when there are few duplicate entries
707 -
does this take the second distinct number in the list? If so, thanks! I really like that approach
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q)list1:4 7 9 7 2 8 9 q)ru:@[;1] desc distinct:: /runner-up q)ru list1 8
Using general list notation (items separated by semicolons and embraced by parentheses) suggests a list is general; better to write a vector as a vector literal.
Composition
ruis a sequence of three unaries. In evaluation order: select the distinct items; sort descending; select second item. -
Nicely timed question! Im just up to the chapter on composition in the book Im writing, Vector Programming in Q, and I need to settle a few questions about it myself. For example, is there ever a reason to prefer Compose to a train suffixed by
@or::?Nick Psaris commented recently that he favours
::because it composes directly rather than forming a projection. (I doubt the evaluation overhead is significant, but I get the point.)
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