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xrank

Group by value

x xrank y     xrank[x;y]

Where

  • x is a long atom
  • y is of sortable type

returns for each item in y the bucket into which it falls, represented as a long from 0 to x-1.

If the total number of items is evenly divisible by x, then each bucket will have the same number of items; otherwise the first items of the result are longer.

xrank is right-uniform.

q)4 xrank til 8          / equal size buckets
0 0 1 1 2 2 3 3
q)4 xrank til 9          / first bucket has extra
0 0 0 1 1 2 2 3 3
q)
q)3 xrank 1 37 5 4 0 3   / outlier 37 does not get its own bucket
0 2 2 1 0 1
q)3 xrank 1 7 5 4 0 3    / same as above
0 2 2 1 0 1

Example using stock data:

q)show t:flip `val`name!((20?20);(20?(`MSFT`ORCL`CSCO)))
val name
--------
17  MSFT
1   CSCO
14  CSCO
13  ORCL
13  ORCL
9   ORCL
...

q)select Min:min val,Max:max val,Count:count i by bucket:4 xrank val from t
bucket| Min Max Count
------| -------------
0     | 0   7   5
1     | 9   12  5
2     | 13  15  5
3     | 15  17  5

Duplicate keys in a dictionary or duplicate column names in a table will cause sorts and grades to return unpredictable results.

Sorting