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desc, idesc, xdesc

Sort and grade: descending

Q chooses from a variety of algorithms, depending on the type and data distribution.

desc

Descending sort

desc x    desc[x]

Returns x sorted into descending order. The function is uniform. The sort is stable: it preserves order between equals.

Where x is a

  • vector, it is returned sorted
  • mixed list, the result is sorted within datatype
  • dictionary or table, the result is sorted by the first key value or column (respectively).
q)desc 2 1 3 4 2 1 2                    / vector
4 3 2 2 2 1 1

q)desc (1;1b;"b";2009.01.01;"a";0)      / mixed list
2009.01.01
"b"
"a"
1
0
1b

q)t:([]a:3 4 1;b:`a`d`s)                / table
q)desc t
a b
---
4 d
3 a
1 s

idesc

Descending grade

idesc x    idesc[x]

Where x is a list or dictionary, returns the indices needed to sort list it in descending order. 

q)L:2 1 3 4 2 1 2
q)idesc L
3 2 0 4 6 1 5
q)L idesc L
4 3 2 2 2 1 1
q)(desc L)~L idesc L
1b
q)idesc `a`c`b!1 2 3
`b`c`a

xdesc

Sorts a table in descending order of specified columns. The sort is by the first column specified, then by the second column within the first, and so on.

x xdesc y    xdesc[x;y]

Where x is a symbol vector of column names defined in y, which is passed by

  • value, returns
  • reference, updates

y sorted in descending order by x.

The `s# attribute is not set. The sort is stable, i.e. it preserves order amongst equals.

q)\l sp.q
q)s
s | name  status city
--| -------------------
s1| smith 20     london
s2| jones 10     paris
s3| blake 30     paris
s4| clark 20     london
s5| adams 30     athens
q)`city xdesc s                 / sort descending by city
s | name  status city
--| -------------------
s2| jones 10     paris
s3| blake 30     paris
s1| smith 20     london
s4| clark 20     london
s5| adams 30     athens
q)meta `city xdesc s            / `s# attribute not set
c     | t f a
------| -----
s     | s
name  | s
status| i
city  | s

Duplicate column names xdesc signals dup if it finds duplicate columns in the right argument. (Since V3.6 2019.02.19.)

.Q.id (sanitize)

Sorting data on disk

xdesc can sort data on disk directly, without loading the entire table into memory: see xasc.

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

asc, iasc, xasc
Dictionaries & tables, Sorting