Circles in a Circle, 1923
Everything begins with a dot.
— W.W. Kandinsky
.
Apply, Index, Trap
@
Apply At, Index At, Trap At
 Apply a function to a list of arguments
 Get items at depth in a list
 Trap errors
rank  syntax  function semantics  list semantics 

2  v . vx .[v;vx] 
Apply Apply v to list vx of arguments 
Index Get item/s vx at depth from v 
2  u @ ux @[u;ux] 
Apply At Apply unary u to argument ux 
Index At Get items ux from u 
3  .[g;gx;e] 
Trap Try g . gx ; catch with e 

3  @[f;fx;e] 
Trap At Try f@fx ; catch with e 
Where
e
is an expression, typically a functionf
is a unary function andfx
in its domaing
is a function of rank n andgx
an atom or list of count n with items in the domains ofg
v
is a value of rank n (or a handle to one) andvx
a list of count n with items in the domains ofv
u
is a unary value (or a handle to one) andux
in its domain
Amend, Amend At
For the ternary and quaternary forms
.[d; i; u] @[d; i; u]
.[d; i; v; vy] @[d; i; v; vy]
where
d
is a list or dictionary, or a handle to a list, dictionary or datafilei
indexesd
asd . i
ord @ i
u
is a unary withd
in its domainv
is a binary withd
andvy
is in its left and right domains
see Amend and Amend At.
Apply, Index
v . vx
evaluates value v
on the n arguments listed in vx
.
q)add
0 1 2 3
1 2 3 4
2 3 4 5
3 4 5 6
q)add . 2 3 / add[2;3] Index
5
q)(+) . 2 3 / +[2;3] Apply
5
q).[+;2 3]
5
q).[add;2 3]
5
If v
has rank n, then vx
has n items and v
is evaluated as:
v[vx[0]; vx[1]; …; vx[1+count vx]]
If v
has rank 2 then vx
has 2 items and v
is applied to the first argument vx[0]
and the second argument vx[1]
.
v[vx[0];vx[1]]
If v
has 1 argument then vx
has 1 item and v
is applied to the argument vx[0]
.
v[vx[0]]
Q for Mortals §6.5.3 Indexing at Depth
Nullaries
Nullaries (functions of rank 0) are handled differently. The pattern above suggests that the empty list ()
would be the argument list to nullary v
, but v . ()
is a case of Index, where empty vx
always selects v
.
Apply for nullary v
is denoted by v . enlist[::]
, i.e. the right argument is the enlisted null.
For example:
q)a: 2 3
q)b: 10 20
q){a + b} . enlist[::]
12 23
Index
d . i
returns an item from list or dictionary d
as specified by successive items in list i
.
The result is found in d
at depth count i
as follows.
The list i
is a list of successive indexes into d
. i[0]
must be in the domain of d@
. It selects an item of d
, which is then indexed by i[1]
, and so on.
( (d@i[0]) @ i[1] ) @ i[2]
…
q)d
(1 2 3;4 5 6 7)
(8 9;10;11 12)
(13 14;15 16 17 18;19 20)
q)d . enlist 1 / select item 1, i.e. d@1
8 9
10
11 12
q)d . 1 2 / select item 2 of item 1
11 12
q)d . 1 2 0 / select item 0 of item 2 of item 1
11
Index At
The selections at each level are individual applications of Index At: first, item d@i[0]
is selected, then (d@i[0])@i[1]
, then ((d@i[0])@ i[1])@ i[2]
, and so on.
These expressions can be rewritten using Over applied to Index At; the first is d@/i[0]
, the second is d@/i[0 1]
, and the third is d@/i[0 1 2]
.
In general, for a vector i
of any count, d . i
is identical to d@/i
.
q)((d @ 1) @ 2) @ 0 / selection in terms of a series of @s
11
q)d @/ 1 2 0 / selection in terms of @Over
11
Cross sections
Index is crosssectional when the items of i
are lists. That is, itemsatdepth in d
are indexed for paths made up of all combinations of atoms of i[0]
and atoms of i[1]
and atoms of i[2]
, and so on to the last item of i
.
The simplest case of crosssectional index occurs when the items of i
are vectors. For example, d .(2 0;0 1)
selects items 0 and 1 from both items 2 and 0:
q)d . (2 0; 0 1)
13 14 15 16 17 18
1 2 3 4 5 6 7
q)count each d . (2 0; 0 1)
2 2
Note that items appear in the result in the same order as the indexes appear in i
.
The first item of i
selects two items of d
, as in d@i[0]
. The second item of i
selects two items from each of the two items just selected, as in (d@i[0])@'i[1]
. Had there been a third vector item in i
, say of count 5, then that item would select five items from each of the four itemsatdepth 1 just selected, as in ((d@i[0])@'i[1])@''i[2]
, and so on.
When the items of i
are vectors the result is rectangular to at least depth count i
, depending on the regularity of d
, and the k
th item of its shape vector is (count i)[k]
for every k
less than count i
. That is, the first count i
items of the shape of the result are count each i
.
More general crosssectional indexing occurs when the items of i
are rectangular lists, not just vectors, but the situation is much like the simpler case of vector items.
Nulls in i
Nulls in i
mean “select all”: if i[0]
is null, then continue on with d
and the rest of i
, i.e. 1_i
; if i[1]
is null, then for every selection made through i[0]
, continue on with that selection and the rest of i
, i.e. 2_i
; and so on. For example, d .(::;0)
means that the 0th item of every item of d
is selected.
q)d
(1 2 3;4 5 6 7)
(8 9;10;11 12)
(13 14;15 16 17 18;19 20)
q)d . (::;0)
1 2 3
8 9
13 14
Another example, this time with i[1]
equal to null:
q)d . (0 2;::;1 0)
(2 1;5 4)
(14 13;16 15;20 19)
Note that d .(::;0)
is the same as d .(0 1 2;0)
, but in the last example, there is no value that can be substituted for null in (0 2;;1 0)
to get the same result, because when item 0 of d
is selected, null acts like 0 1
, but when item 2 of d
is selected, it acts like 0 1 2
.
The general case of a nonnegative integer list i
In the general case, when the items of i
are nonnegative integer atoms or lists, or null, the structure of the result can be thought of as cascading structures of the items of i
. That is, with nulls aside, the result is structurally like i[0]
, except that wherever there is an atom in i[0]
, the result is structurally like i[1]
, except that wherever there is an atom in i[1]
, the result is structurally like i[2]
, and so on.
The general case of Index can be defined recursively in terms of Index At by partitioning the list i
into its first item and the rest:
Index:{[d;F;R]
$[ F~::; Index[d; first R; 1 _ R];
0 =count R; d @ F;
0>type F; Index[d @ F; first R; 1 _ R]
Index[d;; R]'F ]}
That is, d . i
is Index[d;first i;1_i]
.
To work through the definition, start with F
as the first item of i
and R
as the remainder. At each step in the recursion:
 if
F
is null then select all ofd
and continue on, with the first item of the remainderR
as the newF
and the remainder ofR
as the new remainder;  otherwise, if the remainder is the empty vector apply Index At (the right argument
F
is now the last item ofi
), and we are done;  otherwise, if
F
is an atom, apply Index At to select that item ofd
and continue on in the same way as whenF
is null;  otherwise, apply Index with fixed arguments
d
andR
, but independently to the items of the listF
.
Dictionaries and symbolic indexing
If i
is a symbol atom then d
must be a dictionary or handle of a directory on the Ktree, and d . i
selects the value of the entry named in i
. For example, if:
dir:`a`b!(2 3 4;"abcdefg")
then `dir . enlist`b
is "abcdefg"
and `dir . (`b;1 3 5)
is "bdf"
.
If i
is a list whose items are nonnegative integer atoms and symbol atoms, then just like the nonnegative integer vector case, d . i
is a single item at depth count i
in d
. The difference is that wherever a symbol appears in i
, say as the kth item, the selection up to the kth item must produce a dictionary or a handle of a directory. Selection by the kth item is the value of an entry in that dictionary or directory, and further selections go on from there. For example:
q)(1;`a`b!(2 3 4;10 20 30 40)) . (1; `b; 2)
30
As we have seen above for the general case, every atom in the k
th item of i
must be a valid index of all items at depth k
selected by d . k # i
. Moreover, symbols can only select from dictionaries and directories, and integers cannot.
Consequently, if the k
th item of i
contains a symbol atom, then all items selected by d . k # i
must be dictionaries or handles of directories, and therefore all atoms in the k
th item of i
must be symbols.
It follows that each item of i
must be made up entirely of nonnegative integer atoms, or entirely of symbol atoms, and if the k
th item of i
is made up of symbols, then all items at depth k
in d
selected by the first k
items of i
must be dictionaries.
Note that if d
is either a dictionary or handle to a directory then d . enlist key d
is a list of values of all the entries.
Step dictionaries
Where d
is a dictionary, d@i
or d[i]
or d i
returns for each item of i
that is outside the domain of d
a null of the same type as the keys.
q)d:`cat`cow`dog`sheep!`chat`vache`chien`mouton
q)d
cat  chat
cow  vache
dog  chien
sheep mouton
q)d `sheep`snake`cat`ant
`mouton``chat`
q)
q)e:(10*til 10)!til 10
q)e
0  0
10 1
20 2
30 3
40 4
50 5
60 6
70 7
80 8
90 9
q)e 80 35 20 10
8 0N 2 0N
A step dictionary has the sorted attribute set.
Its keys are a sorted vector.
Where s
is a step dictionary, and i[k]
are the items of i
that are outside the domain of d
, the value/s for d@i@k
are the values for the highest keys that are lower than i k
.
q)d:`cat`cow`dog`sheep!`chat`vache`chien`mouton
q)ds:`s#d
q)ds~d
1b
q)ds `sheep`snake`cat`ant
`mouton`mouton`chat`
q)
q)es:`s#e
q)es~e
1b
q)es 80 35 20 10
8 3 2 0N
Set Attribute
qrious kdb+: Step Dictionaries
Apply At, Index At
@
is syntactic sugar for the case where u
is a unary and ux
a 1item list.
u@ux
is always equivalent to u . enlist ux
.
Brackets are syntactic sugar
The brackets of an argument list are also syntactic sugar. Nothing can be expressed with brackets that cannot also be expressed using .
.
You can use the derived function @\:
to apply a list of unary values to the same argument.
q){`o`h`l`c!(first;max;min;last)@\:x}1 2 3 4 22 / open, high, low, close
o 1
h 22
l 1
c 22
Composition
A sequence of unaries u
, v
, w
… can be composed with Apply At as u@v@w@
.
All but the last @
may be elided: u v w@
.
q)tc:til count@ / indexes of a list
q)tc "abc"
"0 1 2"
The last value in the sequence can have higher rank if projected as a unary by Apply.
q)di:reciprocal(%). / divide into
q)di 2 3 / divide 2 into 3
1.5
Trap
In the ternary, if evaluation of the function fails, the expression is evaluated. (Compare try/catch in some other languages.)
q).[+;"ab";`ouch]
`ouch
If the expression is a function, it is evaluated on the text of the signalled error.
q).[+;"ab";{"Wrong ",x}]
"Wrong type"
For a successful evaluation, the ternary returns the same result as the binary.
q).[+;2 3;{"Wrong ",x}]
5
Trap At
@[f;fx;e]
is equivalent to .[f;enlist fx;e]
.
Use Trap At as a simpler form of Trap, for unary values.
Limit of the trap
Trap catches only errors signalled in the applications of f
or g
. Errors in the evaluation of fx
or gg
themselves are not caught.
q)@[2+;"42";`err]
`err
q)@[2+;"42"+3;`err]
'type
[0] @[2+;"42"+3;`err]
^
When e
is not a function
If e
is a function it will be evaluated only if f
or g
fails. It will however be parsed before any of the other expressions are evaluated.
q)@[2+;"42";{)}]
')
[0] @[2+;"42";{)}]
^
If e
is any other kind of expression it will always be evaluated – and first, in the usual righttoleft sequence. In this respect Trap and Trap At are unlike try/catch in other languages.
q)@[string;42;a:100] / expression not a function
"42"
q)a // but a was assigned anyway
100
q)@[string;42;{b::99}] / expression is a function
"42"
q)b // not evaluated
'b
[0] b
^
For most purposes, you will want e
to be a function.
Q for Mortals §10.1.8 Protected Evaluation
Errors signalled
index an atom in vx or ux is not an index to an itematdepth in d
rank the count of vx is greater than the rank of v
type v or u is a symbol atom, but not a handle to an value
type an atom of vx or ux is not an integer, symbol or null