Next: Iteration Clauses, Previous: Loop Examples, Up: Loop Facility [Contents][Index]
Most loops are governed by one or more for
clauses.
A for
clause simultaneously describes variables to be
bound, how those variables are to be stepped during the loop,
and usually an end condition based on those variables.
The word as
is a synonym for the word for
. This
word is followed by a variable name, then a word like from
or across
that describes the kind of iteration desired.
In Common Lisp, the phrase being the
sometimes precedes
the type of iteration; in this package both being
and
the
are optional. The word each
is a synonym
for the
, and the word that follows it may be singular
or plural: ‘for x being the elements of y’ or
‘for x being each element of y’. Which form you use
is purely a matter of style.
The variable is bound around the loop as if by let
:
(setq i 'happy) (cl-loop for i from 1 to 10 do (do-something-with i)) i ⇒ happy
for var from expr1 to expr2 by expr3
This type of for
clause creates a counting loop. Each of
the three sub-terms is optional, though there must be at least one
term so that the clause is marked as a counting clause.
The three expressions are the starting value, the ending value, and
the step value, respectively, of the variable. The loop counts
upwards by default (expr3 must be positive), from expr1
to expr2 inclusively. If you omit the from
term, the
loop counts from zero; if you omit the to
term, the loop
counts forever without stopping (unless stopped by some other
loop clause, of course); if you omit the by
term, the loop
counts in steps of one.
You can replace the word from
with upfrom
or
downfrom
to indicate the direction of the loop. Likewise,
you can replace to
with upto
or downto
.
For example, ‘for x from 5 downto 1’ executes five times
with x
taking on the integers from 5 down to 1 in turn.
Also, you can replace to
with below
or above
,
which are like upto
and downto
respectively except
that they are exclusive rather than inclusive limits:
(cl-loop for x to 10 collect x) ⇒ (0 1 2 3 4 5 6 7 8 9 10) (cl-loop for x below 10 collect x) ⇒ (0 1 2 3 4 5 6 7 8 9)
The by
value is always positive, even for downward-counting
loops. Some sort of from
value is required for downward
loops; ‘for x downto 5’ is not a valid loop clause all by
itself.
for var in list by function
This clause iterates var over all the elements of list,
in turn. If you specify the by
term, then function
is used to traverse the list instead of cdr
; it must be a
function taking one argument. For example:
(cl-loop for x in '(1 2 3 4 5 6) collect (* x x)) ⇒ (1 4 9 16 25 36) (cl-loop for x in '(1 2 3 4 5 6) by 'cddr collect (* x x)) ⇒ (1 9 25)
for var on list by function
This clause iterates var over all the cons cells of list.
(cl-loop for x on '(1 2 3 4) collect x) ⇒ ((1 2 3 4) (2 3 4) (3 4) (4))
for var in-ref list by function
This is like a regular in
clause, but var becomes
a setf
-able “reference” onto the elements of the list
rather than just a temporary variable. For example,
(cl-loop for x in-ref my-list do (cl-incf x))
increments every element of my-list
in place. This clause
is an extension to standard Common Lisp.
for var across array
This clause iterates var over all the elements of array, which may be a vector or a string.
(cl-loop for x across "aeiou" do (use-vowel (char-to-string x)))
for var across-ref array
This clause iterates over an array, with var a setf
-able
reference onto the elements; see in-ref
above.
for var being the elements of sequence
This clause iterates over the elements of sequence, which may
be a list, vector, or string. Since the type must be determined
at run-time, this is somewhat less efficient than in
or
across
. The clause may be followed by the additional term
‘using (index var2)’ to cause var2 to be bound to
the successive indices (starting at 0) of the elements.
This clause type is taken from older versions of the loop
macro,
and is not present in modern Common Lisp. The ‘using (sequence …)’
term of the older macros is not supported.
for var being the elements of-ref sequence
This clause iterates over a sequence, with var a setf
-able
reference onto the elements; see in-ref
above.
for var being the symbols [of obarray]
This clause iterates over symbols, either over all interned symbols or over all symbols in obarray. The loop is executed with var bound to each symbol in turn. The symbols are visited in an unspecified order.
As an example,
(cl-loop for sym being the symbols when (fboundp sym) when (string-match "^map" (symbol-name sym)) collect sym)
returns a list of all the functions whose names begin with ‘map’.
The Common Lisp words external-symbols
and present-symbols
are also recognized but are equivalent to symbols
in Emacs Lisp.
Due to a minor implementation restriction, it will not work to have
more than one for
clause iterating over symbols, hash tables,
keymaps, overlays, or intervals in a given cl-loop
. Fortunately,
it would rarely if ever be useful to do so. It is valid to mix
one of these types of clauses with other clauses like for … to
or while
.
for var being the hash-keys of hash-table
for var being the hash-values of hash-table
This clause iterates over the entries in hash-table with var bound to each key, or value. A ‘using’ clause can bind a second variable to the opposite part.
(cl-loop for k being the hash-keys of h using (hash-values v) do (message "key %S -> value %S" k v))
for var being the key-codes of keymap
for var being the key-bindings of keymap
This clause iterates over the entries in keymap.
The iteration does not enter nested keymaps but does enter inherited
(parent) keymaps.
A using
clause can access both the codes and the bindings
together.
(cl-loop for c being the key-codes of (current-local-map) using (key-bindings b) do (message "key %S -> binding %S" c b))
for var being the key-seqs of keymap
This clause iterates over all key sequences defined by keymap and its nested keymaps, where var takes on values which are vectors. The strings or vectors are reused for each iteration, so you must copy them if you wish to keep them permanently. You can add a ‘using (key-bindings …)’ clause to get the command bindings as well.
for var being the overlays [of buffer] …
This clause iterates over the “overlays” of a buffer
(the clause extents
is synonymous
with overlays
). If the of
term is omitted, the current
buffer is used.
This clause also accepts optional ‘from pos’ and
‘to pos’ terms, limiting the clause to overlays which
overlap the specified region.
for var being the intervals [of object] …
This clause iterates over all intervals of a buffer or string with
constant text properties. The variable var will be bound to
conses of start and end positions, where one start position is always
equal to the previous end position. The clause allows of
,
from
, to
, and property
terms, where the latter
term restricts the search to just the specified property. The
of
term may specify either a buffer or a string. See (elisp)Text
Properties.
for var being the frames
This clause iterates over all Emacs frames. The clause screens
is
a synonym for frames
. The frames are visited in
next-frame
order starting from selected-frame
.
for var being the windows [of frame]
This clause iterates over the windows (in the Emacs sense) of
the current frame, or of the specified frame. It visits windows
in next-window
order starting from selected-window
(or frame-selected-window
if you specify frame).
This clause treats the minibuffer window in the same way as
next-window
does. For greater flexibility, consider using
walk-windows
instead.
for var being the buffers
This clause iterates over all buffers in Emacs. It is equivalent to ‘for var in (buffer-list)’.
for var = expr1 then expr2
This clause does a general iteration. The first time through the loop, var will be bound to expr1. On the second and successive iterations it will be set by evaluating expr2 (which may refer to the old value of var). For example, these two loops are effectively the same:
(cl-loop for x on my-list by 'cddr do …) (cl-loop for x = my-list then (cddr x) while x do …)
Note that this type of for
clause does not imply any sort
of terminating condition; the above example combines it with a
while
clause to tell when to end the loop.
If you omit the then
term, expr1 is used both for
the initial setting and for successive settings:
(cl-loop for x = (random) when (> x 0) return x)
This loop keeps taking random numbers from the (random)
function until it gets a positive one, which it then returns.
If you include several for
clauses in a row, they are
treated sequentially (as if by let*
and setq
).
You can instead use the word and
to link the clauses,
in which case they are processed in parallel (as if by let
and cl-psetq
).
(cl-loop for x below 5 for y = nil then x collect (list x y)) ⇒ ((0 nil) (1 1) (2 2) (3 3) (4 4)) (cl-loop for x below 5 and y = nil then x collect (list x y)) ⇒ ((0 nil) (1 0) (2 1) (3 2) (4 3))
In the first loop, y
is set based on the value of x
that was just set by the previous clause; in the second loop,
x
and y
are set simultaneously so y
is set
based on the value of x
left over from the previous time
through the loop.
Another feature of the cl-loop
macro is destructuring,
similar in concept to the destructuring provided by defmacro
(see Argument Lists).
The var part of any for
clause can be given as a list
of variables instead of a single variable. The values produced
during loop execution must be lists; the values in the lists are
stored in the corresponding variables.
(cl-loop for (x y) in '((2 3) (4 5) (6 7)) collect (+ x y)) ⇒ (5 9 13)
In loop destructuring, if there are more values than variables
the trailing values are ignored, and if there are more variables
than values the trailing variables get the value nil
.
If nil
is used as a variable name, the corresponding
values are ignored. Destructuring may be nested, and dotted
lists of variables like (x . y)
are allowed, so for example
to process an alist
(cl-loop for (key . value) in '((a . 1) (b . 2)) collect value) ⇒ (1 2)
Next: Iteration Clauses, Previous: Loop Examples, Up: Loop Facility [Contents][Index]