List operations.

Some functions are flagged as not tail-recursive. A tail-recursive function uses constant stack space, while a non-tail-recursive function uses stack space proportional to the length of its list argument, which can be a problem with very long lists. When the function takes several list arguments, an approximate formula giving stack usage (in some unspecified constant unit) is shown in parentheses.

The above considerations can usually be ignored if your lists are not longer than about 10000 elements.

Some functions are flagged as not tail-recursive. A tail-recursive function uses constant stack space, while a non-tail-recursive function uses stack space proportional to the length of its list argument, which can be a problem with very long lists. When the function takes several list arguments, an approximate formula giving stack usage (in some unspecified constant unit) is shown in parentheses.

The above considerations can usually be ignored if your lists are not longer than about 10000 elements.

Return the length (number of elements) of the given list.

Return the first element of the given list. Raise

`Failure "hd"`

if the list is empty.
Return the given list without its first element. Raise

`Failure "tl"`

if the list is empty.
Return the

`n`

-th element of the given list.
The first element (head of the list) is at position 0.
Raise `Failure "nth"`

if the list is too short.
Raise `Invalid_argument "List.nth"`

if `n`

is negative.
List reversal.

Catenate two lists. Same function as the infix operator

`@`

.
Not tail-recursive (length of the first argument). The `@`

operator is not tail-recursive either.`List.rev_append l1 l2`

reverses `l1`

and concatenates it to `l2`

.
This is equivalent to `List.rev`

` l1 @ l2`

, but `rev_append`

is
tail-recursive and more efficient.
Concatenate a list of lists. The elements of the argument are all
concatenated together (in the same order) to give the result.
Not tail-recursive
(length of the argument + length of the longest sub-list).

Same as

`concat`

. Not tail-recursive
(length of the argument + length of the longest sub-list).`List.iter f [a1; ...; an]`

applies function `f`

in turn to
`a1; ...; an`

. It is equivalent to
`begin f a1; f a2; ...; f an; () end`

.
Same as

**Since** 4.00.0

`List.iter`

, but the function is applied to the index of
the element as first argument (counting from 0), and the element
itself as second argument.`List.map f [a1; ...; an]`

applies function `f`

to `a1, ..., an`

,
and builds the list `[f a1; ...; f an]`

with the results returned by `f`

. Not tail-recursive.
Same as

**Since** 4.00.0

`List.map`

, but the function is applied to the index of
the element as first argument (counting from 0), and the element
itself as second argument. Not tail-recursive.`List.fold_left f a [b1; ...; bn]`

is
`f (... (f (f a b1) b2) ...) bn`

.`List.fold_right f [a1; ...; an] b`

is
`f a1 (f a2 (... (f an b) ...))`

. Not tail-recursive.`List.iter2 f [a1; ...; an] [b1; ...; bn]`

calls in turn
`f a1 b1; ...; f an bn`

.
Raise `Invalid_argument`

if the two lists have
different lengths.`List.map2 f [a1; ...; an] [b1; ...; bn]`

is
`[f a1 b1; ...; f an bn]`

.
Raise `Invalid_argument`

if the two lists have
different lengths. Not tail-recursive.`List.fold_left2 f a [b1; ...; bn] [c1; ...; cn]`

is
`f (... (f (f a b1 c1) b2 c2) ...) bn cn`

.
Raise `Invalid_argument`

if the two lists have
different lengths.`List.fold_right2 f [a1; ...; an] [b1; ...; bn] c`

is
`f a1 b1 (f a2 b2 (... (f an bn c) ...))`

.
Raise `Invalid_argument`

if the two lists have
different lengths. Not tail-recursive.`for_all p [a1; ...; an]`

checks if all elements of the list
satisfy the predicate `p`

. That is, it returns
`(p a1) && (p a2) && ... && (p an)`

.`exists p [a1; ...; an]`

checks if at least one element of
the list satisfies the predicate `p`

. That is, it returns
`(p a1) || (p a2) || ... || (p an)`

.
Same as

`List.for_all`

, but for a two-argument predicate.
Raise `Invalid_argument`

if the two lists have
different lengths.
Same as

`List.exists`

, but for a two-argument predicate.
Raise `Invalid_argument`

if the two lists have
different lengths.`mem a l`

is true if and only if `a`

is equal
to an element of `l`

.
Same as

`List.mem`

, but uses physical equality instead of structural
equality to compare list elements.`find p l`

returns the first element of the list `l`

that satisfies the predicate `p`

.
Raise `Not_found`

if there is no value that satisfies `p`

in the
list `l`

.`filter p l`

returns all the elements of the list `l`

that satisfy the predicate `p`

. The order of the elements
in the input list is preserved.`partition p l`

returns a pair of lists `(l1, l2)`

, where
`l1`

is the list of all the elements of `l`

that
satisfy the predicate `p`

, and `l2`

is the list of all the
elements of `l`

that do not satisfy `p`

.
The order of the elements in the input list is preserved.`assoc a l`

returns the value associated with key `a`

in the list of
pairs `l`

. That is,
`assoc a [ ...; (a,b); ...] = b`

if `(a,b)`

is the leftmost binding of `a`

in list `l`

.
Raise `Not_found`

if there is no value associated with `a`

in the
list `l`

.
Same as

`List.assoc`

, but uses physical equality instead of structural
equality to compare keys.
Same as

`List.assoc`

, but simply return true if a binding exists,
and false if no bindings exist for the given key.
Same as

`List.mem_assoc`

, but uses physical equality instead of
structural equality to compare keys.`remove_assoc a l`

returns the list of
pairs `l`

without the first pair with key `a`

, if any.
Not tail-recursive.
Same as

`List.remove_assoc`

, but uses physical equality instead
of structural equality to compare keys. Not tail-recursive.
Transform a list of pairs into a pair of lists:

`split [(a1,b1); ...; (an,bn)]`

is `([a1; ...; an], [b1; ...; bn])`

.
Not tail-recursive.
Transform a pair of lists into a list of pairs:

`combine [a1; ...; an] [b1; ...; bn]`

is
`[(a1,b1); ...; (an,bn)]`

.
Raise `Invalid_argument`

if the two lists
have different lengths. Not tail-recursive.
Sort a list in increasing order according to a comparison
function. The comparison function must return 0 if its arguments
compare as equal, a positive integer if the first is greater,
and a negative integer if the first is smaller (see Array.sort for
a complete specification). For example,

The current implementation uses Merge Sort. It runs in constant heap space and logarithmic stack space.

`Pervasives.compare`

is a suitable comparison function.
The resulting list is sorted in increasing order.
`List.sort`

is guaranteed to run in constant heap space
(in addition to the size of the result list) and logarithmic
stack space.
The current implementation uses Merge Sort. It runs in constant heap space and logarithmic stack space.

Same as

The current implementation uses Merge Sort. It runs in constant heap space and logarithmic stack space.

`List.sort`

, but the sorting algorithm is guaranteed to
be stable (i.e. elements that compare equal are kept in their
original order) .
The current implementation uses Merge Sort. It runs in constant heap space and logarithmic stack space.

Merge two lists:
Assuming that

`l1`

and `l2`

are sorted according to the
comparison function `cmp`

, `merge cmp l1 l2`

will return a
sorted list containting all the elements of `l1`

and `l2`

.
If several elements compare equal, the elements of `l1`

will be
before the elements of `l2`

.
Not tail-recursive (sum of the lengths of the arguments).