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Slicing
=======
.. module:: fun
This section contains functions to make subsequences from iterators.
Basic
-----
.. function:: nth(n, gen, param, state)
iterator:nth(n)
:param uint n: a sequential number (indexed starting from ``1``,
like Lua tables)
:returns: **n**-th element of ``gen, param, state`` iterator
This function returns the **n**-th element of ``gen, param, state``
iterator. If the iterator does not have **n** items then ``nil`` is returned.
Examples:
.. code-block:: lua
> print(nth(2, range(5)))
2
> print(nth(10, range(5)))
nil
> print(nth(2, {"a", "b", "c", "d", "e"}))
b
> print(nth(2, enumerate({"a", "b", "c", "d", "e"})))
2 b
This function is optimized for basic array and string iterators and has
``O(1)`` complexity for these cases.
.. function:: head(gen, param, state)
iterator:head()
:returns: a first element of ``gen, param, state`` iterator
Extract the first element of ``gen, param, state`` iterator.
If the iterator is empty then an error is raised.
Examples:
.. code-block:: lua
> print(head({"a", "b", "c", "d", "e"}))
a
> print(head({}))
error: head: iterator is empty
> print(head(range(0)))
error: head: iterator is empty
> print(head(enumerate({"a", "b"})))
1 a
.. function:: car(gen, param, state)
An alias for :func:`head`.
.. function:: tail(gen, param, state)
iterator:tail()
:returns: ``gen, param, state`` iterator without a first element
Return a copy of ``gen, param, state`` iterator without its first element.
If the iterator is empty then an empty iterator is returned.
Examples:
.. code-block:: lua
> each(print, tail({"a", "b", "c", "d", "e"}))
b
c
d
e
> each(print, tail({}))
> each(print, tail(range(0)))
> each(print, tail(enumerate({"a", "b", "c"})))
2 b
3 c
.. function:: cdr(gen, param, state)
An alias for :func:`tail`.
Subsequences
------------
.. function:: take_n(n, gen, param, state)
iterator:take_n(n)
:param n: a number of elements to take
:type n: uint
:returns: an iterator on the subsequence of first **n** elements
Examples:
.. code-block:: lua
> each(print, take_n(5, range(10)))
1
2
3
4
5
> each(print, take_n(5, enumerate(duplicate('x'))))
1 x
2 x
3 x
4 x
5 x
.. function:: take_while(predicate, gen, param, state)
iterator:take_while(predicate)
:type predicate: function(...) -> bool
:returns: an iterator on the longest prefix of ``gen, param, state``
elements that satisfy **predicate**.
Examples:
.. code-block:: lua
> each(print, take_while(function(x) return x < 5 end, range(10)))
1
2
3
4
> each(print, take_while(function(i, a) return i ~=a end,
enumerate({5, 3, 4, 4, 2})))
1 5
2 3
3 4
.. seealso:: :func:`filter`
.. function:: take(n_or_predicate, gen, param, state)
iterator:take(n_or_predicate)
An alias for :func:`take_n` and :func:`take_while` that autodetects
required function based on **n_or_predicate** type.
.. function:: drop_n(n, gen, param, state)
iterator:drop_n(n)
:param n: the number of elements to drop
:type n: uint
:returns: ``gen, param, state`` iterator after skipping first **n**
elements
Examples:
.. code-block:: lua
> each(print, drop_n(2, range(5)))
3
4
5
> each(print, drop_n(2, enumerate({'a', 'b', 'c', 'd', 'e'})))
3 c
4 d
5 e
.. function:: drop_while(predicate, gen, param, state)
iterator:drop_while(predicate)
:type predicate: function(...) -> bool
:returns: ``gen, param, state`` after skipping the longest prefix
of elements that satisfy **predicate**.
Examples:
.. code-block:: lua
> each(print, drop_while(function(x) return x < 5 end, range(10)))
5
6
7
8
9
10
.. seealso:: :func:`filter`
.. function:: drop(n_or_predicate, gen, param, state)
iterator:drop(n_or_predicate)
An alias for :func:`drop_n` and :func:`drop_while` that autodetects
required function based on **n_or_predicate** type.
.. function:: span(n_or_predicate, gen, param, state)
iterator:span(n_or_predicate)
:type n_or_predicate: function(...) -> bool or uint
:returns: iterator, iterator
Return an iterator pair where the first operates on the longest prefix
(possibly empty) of ``gen, param, state`` iterator of elements that
satisfy **predicate** and second operates the remainder of
``gen, param, state`` iterator.
Equivalent to:
.. code-block:: lua
return take(n_or_predicate, gen, param, state),
drop(n_or_predicate, gen, param, state);
Examples:
.. code-block:: lua
> each(print, zip(span(function(x) return x < 5 end, range(10))))
1 5
2 6
3 7
4 8
> each(print, zip(span(5, range(10))))
1 6
2 7
3 8
4 9
5 10
.. note:: ``gen, param, state`` must be pure functional to work properly
with the function.
.. seealso:: :func:`partition`
.. function:: split(n_or_predicate, gen, param, state)
An alias for :func:`span`.
.. function:: split_at(n, gen, param, state)
An alias for :func:`span`.
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