# queue

### MODULE

queue

### MODULE SUMMARY

Abstract Data Type for FIFO Queues

### DESCRIPTION

This module implements FIFO queues in an efficient manner.

All operations has an amortised O(1) running time, except `len/1`, `reverse/1`, `join/2` and `split/2` that probably are O(n).

### EXPORTS

`cons(Item, Q1) -> Q2`

Types:

`Item = term()`
`Q1 = Q2 = queue()`

Inserts `Item` at the head of queue `Q1`. Returns the new queue `Q2`.

`daeh(Q) -> Item`

The same as `last(Q)` and the opposite of `head(Q)`.

`from_list(L) -> queue()`

Types:

`L = list()`

Returns a queue containing the items in `L`, in the same order - the head item of the list will be the head item of the queue.

`head(Q) -> Item`

Types:

`Item = term()`
`Q = queue()`

Returns `Item` from the head of queue `Q`.

Fails with reason `empty` if `Q` is empty.

`in(Item, Q1) -> Q2`

Types:

`Item = term()`
`Q1 = Q2 = queue()`

Inserts `Item` at the tail of queue `Q1`. Returns a new queue `Q2`. This is the same as `snoc(Q1, Item)`.

`in_r(Item, Q1) -> Q2`

Types:

`Item = term()`
`Q1 = Q2 = queue()`

Inserts `Item` at the head of queue `Q1`. Returns a new queue `Q2`. This is the same as `cons(Item, Q1)`.

`init(Q1) -> Q2`

Types:

`Item = term()`
`Q1 = Q2 = queue()`

Returns a queue `Q2` that is the result of removing the last item from `Q1`. This is the opposite of `tail(Q1)`.

Fails with reason `empty` if `Q1` is empty.

`is_empty(Q) -> true | false`

Types:

`Q = queue()`

Tests if `Q` is empty and returns `true` if so and `false` otherwise.

`join(Q1, Q2) -> Q3`

Types:

`Q1 = Q2 = Q3 = queue()`

Returns a queue `Q3` that is the result of joining `Q1` and `Q2` with `Q1` before (at the head) `Q2`.

`lait(Q1) -> Q2`

The same as `init(Q1)` and the opposite of `tail(Q1)`.

`last(Q) -> Item`

Types:

`Item = term()`
`Q = queue()`

Returns the last item of queue `Q`. This is the opposite of `head(Q)`.

Fails with reason `empty` if `Q` is empty.

`len(Q) -> N`

Types:

`Q = queue()`
`N = integer()`

Calculates and returns the length of queue `Q`.

`new() -> Q`

Types:

`Q = queue()`

Returns an empty queue.

`out(Q1) -> Result`

Types:

`Result = {{value, Item}, Q2} | {empty, Q1}`
`Q1 = Q2 = queue()`

Removes the head item from the queue `Q1`. Returns the tuple `{{value, Item}, Q2}`, where `Item` is the item removed and `Q2` is the new queue. If `Q1` is empty, the tuple `{empty, Q1}` is returned.

`out_r(Q1) -> Result`

Types:

`Result = {{value, Item}, Q2} | {empty, Q1}`
`Q1 = Q2 = queue()`

Removes the last item from the queue `Q1`. Returns the tuple `{{value, Item}, Q2}`, where `Item` is the item removed and `Q2` is the new queue. If `Q1` is empty, the tuple `{empty, Q1}` is returned.

`reverse(Q1) -> Q2`

Types:

`Q1 = Q2 = queue()`

Returns a queue `Q2` that contains the items of `Q1` in the reverse order.

`snoc(Q1, Item) -> Q2`

Types:

`Item = term()`
`Q1 = Q2 = queue()`

Inserts `Item` as the last item of queue `Q1`. Returns the new queue `Q2`. This is the opposite of `cons(Item, Q1)`.

`split(N, Q1) -> {Q2,Q3}`

Types:

`N = integer()`
`Q1 = Q2 = Q3 = queue()`

Splits `Q1` into a queue `Q2` of length `N` with items from the head end, and a queue `Q3` with the rest of the items.

`tail(Q1) -> Q2`

Types:

`Item = term()`
`Q1 = Q2 = queue()`

Returns a queue `Q2` that is the result of removing the head item from `Q1`.

Fails with reason `empty` if `Q1` is empty.

`to_list(Q) -> list()`

Types:

`Q = queue()`

Returns a list of the items in the queue, with the head item of the queue as the head of the list.

### AUTHORS

Claes Wikström - support@erlang.ericsson.se
Raimo Niskanen - support@erlang.ericsson.se

stdlib 1.14.3
Copyright © 1991-2007 Ericsson AB