Today’s Erlang Thursday is on queue:split/2 from the queue modules Original API.

queue:split/2 takes two arguments. The first argument being a integer N from 0 to X, where X is number of items in the queue, and the second argument is the queue that we wish to split. The return value is a two-tuple with a the first item being a queue of the first N items, and the second item in the tuple is a queue of the rest of the items.

QueueOne = queue:from_list([a, 1, b, 2, c, 3, 4]).
% {[4,3,c],[a,1,b,2]}
queue:split(4, QueueOne).
% {{[2],[a,1,b]},{[4,3],[c][/c]}}
queue:split(0, QueueOne).
% {{[],[]},{[4,3,c],[a,1,b,2]}}
queue:split(1, QueueOne).
% {{[],[a]},{[4,3,c],[1,b,2]}}
queue:split(7, QueueOne).
% {{[4,3,c],[a,1,b,2]},{[],[]}}
queue:split(15, QueueOne).
% ** exception error: bad argument
%      in function  queue:split/2
%         called as queue:split(15,{[4,3,c],[a,1,b,2]})
{SplitFirst, SplitSecond} = queue:split(3, QueueOne).
% {{[b,1],[a]},{[4,3,c],[2]}}
SplitFirst.
% {[b,1],[a]}
SplitSecond.
% {[4,3,c],[2]}
queue:peek(SplitFirst).
% {value,a}
queue:peek(SplitSecond).
% {value,2}

Erlang also provides a queue:join/2 function that takes two queues, and returns a new queue, with the queue that was passed as the second argument appended to the queue passed in as the first argument.

queue:join(SplitFirst, SplitSecond).
% {[4,3,c],[a,1,b,2]}
queue:join(SplitSecond, SplitFirst).
% {[b,1],[2,c,3,4,a]}
queue:join(queue:new(), SplitFirst).
% {[b,1],[a]}
queue:join(queue:new(), queue:new()).
% {[],[]}

–Proctor