Matlab - Queue data structure - matlab

Question

I am working on implementing queue data structures using cell arrays in Matlab. I am trying to write functions to advance the queue by one, as well as search the queue for a specific item. At the moment the function looks like this (car types are the example data).
function q = createQueue()
q={};
q = enqueue(q,'Hilux');
q = enqueue(q,'E-Type');
q = enqueue(q,'Beetle');
q = enqueue(q,'Enzo');
q = enqueue(q,'Boxter');
q = dequeue(q)
q = searchQueue(q,'Boxter')
end
% Adds an item to the end of the queue. Returns the new queue.
function q = enqueue(q,item)
q{end+1} = item;
end
function [q item] = dequeue(q)
q{1} = {};
q{1} = q{2};
q{2} = q{3};
q{3} = q{4};
q{4} = q{5};
q{5} = {};
end
function answer = searchQueue(q, item)
for i = 1:length(q)
if q{i} == item
answer = fprintf('FOUND AT INDEX %d',i);
break
else
disp('-1')
end
end
end
Currently, the dequeue function leaves an empty cell, rather than removing the cell entirely. Is this able to be avoided? The searchQueue function also returns an error, and i am lost as to why. Thanks

Answer 1

Here's a rough first cut at it using matlab's object oriented capabilities. The reason for creating a class is to get reference semantics from the handle type, which allows for the dequeue/enqueue functions to modify the cell array directly, removing the need for reassignment.
I believe the code example below answers your main question about how to dequeue without leaving an empty cell (could be used without the OOP approach to the same effect).
To answer your question about what's wrong with your search: 1) the comparison q{i} == item gives you problems because == compares characters (which fails if there is a string size mismatch) whereas you wanted to use isequal(); 2) you wanted sprintf rather than fprintf; and 3) while not strictly wrong, the else in your loop happens on every item that doesn't match, which is probably not what you wanted.
classdef Queue < handle
properties
data = {}
end
methods
function q = Queue()
% TODO: do something in constructor if desired
end
function item = dequeue(q)
% TODO: bounds check
item = q.data(1);
q.data = q.data(2:end);
end
function enqueue(q,item)
q.data{end+1} = item;
end
function answer = search(q, item)
% TODO: check for multiple occurrences
answer = 'NOT FOUND';
for i = 1:length(q.data)
if isequal(q.data{i},item)
answer = sprintf('FOUND AT INDEX %d',i);
break
end
end
end
end
end