"the variable assigned to a may be unused"

Mattlab just returns the name of my function when i run it and the warning message that comes up currently says "the variable assigned to a may be unused" but im not sure what im doing wrong.

%the heap sort fuction 
function sorted = heapsort(x)

%first we must build initial max-heap from x 
%we need to find the length of the array 
n = length(x);

%and create an initial max-heap with the given parameters of (array,
%length)
maxheap = buildmaxheap(x,n);

%initialise the array for the sorted elements
sorted = zeros(1,n);

% now the heapsort algorithm is as follows 
for i = 1:n
    k = n+1-i;     %to find largest i in array
    %put (n+1-i)th largest element in place 
    x = swap(x,1,k);
    %adding final value to the sorted array 
    sorted(k) = maxheap(k);
    %removing final  value 
    maxheap(k) =[];
     
    %create the next maxheap
    n = n - 1;
    maxheap = buildmaxheap(maxheap,i,n);
end

end

%the build max heap function
function maxheap = buildmaxheap(x,n)
a = x;
for i = floor(n/2):-1:1
    %call heapify to make it sorted
    a = maxheapify(a,i,n);
 
end
maxheap = a ;
end



%heapify function
function a = maxheapify(x,i,n)
%calculating the indicies of the child nodes
lhn = 2*i;
rhn = 2*i+1;
a = x;
%comparing left hand child node
if ((lhn <= n) && (a(lhn) > a(i))) %l<=n to check its not a leaf and a(l)>a(i) to find the biggest of the two
    biggest = lhn;
else
    biggest = i; %setting biggest found value to compare with rhn 
end 
%comparing right hand child node
if ((rhn <= n) && (a(rhn) > a(biggest))) %comparing rhn with biggest found previously
    biggest = rhn;
end 
%checking if it is complete
if biggest ~= i 
    %swap
    a = swap(a,i,biggest);
%recursively call itself if its not finished
a = maxheapify(a,biggest,n);

end 
a =x ;
end


function x = swap(x,i,j)
%swap elements i and j in vector(or this case array) x 

val = x(i);
x(i) = x(j);
x(j) = val;
end

Answer 1

[dpb]

In

%the build max heap function
function maxheap = buildmaxheap(x,n)
  a = x;
  for i = floor(n/2):-1:1
    %call heapify to make it sorted
    a = maxheapify(a,i,n);
  end
  maxheap = a ;
end

the x isn't used after the assign...I currently have lost access at least temporarily so can't test; not sure if that's what's generating the warning or not; you forgot to put the error message in full context of the calling code and the traceback so have to guess precisely where it was triggered.

[Ghazal Ramzan ali]

im not sure what you mean, how would i resolve this?

Answer 2

[Image Analyst]

In this code:

if biggest ~= i
	%swap
	a = swap(a,i,biggest);
	%recursively call itself if its not finished
	a = maxheapify(a,biggest,n);
end
a =x ;

let's assume that the if criteria passes and you then execute an assignment to a:

a = maxheapify(a,biggest,n);

Well, as soon as that's finished, you go and assign x to a, overwriting the a you just created:

a =x ;

So the first assignment of the output of maxheapify() to a never gets used. It's assigned but immediately overwritten. So the warning (it's not an error) just notifies you of that situation in case that's not what you wanted.

[Ghazal Ramzan ali]

Thank you so much, the code no longer has any error warnings however it still wont run and is saying there is an error on line 6 with n = length(x)?

[Image Analyst]

What did you pass in to heapsort() for x? You didn't just click the green run triangle without giving anything for x did you?