Undefined function 'lt' for input arguments of type 'cell'. in using cell

Question

My teacher asked us to write a program for Huffmam Coding in MATLAB, but I met a problem:

Undefined function 'lt' for input arguments of type 'cell'.

Error in work5455 (line 49)

Which is this line:

if(cell{k+1}(1)<cell{k}(1)&&cell{k}(2)==-1)

Since there is no 'lt',how can I solve it? My teacher is just too busy to answer my e-mail..... Here`s the code(not finished),thank you very much!

fprintf('Reading data...')
data=imread('C:\Users\dell\Desktop\2.png');
data=rgb2gray(data);
data=uint8(data);
fprintf('Done!\n')

if~isa(data,'uint8'),
    error('input argument must be a uint8 vector')
end
f=repmat(0,1,256);
len=length(data);
for j=0:255
    for i=1:len
        if data(i)==j;
            f(j)=f(j)+1;
        end
    end
end

f=double(f./len);

simbols=find(f~=0);
f=f(simbols);
[f,sortindex]=sort(f)
simbols=simbols(sortindex)

len=length(simbols);
codeword=cell(1,len);
huffnode=cell(1,2*len);
for i=1:len
    cell{i}={f(i),-1,-1,-1};
end
for i=len+1:2*len-1
    cell{i}={-1,-1,-1,-1};
end

m=len
%for i=1:len
for j=1:len+i-1
    m=m-1;
    for k=1:m
        if(cell{k+1}(1)<cell{k}(1)&&cell{k}(2)==-1)
            cell{2*len}=cell{k+1};
            cell{k+1}=cell{k};
            cell{k}=cell{2*len};
        end
    end
end

Answer 1

The Output of cell{k+1}(1) is a 1x1 cell. Since you cannot compare cells to other cells, you have to make a datatype conversion using cell2mat to get the number itself before. Using

if(cell2mat(cell{k+1}(1))<cell2mat(cell{k}(1))&&cell2mat(cell{k}(2))==-1)

should solve your Problem.
Since cell is a built-in function in matlab I would highly recommend renaming your variable. As @AndrasDeak stated in the comments you might get Problems later on otherwise. In General, you should never use built-in function-names as your variable- or function-names. Please note that i and j represent the imaginary unit in matlab and thus should not be used as Iteration variables either.