Undefined function or variable "sum".

I have the following code:

function outdata=pixelate(fname)
imdata=rgb2gray(imread(fname));
outdata=zeros(size(imdata))+255;
for i=1:size(imdata,1)
  indi=[i-1 i i+1];
  indi(indi<=0)=[];
  indi(indi>size(imdata,1))=[];
  isum=sum(indi,:);
  isize=length(indi);
  for j=1:size(imdata,2)
    indj=[i-1 i i+1];
    indj(indj<=0)=[];
    indj(indj>size(imdata,1))=[];
    jsize=length(indj);
    avg=sum(isum(indj))/(isize*jsize);
    if avg<255*0.5
      outdata(i,j)=0;
    end
  end
end
end

The code analyzer generates a warning on line 8 saying 'The variable 'sum' might be used before it is defined.' The when I run the code I get the following:

Undefined function or variable "sum".
Error in pixelate (line 10)
  isum=sum(indi,:);

After a little googling, I tried the following while in debug mode (stopped at line 8):

K>> which sum
built-in (C:\Program Files\MATLAB\R2014a\toolbox\matlab\datafun\@uint8\sum)  % uint8 method
K>> which -all sum
built-in (C:\Program Files\MATLAB\R2014a\toolbox\matlab\datafun\@uint8\sum)    % uint8 method
built-in (C:\Program Files\MATLAB\R2014a\toolbox\matlab\datafun\@uint16\sum)   % Shadowed uint16 method
built-in (C:\Program Files\MATLAB\R2014a\toolbox\matlab\datafun\@uint32\sum)   % Shadowed uint32 method
built-in (C:\Program Files\MATLAB\R2014a\toolbox\matlab\datafun\@uint64\sum)   % Shadowed uint64 method
built-in (C:\Program Files\MATLAB\R2014a\toolbox\matlab\datafun\@int8\sum)     % Shadowed int8 method
built-in (C:\Program Files\MATLAB\R2014a\toolbox\matlab\datafun\@int16\sum)    % Shadowed int16 method
built-in (C:\Program Files\MATLAB\R2014a\toolbox\matlab\datafun\@int32\sum)    % Shadowed int32 method
built-in (C:\Program Files\MATLAB\R2014a\toolbox\matlab\datafun\@int64\sum)    % Shadowed int64 method
built-in (C:\Program Files\MATLAB\R2014a\toolbox\matlab\datafun\@single\sum)   % Shadowed single method
built-in (C:\Program Files\MATLAB\R2014a\toolbox\matlab\datafun\@char\sum)     % Shadowed char method
built-in (C:\Program Files\MATLAB\R2014a\toolbox\matlab\datafun\@double\sum)   % Shadowed double method
built-in (C:\Program Files\MATLAB\R2014a\toolbox\matlab\datafun\@logical\sum)  % Shadowed logical method
C:\Program Files\MATLAB\R2014a\toolbox\symbolic\symbolic\@sym\sum.m            % Shadowed sym method
C:\Program Files\MATLAB\R2014a\toolbox\matlab\timeseries\@timeseries\sum.m     % Shadowed timeseries method
K>> whos sum
K>>

I have tried restarting matlab. I have also tried copying the code and pasting it to a new file. I have not tried restarting the computer, though I will do that when it will not hurt too much.

Answer 1

[Fangjun Jiang]

check the syntax of the sum() function

Syntax

B = sum(A)
B = sum(A,dim)
B = sum(..., 'double')
B = sum(..., dim,'double')
B = sum(..., 'native')
B = sum(..., dim,'native')

Your statement isum=sum(indi,:) is invalid. Maybe you mean isum=sum(indi(:));

[Craig]

I saw the typo while trying to rewrite it to remove the 'sum' command entirely. A little embarrassing for me. :)

[KAE]

Don't feel embarrassed: I had the same kind of typo and the error message confused things. It seems like sum should crash instead of saying the function is undefined.

[Adam]

The error is a symptom of the, often annoying, similar syntax for calling a function or indexing into a variable, using parenthesis.

Matlab tries to be intelligent and sees the indexing syntax and thus assumes 'sum' is intended as a variable rather than the function, which is why the error occurs and with the wording it has - the variable 'sum' does indeed not exist to be indexed into.

But yes, it is certainly one of those errors that can be very confusing when you aren't used to it or expecting it (and indeed, are we ever expecting such errors?!)

[Anthony Dave]

Someone code "sum" as a variavle. I rename the var, and it works well.

Answer 2

[Adam]

isum=sum(indi,:);

is not valid syntax for the builtin sum function as far as I am aware.

It is a syntax that would be consistent with 'sum' representing a matrix variable which, of course, is undefined, hence the error.