out of memory error using ndgrid

a=1:500;
b=1:4;
c=0:2;
d=0:1;
e=0:1;
f=1:15;

I tried running a code, which is

[a, b, c, d, e, f]=ndgrid(sets{:});

but it shows that

Error using repmat
Out of memory. Type HELP MEMORY for your options.

How can I solve this problem? Can I make the code simpler so that it does not take that much memory?

Answer 1

[Walter Roberson]

We do not know. What are the values in sets ? What do you do with the data afterwards?

If sets = {a, b, c, d, e, f} according to the initial assignments of a, b, c, d, e, f that you show, then the total memory for the output of ndgrid is not much, less than 18 megabytes.

However, if you were working in a loop like

a=1:500;
b=1:4;
c=0:2;
d=0:1;
e=0:1;
f=1:15;

for K = 1 : 5
  sets = {a, b, c, d, e, f};
  [a, b, c, d, e, f] = ndgrid(sets{:});
  ...
end

then after you do the ndgrid once, because you are overwriting a, b, c, d, e, f, on the second iteration of the loop, the variables would have 360000 elements each, and you would run out of memory trying to ndgrid that .

Re-using variable names for different purposes is poor programming practice that leads to this kind of problem.

[SANG GYU LEE]

I am trying to make a code which shows every combination of input matrix. In this case, the output will be a 6 column (from a to f) matrix.

All arrays from a to f contain possible numbers that I might be selected. For the case I gave as an example, the output might be [1 1 0 0 0 1; 1 1 0 0 0 2;1 1 0 0 0 3;...;1 1 0 0 0 15; 1 1 0 0 1 1; ...; 500 4 2 1 1 15].

[Walter Roberson]

nset = length(sets);
[T1{1:nset] = ndgrid(sets{:});
output = zeros(numel(T1{1}), nset, class(T1{1}));  %corrected
for K = 1 : nset
  output(:,K) = reshape(T1{K}, [], 1);
end

If you still run out of memory doing the ndgrid then you probably do not have enough memory to complete the operation.

However, if you are running an older version of MATLAB, where repmat was not a built-in operation, then the way repmat was written was not always the best if you were short on memory. In that particular case you would be better off writing the functionality yourself in terms of loops -- or, better yet, writing the functionality as mex. In current versions, repmat is built in and more efficient. (This is relevant because ndgrid relies on repmat.)

[SANG GYU LEE]

Thank you very much for your help.

I made a following function

function output=testcomb(a, b, c, d, e, f)
sets = {a, b, c, d, e, f};
nset = length(sets);
[T1{1:nset}] = ndgrid(sets{:});
output = zeros(numel(T1{1}, nset, class(T1{1})));
for K = 1 : nset
  output(:,K)=reshape(T1{K},[], 1);
end

I get an error

Error in testcomb (line 7)
  output(:,K)=reshape(T1{K},[], 1);

Could you please help me fix this?

[Walter Roberson]

function output=testcomb(a, b, c, d, e, f)
sets = {a, b, c, d, e, f};
nset = length(sets);
[T1{1:nset}] = ndgrid(sets{:});
output = zeros(numel(T1{1}), nset, class(T1{1}));  %corrected
for K = 1 : nset
  output(:,K)=reshape(T1{K},[], 1);
end

[SANG GYU LEE]

Well, I am sorry. I still have the same error on the line of

output(:,K)=reshape(T1{K},[], 1);

[Walter Roberson]

What values are you passing in? What is the exact error message, everything in red?

I tested the code here with the values you showed earlier,

a=1:500;
b=1:4;
c=0:2;
d=0:1;
e=0:1;
f=1:15;

and it seems to work fine after the correction to the assignment to output that I noted above.

If you are using MS Windows, please also post the output of

memory

[SANG GYU LEE]

I closed Matlab and tried your code again. It works well. Thank you very much for your help. I really appreciate.