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Indexing (querying) the ndgrid output without actually building it

Is it necessary to index the ndgrid output at all without actually plotting the output due to memory issues? Suppose we need to plot the output and query the i-th item

[output{1:n}] = ndgrid(1:k)  
[output{1}(i) ... output{n}(i)]

Can building be avoided?

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2 answers


I believe you are looking for a function ind2sub

:

[o{1:n}] = ind2sub( ones(1,n)*k, ii );

The size of your ndgrid is [k, k, ... k]

( n

times) and you are trying to access the linear index ii

into that dimensional hypertom. k^n

...



PS,
Better not to use i

as variable name in Matlab .

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I got it over with, so I am posting a complete answer. Write the following function in m file:

function output=my_ndgrid(varargin)

in=nargin;
sz=zeros(1,in-1);
for i=1:in-1
    sz(i)=length(varargin{i});
end

ind=varargin{in};

[ndgrid_ind{1:length(sz)}] = ind2sub(sz,ind);

for i=1:length(sz)
    output{i}(ind)=varargin{i}(ndgrid_ind{i});
end

end

following command taken from this answer

[ndgrid_ind{1:length(sz)}] = ind2sub(sz,ind);

In the above function, you can pass as many arguments as you like, just like you go to ndgrid

. It's just that the last argument should be the index (in your case the element i^th

, so the index will be i

).



For example,

a=my_ndgrid(1:3:10,2:2:6,5:1:8,10); %asking for 10th element

It will be saved a{1}(10),...,a{3}(10)

as you like.

You get [4 6 5]

as answer that matches ndgrid

by manually creating .

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