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Error Using Accumarray, Third input SZ must be a full row vector with one element for each column of SUBS

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Ellie
Ellie on 27 Jul 2015
Commented: Walter Roberson on 28 Jul 2015
Hi All, I am having trouble using accumarray. I am getting the following error and I am having trouble figuring out why. Thanks in advance.
Error using accumarray Third input SZ must be a full row vector with one element for each column of SUBS.
Error in approach2 (line 13) p = accumarray(subs, val, sz);
And the following is the code:
d = [3 3 4 4 4 5 6 5 5 4 3 2 3 4 3 2 5 6 6 5 5 4 3];
precision = 1;
f = (round(d/precision)).*precision;
[c, ~, ic] = unique(f);
N = numel(c);
subs = [ic(1:end - 3), ic(2:end-2), ic(3:end-1), ic(4:end)]; val = 1; sz = [N^2, N^2];
p = accumarray(subs, val, sz);
p = bsxfun(@rdivide, p, sum(p, 2));

Answers (2)

Star Strider
Star Strider on 27 Jul 2015
If you don’t want to use the third argument, you don’t need to include it.
See if substituting this for your accumarray call does what you want:
p = accumarray(subs, val);

Walter Roberson
Walter Roberson on 27 Jul 2015
Edited: Walter Roberson on 27 Jul 2015
When you use accumarray, each row of subs is used as an index into array, as if
idx = num2cell(subs(K,:));
result(idx{:}) = val(K);
Your subs is 4 wide, so you are asking to create a 4 dimensional array.
My guess is that you want sz = [N, N, N, N]
You could always reshape() the result to N^2 x N^2
  1 Comment
Walter Roberson
Walter Roberson on 28 Jul 2015
subs_sq = [(subs(:,2) - 1)*N + subs(:,1), (subs(:,4) - 1)*N + subs(:,3)];
p = accumarray(subs_sq, val);
p = bsxfun(@rdivide, p, max(1,sum(p, 2)));
Or alternately,
p = reshape(accumarray(subs, val, [N, N, N, N]), [N^2, N^2]);
p = bsxfun(@rdivide, p, max(1,sum(p, 2)));
Yes this does create a temporary 4-dimensional array, and then immediately reshapes it to 2D. It also has exactly the same effect as the subs_sq version that builds the 2D array directly by assigning a number to each possible 2D vector.

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