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 2015-7-27
评论： Walter Roberson 2015-7-28
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));

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回答（2 个）

Star Strider
Star Strider 2015-7-27
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 2015-7-27
编辑：Walter Roberson 2015-7-27
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 个评论
Walter Roberson
Walter Roberson 2015-7-28
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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