Error using reshape Number of elements must not change. Use [] as one of the size inputs to automatically calculate the appropriate size for that dimension.

Hi,

I have the following code inside of a function:

x = exp(xi);
sz = size(x, 1)-255;
a = reshape(x(1:sz-1,:), [255 256]); 
b = x(sz:end, :);

I am using a minimisation function on this function which throws the following error.

Error using reshape
Number of elements must not change. Use [] as one of the size inputs to automatically calculate the appropriate
size for that dimension.

Error in minimize (line 75)
[f0 df0] = feval(f, X, varargin{:});          % get function value and gradient

I would appreciate any ideas as to why this error might be occuring. Thanks! :)

Answer 1

[Walter Roberson]

sz = size(x, 1)-255;
a = reshape(x(1:sz-1,:), [255 256]); 

So x(1:sz-1,:) would get all except the last 256 rows of x . And then you reshape that to 255 by 256. that is only going to work if the original data was 511 x 256 or certain other "magic" size combinations.

The reason to use things like size(x,1) are to permit flexibility in the size of the array x. But your reshape() is coded in a way that provides no flexibility: it demands that the data be exactly a certain number of elements, which contradicts the request for general size handling gained by asking size()

[Emilia Butters]

Hi Walter, thanks for the help! :)

My original matrix is 65280 x 1 (I pass a matrix of 'ones' of this size into this function each time). I then want to separate the last 256 values of this matrix and assign it to 'b', and take the remaining 65281 and reshape it into 255 x 256.

As I pass the same matrix to this function each time, I don't understand why there is a problem why the shaping. According to what I have just said, should there still be a problem? Or does this mean I have a problem elsewhere?

[Walter Roberson]

255*256 cannot end in a 1. By inspection the 5*6 must end in a 0 in decimal, not in a 1.

[Emilia Butters]

Ah okay thanks! I've fixed it to the following but it still throws the reshape error...

theta_unlog = exp(theta);
sz = size(theta_unlog, 1);
new_sz = sz-256;
new_theta = theta_unlog(1:new_sz,:); 
a = reshape(new_theta, [255 256]); 
b = theta_unlog(new_sz+1:end, :);

[Walter Roberson]

If you want the remaining to have 255*256 elements after you took away 256 then it follows that the original should have 255*256 + 1*256 = 256*256 elements, which is 65536