Why can't I employ operator precedence to write statement like $sum((A.^2)(:))$

Question

I want to compute the L2 norm of a n-d matrix. I want to compute this in a single statement, without introducing temporal variables. But it seems I have to, because if I write it like this, it will complains unbalanced parenthesis,

sqrt(sum((A.^2)(:)))

So I have to introduce a temporal matrix B, to write like this

B = A.^2

sqrt(sum(B(:)))

Is there any technique I can use to avoid this? I also found that if a function returns a matrix, I also cannot write like this

(fun(A))(:)

My main concern is why the operator precedence does not work here.

Which norm exactly are you trying to calculate? Because that's not the definition of L2 norm for matrices. The L2 norm of matrices is the sum of the L2 norms of each row vector. see https://en.wikipedia.org/wiki/Matrix_norm#L2.2C1_and_Lp.2Cq_norms — ibezito, May 24 '16 at 04:59
@drorco I want to calculate L2 norm. Yes, I just calculate the squared sum, I need take a square root of the sum. — Hua, May 24 '16 at 05:05
@David Yes, I figured out this way. I am wondering if it is forbidden in matlab to use two pair of parenthesis next to each other. — Hua, May 24 '16 at 05:07
yes, it is forbidden. You cannot index into an expression that has not been assigned into a variable. — Alex, May 24 '16 at 05:26

score 2 · Accepted Answer · edited May 23 '17 at 11:44

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There are several approaches for solving this in one line. One option is to use the reshape function, to reshape A into a vector, as follows:

sqrt(sum(reshape(A,prod(size(A)),1).^2))

Another option is to use Matlab's builtin function:

sqrt(sum(builtin('_paren', A(:)).^2))

edited May 23 '17 at 11:44

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answered May 24 '16 at 05:07

ibezito

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Yes, this works for 2-d matrix. If I have to handle n-d, I have to use (:), which cause the problem. – Hua May 24 '16 at 05:09

Why can't I employ operator precedence to write statement like $sum((A.^2)(:))$

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