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### 17.4 Sums and Products

— Built-in Function: sum (x, dim)
— Built-in Function: sum (..., 'native')

> Sum of elements along dimension dim. If dim is omitted, it defaults to 1 (column-wise sum).

> As a special case, if x is a vector and dim is omitted, return the sum of the elements.

> If the optional argument 'native' is given, then the sum is performed in the same type as the original argument, rather than in the default double type. For example

>               sum ([true, true])
>                 => 2
>               sum ([true, true], 'native')
>                 => true

— Built-in Function: prod (x, dim)

> Product of elements along dimension dim. If dim is omitted, it defaults to 1 (column-wise products).

> As a special case, if x is a vector and dim is omitted, return the product of the elements.

— Built-in Function: cumsum (x, dim)

> Cumulative sum of elements along dimension dim. If dim is omitted, it defaults to 1 (column-wise cumulative sums).

> As a special case, if x is a vector and dim is omitted, return the cumulative sum of the elements as a vector with the same orientation as x.

— Built-in Function: cumprod (x, dim)

> Cumulative product of elements along dimension dim. If dim is omitted, it defaults to 1 (column-wise cumulative products).

> As a special case, if x is a vector and dim is omitted, return the cumulative product of the elements as a vector with the same orientation as x.

— Built-in Function: sumsq (x, dim)

> Sum of squares of elements along dimension dim. If dim is omitted, it defaults to 1 (column-wise sum of squares).

> As a special case, if x is a vector and dim is omitted, return the sum of squares of the elements.

> This function is conceptually equivalent to computing

>               sum (x .* conj (x), dim)

> but it uses less memory and avoids calling conj if x is real.

— Function File: accumarray (subs, vals, sz, fun, fillval, issparse)
— Function File: accumarray (csubs, vals, ...)

> Create an array by accumulating the elements of a vector into the positions defined by their subscripts. The subscripts are defined by the rows of the matrix subs and the values by vals. Each row of subs corresponds to one of the values in vals.

> The size of the matrix will be determined by the subscripts themselves. However, if sz is defined it determines the matrix size. The length of sz must correspond to the number of columns in subs.

> The default action of `accumarray` is to sum the elements with the same subscripts. This behavior can be modified by defining the fun function. This should be a function or function handle that accepts a column vector and returns a scalar. The result of the function should not depend on the order of the subscripts.

> The elements of the returned array that have no subscripts assoicated with them are set to zero. Defining fillval to some other value allows these values to be defined.

> By default `accumarray` returns a full matrix. If issparse is logically true, then a sparse matrix is returned instead.

> An example of the use of `accumarray` is:

>               accumarray ([1,1,1;2,1,2;2,3,2;2,1,2;2,3,2], 101:105)
>               => ans(:,:,1) = [101, 0, 0; 0, 0, 0]
>                  ans(:,:,2) = [0, 0, 0; 206, 0, 208]