MATLAB array manipulation tips and tricks

CHAPTER 3. BASIC ARRAY PROPERTIES 7which is the essential part of the function mttsize in the MTT Toolbox.Code like the following is sometimes seen, unfortunately. It might be more intuitive than theabove, but it is more fragile since it might use a lot of memory when dims contains a large value.sx = size(x);% get size along all dimensionsn = max(dims(:)) - ndims(x); % number of dimensions to appendsx = [ sx ones(1, n) ];% pad size vectorsiz = sx(dims);% extract dimensions of interestAn unlikely scenario perhaps, but imagine what happens if x and dims both are scalars and thatdims is a billion. The above code would require more than 8 GB of memory. The suggestedsolution further above requires a negligible amount of memory. There is no reason to write fragilecode when it can easily be avoided.3.2 Dimensions3.2.1 Number of dimensionsThe number of dimensions of an array is the number of the highest non-singleton dimension (seesection 3.2.2) but never less than two since arrays in MATLAB always have at least two dimensions.The function which returns the number of dimensions is ndims, so the number of dimensions of anarray x isdx = ndims(x);% number of dimensionsOne may also say that ndims(x) is the largest value of dim, no less than two, for which size(x,dim)is different from one.Here are a few examplesx = ones(2,1)x = ones(2,1,1,1)x = ones(1,0)x = ones(1,2,3,0,0)x = ones(2,3,0,0,1)x = ones(3,0,0,1,2)% 2-dimensional% 2-dimensional% 2-dimensional% 5-dimensional% 4-dimensional% 5-dimensional3.2.2 Singleton dimensionsA “singleton dimension” is a dimension along which the length is one. That is, if size(x,dim)is one, then dim is a singleton dimension. If, in addition, dim is larger than ndims(x), then dimis called a “trailing singleton dimension”. Trailing singleton dimensions are ignored by size andndims.Singleton dimensions may be removed with squeeze. Removing singleton dimensions changesthe size of an array, but it does not change the number of elements in an arrayFlipping an array along a singleton dimension is a null-operation, that is, it has no effect, itchanges nothing.3.3 Number of elementsThe number of elements in an array may be obtained with numel, e.g., numel(x) is the numberof elements in x. The number of elements is simply the product of the length along all dimensions,that is, prod(size(x)).