Question

Two questions:

1) I’ve found a piece of code that says something like cellArr{x}{y}{3,8} = 1.0;, I was wondering what the {3,8} means. The program connections various nodes in a collections of connected graphs together. Here we would say that “in the set of graphs x, the graph y’s connection from 3 to 8 has a vertex label of 1.0”. Still, in general what does the syntax {3,8} mean in MatLab?

2) This probably isn’t the place for this question but should I really be using cell arrays if I know I’m always going to be having vertex values i.e. decimals/floats. Would a matrix be better because I know I’m only going to have a single data type?

Thank you :).

Answer 1

1. Cell arrays can have multiple dimensions, and thus they can be indexed with multiple subscripts like any other multidimensional array. The syntax {3,8} is indexing a (presumably) 2-D cell array, getting the contents of the cell in the third row and eighth column.

2. There are two main reasons to use cell arrays: storing data of different types or storing data of different sizes. Assuming x and y are scalar indices in your example, then cellArr is a cell array with the cell indexed by x containing another cell array, whose cell indexed by y contains a 2-D cell array which stores your vertex labels.

   Now, if your vertex labels are all the same data type and are all just single non-empty (i.e. not []) values, then the 2-D cell array at the lowest level could be turned into a 2-D numeric array, and your indexing will look like this:

   cellArr{x}{y}(3,8) = 1.0;  %# Note the use of () instead of {}
   

   The question now becomes how to handle the two enclosing sets of cell arrays indexed by x and y. If every cell that can be indexed by y contains 2-D numeric arrays all of the same size and type, then that cell array could be turned into a 3-D numeric array that could be indexed like so:

   cellArr{x}(3,8,y) = 1.0;  %# Here I've chosen to use y as the third dimension
   

   Finally, if every cell that can be indexed by x contains 3-D numeric arrays that are again all of the same size and type, then cellArr could be turned into a 4-D numeric array that could be indexed like so:

   numArr(3,8,y,x) = 1.0;
   

   You could change the order of the subscripts (i.e. the dimensions of numArr) to your liking, but I put x and y at the end so that if you were to index a subarray of vertex labels like numArr(:,:,y,x) it will return it as a 2-D array. If you had the indices ordered such that you would index a subarray of vertex labels like numArr(x,y,:,:), it will return the result as a 4-D array with ones for the two leading singleton dimensions (which you would have to remove using functions like SQUEEZE).