Linear indices from colon operator

Any array (let's consider only 2D here) can be indexed using colon operator such

array(p1:p2,p3:p4)

How can one generate list of linear indices (for that particular array) from parameters [p1, p2, p3, p4]?

Answer 1

[Matt J]

 [I,J]=ndgrid(p1:p2,p3:p4);
 
 linear=sub2ind(size(array)  , I,J);

Answer 2

[Steven Lord]

See sub2ind.

[Stephen23]

Note that in general p1:p2 and p3:p4 when used a indices like that will refer to a rectangular block of that array. So in order to get the linear indices of the entire block you will need to provide SUB2IND with subscript indices for the entire block. One way to achieve this is using NDGRID:

array = randi(9,9,9);
p1 = 3;
p2 = 5;
p3 = 2;
p4 = 5;
[X,Y] = ndgrid(p1:p2,p3:p4);
Z = sub2ind(size(array),X,Y);
array(Z)

ans = 3×4

4 7 2 6 9 9 6 7 1 2 4 8

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array(p1:p2,p3:p4)

ans = 3×4

4 7 2 6 9 9 6 7 1 2 4 8

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[Walter Roberson]

@Stephen23

sub2ind() processes the indexes in parallel. All of the indices must be the same size (or else scalars)

sub2ind([4 5],1:2,1:3)

Error using sub2ind (line 61)
Subscript arguments must be scalars or arrays of the same size.

Answer 3

[Catalytic]

K=createArray(size(array),Fill=sparse(0));

K(p1:p2,p3:p4)=1;

out=find(K); %linear indices

[Matt J]

I would probably do instead,

K=createArray(size(array),Fill=sparse(false));

or maybe just use,

K=false(size(array))

depending on the expected size of the output.