ind2sub with arbitrary martix

Hi

I intend to use ind2sub to get the subscripts indexing from linear indexing and I will use if to read data in matrix of arbitrary size.

The problem with this is that you have to know the dimension of the matrix, so that you can call ind2sub with the corresponding number of outputs. What I would like is to get a output vector that has the corresponding number of elements. Is this possible to achieve?

Best regards

Thomas

Answer 1

[Stephen23]

"In the example below: how to set the three output arguments [I1 I2 I3] "automatically" ?"

You use a comma-separated list:

https://www.mathworks.com/matlabcentral/answers/1656435-tutorial-comma-separated-lists-and-how-to-use-them

A = rand(3,6,34);
X = 24;
C = cell(1,ndims(A));
[C{:}] = ind2sub(size(A),X);

Checking:

C{:}

ans = 3

ans = 2

ans = 2

[Thomas]

Yes, this works!

Thanks alot!

[Torsten]

Strange that the output argument from int2sub is not automatically of size 1 x ndims(A) if the call to ind2sub was written as

I = ind2sub(size(A),X);

[Stephen23]

"Strange that the output argument from int2sub is not automatically of size 1 x ndims(A) "

X can be any size, the outputs have exactly the same size as X. The proposed output size would conflict with that.

If you restrict X to scalar only, then the function is very easy to copy and modify.

[Lucas Graham]

I was about to try to gimmick some sort of iteratively generated string and then use eval(), this is so much better. Thanks.

Answer 2

[Torsten]

You mean

A = rand(5,6);
s = size(A)

s = 1×2

5 6

?

[Thomas]

[i1,i2,i3]=ind2sub([3,3,3],8)

i1 = 2

i2 = 3

i3 = 1

[i1,i2,i3,i4]=ind2sub([3,3,3,4],8)

i1 = 2

i2 = 3

i3 = 1

i4 = 1

But I would like to have a function my_ind2sub so that

I=my_ind2sub([3,3,3],8)

->I=[2,3,1]

and

I=my_ind2sub([3,3,3,3],8)

->I=[2,3,1,1]

[Torsten]

I don't know how you automativcally get I according to the matrix size. I asked this question below. I think @Steven Lord will be able to answer this.

Answer 3

[dpb]

Use size(M) as the size argument to ind2sub where M is the matrix...the size argument doesn't have to be a constant expression.

Answer 4

[Steven Lord]

You have to specify the size of the array you want to use the subscripts to index into because the same linear index could generate different subscripts based on the size. Consider two examples where I want the subscripts for element 4.

A1 = reshape(1:8, 2, 4)

A1 = 2×4

1 3 5 7 2 4 6 8

A2 = reshape(1:8, 4, 2)

A2 = 4×2

1 5 2 6 3 7 4 8

A1 and A2 have the same number of elements but different sizes. This means that element 4 of A1 is element (2, 2) and element 4 of A2 is element (4, 1).

[r1, c1] = ind2sub(size(A1), 4)

r1 = 2

c1 = 2

[r2, c2] = ind2sub(size(A2), 4)

r2 = 4

c2 = 1

If I just called ind2sub with the input 4, should the outputs be 2 and 2, 4 and 1, or something else?

But let's take a step back. When you say this:

What I would like is to get a output vector that has the corresponding number of elements.

do you just want to use linear indexing? How are you hoping or planning to use these indices with a "matrix of arbitrary size"?

B1 = A1.^2 % just so the elements aren't 1:8

B1 = 2×4

1 9 25 49 4 16 36 64

B2 = A2.^2

B2 = 4×2

1 25 4 36 9 49 16 64

[B1(4) B2(4)] % Both are 16

ans = 1×2

16 16

[Torsten]

@Steven Lord

Say I have a matrix A I don't know the size of in advance.

How can I prescribe that the array of outputs I1,I2,...In from ind2sub as

[I1,I2,...,In] = ind2sub(sz,ind)

matches the number of dimensions of A ?

In the example below: how to set the three output arguments [I1 I2 I3] "automatically" ?

A = rand(3,6,34);
s = size(A);

s = 1×3

3 6 34

[I1 I2 I3] = ind2sub(s,24)

I1 = 3

I2 = 2

I3 = 2