How to use cat properly

Hi

I'm confused about the properly way of using cat and the help isn't being so helpful. Let's say that I have a variable var1

Name               size
var1          326x377x31x9. 

I want to rearrange "passing down" the last dimension. I know that this words make no sense in a matrix context, but please bear with me.

So the new size would be (31x9=279)

Name              size
var2          326x377x279

I'm currently using

var2 = cat(XX,var1(:,:,:));

If I use XX as 1,2,3 or 4, I get the same result, this is, the size I want. But, what does it mean in this context if I use XX as 1, 2, 3 or 4?

Thanks.

Answer 1

[Matt J]

The cat command is not causing the change that you are seeing in the array shape. What's causing it is the fact that you are indexing with 3 subscripts (:,:,:) instead of 4. Since the cat command only receives 2 arguments it simply returns the 2nd argument unchanged. You could have equivalently generated var2 without cat as follows:

var1=rand(326,377,31,9);
var2=var1(:,:,:);

whos var1 var2

  Name        Size                      Bytes  Class     Attributes

  var1      326x377x31x9            274317264  double              
  var2      326x377x279             274317264  double              

To do something meaingful with cat(), you must give it 3 or more input arguments, e.g.,

var1=eye(3); var2=rand(3);

cat(1, var1,var2) %cat along dimension 1

ans = 6×3

1.0000 0 0 0 1.0000 0 0 0 1.0000 0.0992 0.6564 0.8313 0.5780 0.2544 0.3718 0.1110 0.8576 0.2186

cat(2,var1,var2) %cat along dimension 2 

ans = 3×6

1.0000 0 0 0.0992 0.6564 0.8313 0 1.0000 0 0.5780 0.2544 0.3718 0 0 1.0000 0.1110 0.8576 0.2186

cat(3,var1,var2) %cat along dimension 3 

ans = 
ans(:,:,1) =

     1     0     0
     0     1     0
     0     0     1


ans(:,:,2) =

    0.0992    0.6564    0.8313
    0.5780    0.2544    0.3718
    0.1110    0.8576    0.2186

[Steven Lord]

Instead of using cat take a look at the reshape function.

x = 1:12

x = 1×12

1 2 3 4 5 6 7 8 9 10 11 12

y = reshape(x, 3, 4)

y = 3×4

1 4 7 10 2 5 8 11 3 6 9 12

z = reshape(x, 4, 3).'

z = 3×4

1 2 3 4 5 6 7 8 9 10 11 12

[user20912]

EDIT: I have answer too fast, sorry.

Thanks for your help! I see in what I was wrong. I goint to try with the reshape function.

[Matt J]

The cat command has no role in what you are seeing. When you pass the cat command only 2 arguments it simply returns the 2nd argument unchanged. You could have equivalently generated var2 without cat as follows:

var1=rand(326,377,31,9);
var2=var1(:,:,:);
whos var1 var2

  Name        Size                      Bytes  Class     Attributes

  var1      326x377x31x9            274317264  double              
  var2      326x377x279             274317264  double              

or, more generally, you could have used reshape() as Steven mentioned.

[user20912]

Dear Matt, when you say

or, more generally, you could have used reshape() as Steven mentioned.

do you mean that

var2=var1(:,:,:);

is the same as the following?

[x,y,~] = size(var1)
var2 = reshape(var1,x,y,[])

Because my results are not changing.

Thanks in advance

[Matt J]

Yes, it's the same.