How do I swap 2 rows of a cell array?

I have a 3x2 cell array and I need to swap row 2 and row 3. How can I solve this simple problem? Is there a function to do so that I don't know?

Answer 1

[dpb]

One way...

>> C=num2cell(randi(10,3,2))
C =
  3×2 cell array
    {[5]}    {[1]}
    {[9]}    {[5]}
    {[6]}    {[5]}
>> C(2:3,:)=flipud(C(2:3,:))
C =
  3×2 cell array
    {[5]}    {[1]}
    {[6]}    {[5]}
    {[9]}    {[5]}
>> 

[Pietro Fedrizzi]

Thank you!

[dpb]

NB: The above reverses any contiguous section in general; Bruno's solution reorders the specific listed row indices into the new order.

Which to use depends on circumstances/needs of the application.

[Pietro Fedrizzi]

Got it. Thanks again for your answer and time.

[Bruno Luong]

Actually I don't quite understand dpb comment about "contiguous section". dpb's code and mine do exactly the same thing so applicable for any circumstance and application, beside mine I don't need a flip.

[dpb]

Does the same thing for the given example, yes.

Change the indices to 2 and 4 (presuming at least four rows in the array, of course) and they don't do the same thing at all.

By using the colon operator my code picks sequential lines -- and will for whatever indices are used.

That was done because the OP's ? did have sequential lines.

Using flip can be avoided but requires writing the object row indexing vector with a -1 step factor. Using flip saves that exercise so the same indexing expression is on both LH and RH sides of the assignment.

The alternative of listing a vector of rows allows for discontiguous rows to be rearranged which the above doesn't but requires writing the two indexing vectors independently.

Again, it all depends upon the need of the application as to which is more appropriate; for the specific case of only two lines the general solution is undoubtedly overkill.

[Bruno Luong]

"Change the indices to 2 and 4 (presuming at least four rows in the array, of course) and they don't do the same thing at all."

??? I just don't know what your are trying to say here. They do the same thing to my book

C=num2cell(randi(10,5,2))

C = 5×2 cell array
    {[ 1]}    {[6]}
    {[ 1]}    {[4]}
    {[ 6]}    {[2]}
    {[ 3]}    {[8]}
    {[10]}    {[4]}

Corg = C;
% dpb method
C([2 4],:)=flipud(C([2 4],:))

C = 5×2 cell array
    {[ 1]}    {[6]}
    {[ 3]}    {[8]}
    {[ 6]}    {[2]}
    {[ 1]}    {[4]}
    {[10]}    {[4]}

C=Corg;
% Bruno method
C([2 4],:)=C([4 2],:)

C = 5×2 cell array
    {[ 1]}    {[6]}
    {[ 3]}    {[8]}
    {[ 6]}    {[2]}
    {[ 1]}    {[4]}
    {[10]}    {[4]}

[dpb]

You wrote above

% dpb method
C([2 4],:)=flipud(C([2 4],:))

but that is NOT the code I wrote; you elided the colon that selects contiguous rows.

What I actually wrote in the original answer was

C([2:3],:)=flipud(C([2:3],:));

So, when change the 3 to a 4 one will get 3 rows instead of just two because I assumed (given the OP's example) there could be a more general case of wanting more than just two rows.

You just missed seeing the other colon, Bruno...

Answer 2

[Bruno Luong]

C([2 3],:) = C([3 2],:);

[Pietro Fedrizzi]

Thanks!