Split array according to one column values

I have an array like this:

A = {10 1 1; 10 1 2; 10 1 1; 10 2 1; 10 1 3; 10 2 4; 10 2 5}

I want to split into arrays based on the second column value. The output should look like this:

B = {10 1 1; 10 1 2; 10 1 1; 10 1 3}

C = {10 2 1; 10 2 4; 10 2 5}

How can I do this ?

[Jack Lavender]

What would you recommend if you have a large array and an unknown number of different values in the indexing column?

[Stephen23]

"What would you recommend if you have a large array and an unknown number of different values in the indexing column?"

https://www.mathworks.com/matlabcentral/answers/412088-split-array-according-to-one-column-values#comment_593120

https://www.mathworks.com/matlabcentral/answers/412088-split-array-according-to-one-column-values#comment_2327420

Answer 1

[Stephen23]

Your life would be much easier if you stored numeric data in numeric arrays:

>> A = [10 1 1; 10 1 2; 10 1 1; 10 2 1; 10 1 3; 10 2 4; 10 2 5]
A =
   10    1    1
   10    1    2
   10    1    1
   10    2    1
   10    1    3
   10    2    4
   10    2    5
>> B = A(A(:,2)==1,:)
B =
   10    1    1
   10    1    2
   10    1    1
   10    1    3
>> C = A(A(:,2)==2,:)
C =
   10    2    1
   10    2    4
   10    2    5

[Rui Carapinha]

Thanks for the fast response :)

What I represented has a example, I can have several values in the 2nd column (sorry if I didn't explained this). That way I would have to know in the first hand the values I have.

[Stephen23]

@Rui Carapinha: there are several approaches. Here is one simple solution using unique and arrayfun:

>> A = [10,1,1;10,1,2;10,1,1;10,2,1;10,1,3;10,2,4;10,2,5]
A =
    10     1     1
    10     1     2
    10     1     1
    10     2     1
    10     1     3
    10     2     4
    10     2     5
>> U = unique(A(:,2));
>> C = arrayfun(@(n)A(A(:,2)==n,:),U,'uni',0);
>> C{:}
ans =
    10     1     1
    10     1     2
    10     1     1
    10     1     3
ans =
    10     2     1
    10     2     4
    10     2     5

[Stephen23]

Another approach using FINDGROUPS and SPLITAPPLY:

A = [10,1,1;10,1,2;10,1,1;10,2,1;10,1,3;10,2,4;10,2,5]

A = 7×3

10 1 1 10 1 2 10 1 1 10 2 1 10 1 3 10 2 4 10 2 5

G = findgroups(A(:,2));
C = splitapply(@(m){m},A,G)

C = 2×1 cell array
    {4×3 double}
    {3×3 double}

C{:}

ans = 4×3

10 1 1 10 1 2 10 1 1 10 1 3

ans = 3×3

10 2 1 10 2 4 10 2 5

[Steven Lord]

You could potentially also use groupsummary and use it to perform whatever operations you'd perform on the matrices stored in the cells of C.

Answer 2

[LuKr]

B=A(cellfun(@(x) isequal(1,x),{A{:,2}}),:)
C=A(cellfun(@(x) isequal(2,x),{A{:,2}}),:)

Maybe like this?

Answer 3

[Hermes Suen]

You should be able to do this through logical indexing, and it may be easier if it was not a cell array but a normal numerical array. Define the matrix as such, without the curly braces:

A = [10 1 1; 10 1 2; 10 1 1; 10 2 1; 10 1 3; 10 2 4; 10 2 5];

Then store the second column into a separate variable:

cols = A(:,2);

This takes the second column of the A matrix and stores it into a column vector called "cols". From there you can use logical indexing as follows:

ones = cols ==1;
twos = cols ==2;

The output of the above two commands should be this:

ones =

7×1 logical array

   1
   1
   1
   0
   1
   0
   0

"Ones" will have a "1" whenever the column vector has a "1" and has a "0" whenever the column vector has anything other than a 1. The same logic then applies to the variable "twos"

From here you can then call the following two commands to get what you are looking for:

B = A(ones,:)
C = A(twos,:)

This is logical indexing, a neat feature of MATLAB, let me know if you need further clarification or if you have any questions!