how can sort the numbers according to the numbners in the first column

suppose i have a matrix where the first column is x(:,1)=[ 1 1 1 2 2 1 3 3 ] and second column is x(:,2)=[ 2 3 2 4 6 9 7 8 9];

i want different matrices which give me the numbers corresponding to 1, 2 and so on. in this case i need three different matrices where the first one will give me [2 3 2 7]

second will give [4 6] and third is [8 9]

Answer 1

[Stephen23]

A general solution using accumarray:

>> x = [1,2;1,3;1,2;2,4;2,6;1,7;3,8;3,9]
x =
   1   2
   1   3
   1   2
   2   4
   2   6
   1   7
   3   8
   3   9
>> C = accumarray(x(:,1),x(:,2),[],@(v){v});
>> C{:}
ans =
   2
   3
   2
   7
ans =
   4
   6
ans =
   8
   9

[johnson saldanha]

thank you

[johnson saldanha]

and if i have to use the elemets from the particular cell how can i access it

[Stephen23]

C{1}
C{2}
C{3}

https://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-cell-array.html

[johnson saldanha]

now suppose i have the store the maximum, minimum and average from each cell, how can i do that and store it in the same cell but second, third and fourth column respectively.

[Stephen23]

@johnson saldanha: you could simply use a loop. Here are some alternatives:

>> fun = @(v){[min(v),max(v),mean(v)]};
>> C = accumarray(x(:,1),x(:,2),[],fun);
>> C{1} % min,max,average
ans =
   2.0000   7.0000   3.5000
>> C{2} % min,max,average
ans =
   4   6   5
>> C{3} % min,max,average
ans =
   8.0000   9.0000   8.5000

Or you could do something a bit different, and just put the values into one matrix:

>> x = [1,2;1,3;1,2;2,4;2,6;1,7;3,8;3,9];
>> mx = accumarray(x(:,1),x(:,2),[],@max);
>> mn = accumarray(x(:,1),x(:,2),[],@min);
>> av = accumarray(x(:,1),x(:,2),[],@mean);
>> out = [x,mx(x(:,1)),mn(x(:,1)),av(x(:,1))]
out =
   1.0000   2.0000   7.0000   2.0000   3.5000
   1.0000   3.0000   7.0000   2.0000   3.5000
   1.0000   2.0000   7.0000   2.0000   3.5000
   2.0000   4.0000   6.0000   4.0000   5.0000
   2.0000   6.0000   6.0000   4.0000   5.0000
   1.0000   7.0000   7.0000   2.0000   3.5000
   3.0000   8.0000   9.0000   8.0000   8.5000
   3.0000   9.0000   9.0000   8.0000   8.5000
%  x(:,1)   x(:,2)   max      min      average
   

But really the best solution would be to use a table, which are designed exactly to do the kind of processing that you are trying to do:

https://www.mathworks.com/help/matlab/matlab_prog/grouping-variables-for-splitting-data.html

Answer 2

[madhan ravi]

By logical indexing you can do it:

a=x(:,1)
b=x(:,2)
first=b(a==1)
second=b(a==2)
third=b(a==3)

Note: It is assumed that x(:,1) and x(:,2) are column vectors and should contain equal number of elements because it’s extracted from a matrix and the above produces the result you need.

[johnson saldanha]

there are n numbers in the first column. not just 1,2 and 3. i want the solution for max of x(:,1);

[madhan ravi]

It works for n numbers right run a loop and use iterator in the place of 1,2and 3 and store the output in a cell whereby if you use celldisp(result) at the end , it will produce your desired result.

[johnson saldanha]

how to store the output in a cell

[madhan ravi]

result=cell(1,n) %before loop
result{i}=a(b==i); %inside loop
celldisp(result)  %outside loop

[johnson saldanha]

thanks