How may I calculate the number of rows in each group?

Hi! If I have a matrix:

M= [1 50 60 70 50 40
    2 NaN 10 20 10 10
    3 NaN 20 NaN NaN NaN
    1 NaN 60 30 40 50
    2 10 20 10 20 NaN
    1 30 20 40 NaN 50
    2 NaN 50 50 NaN NaN]

The first column indicates to which group the rows belong to. "1, 2, 3" (row 1, 2, 3) are Group 1, "1, 2" (row 4, 5 ) are Group 2, the next "1, 2" ( row 6, 7) are Group 3.

I am trying to find the number of rows in each group. The desired result would be:

N= [3
    2
    2]

Could someone please teach me how to do this? It seems that I can't get my head around it. I think the first step can be splitting the groups:

g=nan(size(M,1),1);
ix=(M(:,1)==1);
g(ix)=[1:sum(ix)];
g=fillmissing(g,'previous');

but I am not sure where to go from here.

[KSSV]

Whats the logic for this?

"1, 2, 3" (row 1, 2, 3) are Group 1, "1, 2" (row 4, 5 ) are Group 2, the next "1, 2" ( row 6, 7) are Group 3

[Dyuman Joshi]

So when the value in first column is 1, a new group starts? If so, @Voss's answer should be good to go.

If not, then specify a general criteria for splitting into groups.

Answer 1

[Voss]

diff(find([M(:,1); 1] == 1))

[Julia]

Thank you! Your answer helps a lot.

Answer 2

[Bhavana Ravirala]

Hi Julia,

According to my understanding, the first element of the row is deciding the group. You can use the below code to know the number of rows in each group.

N=M(:,1);   % getting the first column of elements which we are using to divide the groups
k=length(N); % gives the number of rows in the matrix
j=[];
o=0;
for i=1:k
if(n(i)==1)
     if(o~=0)
        j=[j,o];
    end
o=1;
else   
o=o+1;
if i==k
        j=[j,o];
    end
end
end

Hope this helps!!

[Julia]

Thank you! I'll look into the use of for loop.