Counting how many times a number occured after a specific number

Lets say I have a sequence of numbers ranging from 1 to 4, S = [ 3 2 2 4 3 1 ]. I want a 4x4 matrix which tells me how many times I went from say 3 to 2 or 4 to 1. It would look like this: M = [ 0 0 0 0; 0 1 0 1; 1 1 0 0; 0 0 1 0]. Sounds simple, but I'm out of thoughts. Thanks.

[darova]

Can you explain more? I don't understand

Answer 1

[Kelly Kearney]

This is a good use case for accumarray:

S = [ 3 2 2 4 3 1];

[seq, ~, g] = unique([S(1:end-1)' S(2:end)'], 'rows');

n = accumarray(g, ones(size(g)));

M = zeros(4);
idx = sub2ind(size(M), seq(:,1), seq(:,2));
M(idx) = n;

[Sean de Wolski]

You can generate M with accumarray directly!

M2 = accumarray(seq, 1, [4 4])
assert(isequal(M,M2))

[Kelly Kearney]

That only works if there is only one instance of each sequence, though:

S = [ 3 2 2 4 3 1 3 2];
 
[seq, ~, g] = unique([S(1:end-1)' S(2:end)'], 'rows');
 
% Method 1

n = accumarray(g, 1);
 
M = zeros(4);
idx = sub2ind(size(M), seq(:,1), seq(:,2));
M(idx) = n

% Method 2

M2 = accumarray(seq, 1, [4 4])

% Check...

assert(isequal(M,M2))

Result:

M =

0 0 1 0

0 1 0 1

1 2 0 0

0 0 1 0

M2 =

0 0 1 0

0 1 0 1

1 1 0 0

0 0 1 0

Error using testsnippets (line 2638)

Assertion failed.

[Kelly Kearney]

Though apparently the call to unique is unnecessary... possibly what you were suggesting?

S = [ 3 2 2 4 3 1 3 2];
accumarray([S(1:end-1)' S(2:end)'], 1)

ans =

0 0 1 0

0 1 0 1

1 2 0 0

0 0 1 0

I learned something new!

Answer 2

[Andrei Bobrov]

accumarray(hankel(S(1:end-1),S(end-1:end)),1)

Answer 3

[AB WAHEED LONE]

What about m*n matrix

for example , S= [1 2 2 3 1;

1 3 1 3 4;

1 3 4 1 3;

1 3 4 1 3;

1 3 4 1 3];

When i tried to count the same ((1,1),(1,2),(1,3),(1,4),(2,2),(2,3), etc), it just creates the square matrix of max element size.

for example in case of first row X=[1 2 2 3 1], the size of output matrix is 3*3 ,which should have been 4*4.

could you comment on this.