# Counting how many times a number occured after a specific number

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Commented: Kelly Kearney on 10 Apr 2020
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.

#### 1 Comment

darova on 10 Apr 2020
Can you explain more? I don't understand

Kelly Kearney on 10 Apr 2020
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 on 10 Apr 2020
You can generate M with accumarray directly!
M2 = accumarray(seq, 1, [4 4])
assert(isequal(M,M2))
Kelly Kearney on 10 Apr 2020
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 on 10 Apr 2020
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!

Andrei Bobrov on 10 Apr 2020
accumarray(hankel(S(1:end-1),S(end-1:end)),1)