Counting how many times a number occured after a specific number

1 vue (au cours des 30 derniers jours)
Chad
Chad le 10 Avr 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.

Réponse acceptée

Kelly Kearney
Kelly Kearney le 10 Avr 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;
  3 commentaires
Kelly Kearney
Kelly Kearney le 10 Avr 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
Kelly Kearney le 10 Avr 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!

Connectez-vous pour commenter.

Plus de réponses (2)

Andrei Bobrov
Andrei Bobrov le 10 Avr 2020
accumarray(hankel(S(1:end-1),S(end-1:end)),1)

AB WAHEED LONE
AB WAHEED LONE le 14 Déc 2021
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.

Catégories

En savoir plus sur Logical dans Help Center et File Exchange

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by