# Matlab tril function does not work as expected

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Yuzhen Lu on 12 Apr 2020
Answered: Image Analyst on 13 Apr 2020
I am using Matlab2019a. When I was using tril an triu functions for generating triangular matrix, they ony give single values. Here are their function descriptions when type editing commdands:
tril Extract lower triangular part.
tril(X) is the lower triangular part of X.
tril(X,K) is the elements on and below the K-th diagonal
of X . K = 0 is the main diagonal, K > 0 is above the
main diagonal and K < 0 is below the main diagonal.
triu Extract upper triangular part.
triu(X) is the upper triangular part of X.
triu(X,K) is the elements on and above the K-th diagonal of
X. K = 0 is the main diagonal, K > 0 is above the main
diagonal and K < 0 is below the main diagonal.
When typing a simple command:
tril(4)
it gives a single value 4
Why the version of Matlab does not support the offiicially claimed functionalities (both introduced since 2006a) as described at https://www.mathworks.com/help/matlab/ref/triu.html and https://www.mathworks.com/help/matlab/ref/tril.html
James Tursa on 12 Apr 2020
You are going to have to provide examples of why you think they don't work rather that just making a blanket statement.

Image Analyst on 13 Apr 2020
4 is a scalar (or a matrix of 1 row and 1 column) - essentially there is no upper or lower triangular part. There is only one part - one element: 4. So it returns 4, as expected.
So now let's look at an actual matrix (larger than a 1-by-1):
m = magic(7)
t = tril(m)
What does this return for you? For me it returns:
m =
30 39 48 1 10 19 28
38 47 7 9 18 27 29
46 6 8 17 26 35 37
5 14 16 25 34 36 45
13 15 24 33 42 44 4
21 23 32 41 43 3 12
22 31 40 49 2 11 20
t =
30 0 0 0 0 0 0
38 47 0 0 0 0 0
46 6 8 0 0 0 0
5 14 16 25 0 0 0
13 15 24 33 42 0 0
21 23 32 41 43 3 0
22 31 40 49 2 11 20
again, exactly as expected. So now, what does it return for you (evidently it's wrong you say so you won't get what I got) in R2019a, and in the earlier version you used? Sometimes new functions change behavior you know. What was the earlier version you used?