Matlab tril function does not work as expected

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]

You are going to have to provide examples of why you think they don't work rather that just making a blanket statement.

Answer 1

[Image Analyst]

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?