What does this means?

i was working on a class assignment and then i found this one operation i found it confussing.

G= [ 1  2 3; 4 5 6; 7 8 9];
H=  G([1 3], [1 2]);

what does the function H do to the G matrix?

i tried to change the numbers inside the brackets but it shuffles the elements of the matrix G and i dont understand how it works.

is there any documentation referring to this problem?

[Stephen23]

"what does the function H do to the G matrix?"

Nothing: there is no function H, and that indexing does not change G at all.

H is simply the name of a variable that the result of some indexing is assigned to, i.e. H is an array.

"is there any documentation referring to this problem?"

Of course, see the end of the section Indexing Matrices with Two Subscripts here:

https://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html

Indexing is a MATLAB superpower. Basic indexing concepts are introduced here:

https://www.mathworks.com/help/matlab/getting-started-with-matlab.html

Answer 1

[Star Strider]

It returns the elements in from rows 1 and 3 in columns 1 and 2 —

G= [ 1  2 3; 4 5 6; 7 8 9];
H=  G([1 3], [1 2])

H = 2×2

1 2 7 8

.