How can a single matrix element be accessed in Matlab / Octave?

Question

Suppose I have:

>> X = magic(5)
X =

   17   24    1    8   15
   23    5    7   14   16
    4    6   13   20   22
   10   12   19   21    3
   11   18   25    2    9

How do I get i'th element from the second column?

I already figured that indices in (some?) collections in Octave are one-based, but I'm not sure if that holds for matrices, too.

Solution

See the index expressions section of the manual. To get the i'th element from second column:

X(i,2)      # element 'i' from column 2
X(1:end,2)  # the whole 2nd column
X(:,2)      # same thing but shorter
x(:, [2 3]) # whole 2nd and 3rd column

Note that Octave is a language where array elements are in column-major order.

Comments

[carandraug]

@wvxvw Whitespace is irrelevant. Think of the indexing as passing arguments to a function, each argument being a matrix. [2 3] is a vector, 1:5 is also a vector (range), 1:end is also just a range it simply interprets end as the last one. You can do 1:2:end for every two, just as you can with simple ranges.

[carandraug]

@wvxvw give it a try. You already have Octave installed. Does it return the same result?

[carandraug]

@wvxvw It is intended. The : is only used for ranges and selecting whole rows and columns in Octave, it's all the same. About the nickname, I automatically get a notification when commenting on my answer so no need.