Taking the last value of a matrix or vector

Hi. How can I take a last term of a vector - since my vector dimensions change -, and plug it in somewhere else in my code without keep changing the code?

for example right now I do v(6) to get the 6th term. How can I do it for nth term.... Thanks

Answer 1

[Walter Roberson]

v(end)

or

v(length(v))

[Jan]

In older MATLAB version, v(length(v)) has been _significantly_ faster then v(end):

x = rand(1, 100);

MATLAB 6.5:

tic; for i=1:1e6; y=x(end); clear('y'); end; toc

>> 8.31 sec

tic; for i=1:1e6; y=x(length(x)); clear('y'); end; toc

>> 6.45 sec

MATLAB 2009a:

tic; for i=1:1e6; y=x(end); clear('y'); end; toc

>> 10.72 sec

tic; for i=1:1e6; y=x(length(x)); clear('y'); end; toc

>> 11.06 sec

[Walter Roberson]

Were those timings all on the same system? If so that would imply that MATLAB has gotten much slower over the years ?

[Marcus]

In MATLAB R2015a, end is faster than length():

>> x = rand(1, 100);

>> tic; for i=1:1e6; y=x(end); clear('y'); end; toc

Elapsed time is 5.475081 seconds.

>> tic; for i=1:1e6; y=x(length(x)); clear('y'); end; toc

Elapsed time is 5.705095 seconds.

[Sangbok Lee]

R2021a

I've tried it several times.

"end" is always slightly faster than "length()"

I think "end" works the same way using index numbers.

>> x=rand(10000);

>> tic; for i=1:1e7; y=x(end); clear y; end; toc

Elapsed time is 21.722168 seconds.

>> tic; for i=1:1e7; y=x(length(x),length(x)); clear y; end; toc

Elapsed time is 22.073104 seconds.

>> tic; for i=1:1e7; y=x(10000,10000); clear y; end; toc

Elapsed time is 21.818949 seconds.

Answer 2

[Korosh Agha Mohammad Ghasemi]

To access the last element of a vector in MATLAB regardless of its length, you can use the end keyword. This allows you to dynamically reference the last element without having to specify the exact index.

Here’s an example demonstrating how to use the end keyword:

% Example vector
v = [7, 8, 8, 2, 5, 6];

% Access the last element
last_element = v(end);

% Display the last element
disp('The last element is:');
disp(last_element);

% Using the last element somewhere else in your code
result = last_element + 10;  % Example operation
disp('Result of operation with the last element:');
disp(result);

In this example, v(end) retrieves the last element of the vector v regardless of its length. You can then use last_element elsewhere in your code.

If you need to perform operations on the last element in multiple parts of your code, you can encapsulate the logic in a function. Here’s an example:

% Function to get the last element of a vector
function last_element = getLastElement(vector)
    last_element = vector(end);
end

% Example usage
v = [7, 8, 8, 2, 5, 6];
last_element = getLastElement(v);

% Display the last element
disp('The last element is:');
disp(last_element);

% Using the last element somewhere else in your code
result = last_element + 10;  % Example operation
disp('Result of operation with the last element:');
disp(result);

By using the end keyword, you can write more flexible and maintainable code that adapts to changes in the size of your vectors.

[Voss]

"regardless of its length"

Not true. The length must be greater than zero to use end. Here's an example demonstrating the problem:

v = [];
v(end)

Array indices must be positive integers or logical values.