undefine function or variable

ihave a vector positionsa with 20 elements and a variable Alpha_posa and a want to take th ith elemet but this error will kill me

??? Undefined function or method 'Positionsa'
for input arguments of type 'double'.

Error in ==> Untitled at 69
           Alpha_posa=Positionsa(i);

Answer 1

[Brendan Hamm]

Judging by what you have written and the error given it seems that your issue is with the capitalization. MATLAB is case sensitive and therefore you would want:

Alpha_posa = positionsa(i);

[Brendan Hamm]

I would like to reiterate you original statement. You say that you have a 20-by-1 vector positionsa.

Your error states:

??? Undefined function or method ' Positionsa ' for input arguments of type 'double'.

Error in ==> Untitled at 69 Alpha_posa=Positionsa(i);

Given this error, it is clear the command you typed is:

Alpha_posa=Positionsa(i); % Error!

MATLAB is case sensitive, so you need to type:

Alpha_posa = positionsa(i); % This should work if I is also defined

So the following set of commands will work:

positionsa = rand(20,1); % Random vector with 20 elements
i = 5; % An variable for indexing
Alpha_posa = positionsa(i) % Extract the 5th element.

[marwa marwan]

you are so right...samll error can confuse alot thank you for your time