trouble using "mean" function in MATLAB?

consider A=[1 2 3] if i use mean(A(1):A(3)) it gives 2 which is the mean of first element and third element of A.

if A=[3 2 1],and if i use mean(A(1):A(3)) then it says NaN. Why should this occur?shouldnt the command just give the mean of the first and third digit in the array?Any help will be appreciated...

Answer 1

[Azzi Abdelmalek]

Maybe you want

   mean(A(1:3))

Answer 2

[Matt Fig]

You are creating a vector with the elements of A, rather than indexing into A with a vector. Look at what happens:

A = [1 2 3];
B = A(1):A(3)  % Same as:
B2 = 1:3       % Read:  make a vector from 1 to 3 in steps of 1
isequal(B,B2)

A = [3 2 1];
B = A(1):A(3)  % Same as:
B2 = 3:1       % Read: make a vector from 3 to 1 in steps of 1
isequal(B,B2)

Probably what you want to do is index into A with a vector:

A(1:3) % Read: take elements 1 through 3 of A.