Matlab Beginner Needs Help!

Discussion in 'Programmer's Corner' started by ruairdh, Nov 13, 2013.

  1. ruairdh

    Thread Starter New Member

    Nov 13, 2013
    Hi, I'm new to this site and am trying to teach myself matlab. I came across this problem and can't seem to figure it out;

    Write a MATLAB script to derive and plot y(n) below;

    x(n) =[1 1 2 1 3] y(n)=x(n)+y(n-1)

    I have written the following code;

    N = 5;
    x = [1 2 2 1 4];
    n = 0:length(y)-1;

    ...but MATLAB is giving me the following error message;

    Subscript indices must either be real positive integers or logicals.

    Error in test (line 4)


    Any help would be greatly appreciated.

  2. MrChips


    Oct 2, 2009
    Subscript n must begin with n = 1.

    You have another problem because y(n-1) for n = 1 has invalid subscript.
  3. blah2222

    Distinguished Member

    May 3, 2010

    You will need to start 'n' at 1 and have an initial condition for y[n-1] as MATLAB does not like indices of arrays that are less than 1.
  4. WBahn


    Mar 31, 2012
    It's been a couple years since I've touched MatLab, so I'm not even going to try to get the syntax correct. So focus on the idea.

    You have

    N = 5;
    x = [1 2 2 1 4];

    This means that

    x[1:5] is [1 2 2 1 4] (remember, I'm not trying to use MatLab syntax)

    Now, what if we concatenate this with a matrix that has just a single element. Let's assume that I have a function called 'zeroes' that takes two arguments, the number of rows and columns, and returns a matrix that size filled with zeros. Let's further assume that we can contatentate two matrics by using a cat() function.


    x_n_minus_1 = cat(zeroes(1,1), x) would yield [0 1 2 2 1 4];


    x_n = cat(x, zeroes(1,1)) would yield [1 2 2 1 4 0];

    Now I have

    y = x_n + x_n_minus_1

    which will have a length of N+1

    I can put this all on one line

    x_n = cat(x, zeroes(1,1)) + cat(zeroes(1,1), x);

    I know you can do each of these things in MatLab and that, in doing so, you get somethign that generally runs much faster than using a loop. This is called "vectorizing" the problem and anytime you can vectorize a problem and eliminate a loop, the dividend is usually pretty impressive.