Asked by Brooke
on 27 Dec 2011

I have a for loop that gives the values of x(n) -- here n is an index.

d=0.5; x(1)=1; for n=2:10 x(n)= x(n-1)+d; y(n)= log(abs(x(n))); fprintf('y(%d)=%d', n, y(n)) end

I want the loop to also give the values of y(n) = log(abs(x(n))).

However, MATLAB doesn't seem to like this definition and I am presented with the error message: ??? Subscript indices must either be real positive integers or logicals.

What should I do to fix it?

Thanks.

Answer by bym
on 27 Dec 2011

Accepted answer

matlab is ones based, not zeros... x(0) is an error

**[edit]**

works fine for me (added \n in fprintf statement)

d=0.5;

x(1)=1;

for n=2:10 x(n)= x(n-1)+d; y(n)= log(abs(x(n))); fprintf('y(%d)=%d\n', n, y(n)) end y(2)=4.054651e-001 y(3)=6.931472e-001 y(4)=9.162907e-001 y(5)=1.098612e+000 y(6)=1.252763e+000 y(7)=1.386294e+000 y(8)=1.504077e+000 y(9)=1.609438e+000 y(10)=1.704748e+000

Answer by Jan Simon
on 27 Dec 2011

Either `n` is not an integer greater than 0 or the symbols `log` or `abs` have been defined as variables, such that the built-in functions are shadowed. You can test this by using:

dbstop if error

and start the program. Then Matlab stops when the error occurs and you can check the symbols by:

which abs which log

The command `whos` can be helpful also.

Show 3 older comments

Walter Roberson
on 27 Dec 2011

Does that mean that if you comment out the assignment to y(n) that your code will start? If that is the case then you would have gotten a very different error than the one you describe: it takes serious syntax errors to keep the code from running at all.

Please make things faster for yourself by posting the relevant code so that we are not left guessing about what is going on.

Answer by Sean de Wolski
on 27 Dec 2011

x(n)= x(n-1)+d;

when n = 1, the first iteration of the loop, you're trying to reference x(1-1) a.k.a. x(0) which is undefined and the reason you're seeing the above error.

MATLAB and Simulink resources for Arduino, LEGO, and Raspberry Pi test

Learn moreOpportunities for recent engineering grads.

Apply Today
## 2 Comments

## Jan Simon (view profile)

Direct link to this comment:http://www.mathworks.com/matlabcentral/answers/24811#comment_54822

Please post the relevant part of the code instead of describing it.

## Brooke (view profile)

Direct link to this comment:http://www.mathworks.com/matlabcentral/answers/24811#comment_54838

@Jan: I have posted the code.