MATLAB Answers

Brooke

??? Subscript indices must either be real positive integers or logicals

Asked by Brooke
on 27 Dec 2011
Accepted Answer by bym

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.

  2 Comments

Jan Simon
on 27 Dec 2011

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

Brooke
on 27 Dec 2011

@Jan: I have posted the code.

3 Answers

bym
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

  4 Comments

Brooke
on 27 Dec 2011

@Walter and @proecsm:

I'm still getting

EDU>> testing
??? Subscript indices must either be real positive integers or
logicals.

Error in ==> testing at 7
y(n)=log(abs(x(n)));
 
7   y(n)=log(abs(x(n)));

With that code! Why o why? :((

bym
on 27 Dec 2011

do a clear; before running your script

Brooke
on 27 Dec 2011

@proecsm: Thanks, that worked!! :)


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.

  6 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.

Brooke
on 27 Dec 2011

@Walter: Thanks, I have posted the code.


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.

  1 Comment

Brooke
on 27 Dec 2011

@Sean: Sorry, that was a typo, I have corrected it to 2 but the problem still persists.


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