MATLAB Data from loop not saved

Hi everybody, im actually making a little program wich must give me a graph.

Tens_stored = zeros(100,1);
Tens_S = 12;
Res_I = 10;
Res_E = logspace(0,2,100);
for B = logspace(0,2,100);
Tens = (B.*Tens_S)/(B+Res_I);
disp(Tens)
Tens_stored(B) = Tens;
semilogx(Res_E,Tens_stored)
end

But when i launch it, it make an error : Attempted to access Tens_stored(1.04762); index must be a positive integer or logical.

Error in TEST1 (line 8) Tens_stored(B) = Tens; And no one of the values calculated are in the array Tens_Stored. Can somebody help me plz :) Thx

Answer 1

[Simon]

Hi!

You can not use doubles as indices. Do instead:

Res_E = logspace(0,2,100);
for B = 1:length(Res_E);
Tens = (Res_E(B).*Tens_S)/(Res_E(B)+Res_I);
disp(Tens)
Tens_stored(B) = Tens;
end
semilogx(Res_E,Tens_stored)

Plot after the loop! (Is it right that B and Res_E are the same?)

[Patrick Allaglo]

Hi thx for your answer, i just understood the mistake i made... Thx again

Answer 2

[Wayne King]

You can't have a for loop where the index is non-integers, like this

 B = logspace(0,2,100);

because the 2nd element of B is already a non-integer value and you can't access a non-integer element of an array. Think about it. Does it make sense to ask what is the 2.5-th element of a finite-dimensional vector?

Why would you want to space filled elements of a vector logarithmically. even if you could do it with integer values? You would end up with zeros in your vector. For example, if you did

 y(1) = 2;
 y(3)  = 1;

Note that MATLAB makes y a length-3 vector and puts a zero in for the 2nd element.

I'm not sure what you're trying to do here, but maybe something like:

   Tens_stored = zeros(100,1);
   Tens_S = 12;
   Res_I = 10;
   Res_E = logspace(0,2,100);
    for nn = 1:100 
      Tens = (Res_E.*Tens_S)/(B+Res_I);
      Tens_stored(nn) = Tens;
    end

Although the above just gives the same value for Tens_stored at each element of the array, so I'm sure you probably don't want that.

[Patrick Allaglo]

Hi , thx for answering im just beginning Matlab so im a little bit lost. I understand what you tried to say. Im trying to make a program which calculate the real voltage of an electric circuit with 100 of charge values evolving logarithmically between 1 and 100 ohms. :) thx