Why am I getting this error? "Array indices must be positive integers or logical values".

I am getting the error "Array indices must be positive integers or logical values" when I run the code below. It says the error is in the line that I have bolded.

L_SEC0 = 239;

P0 = 1060;

stiff = 440;

L_CC0 = 122;

t_rise = 120;

t_fall = 60;

C0 = 6.25;

a = 400;

b = 150;

L_total = L_SEC0 + L_CC0;

duration = 0.2;

fs = 10,000;

t_step = 1/fs;

time = 0:t_step:duration;

points = duration*fs;

P_twitch_short = zeros(points,1);

P_twitch_long = zeros(points,1);

P_tetanus = zeros(points,1);

SA = zeros(points,1);

L_SEC = L_SEC0;

L_CC = L_CC0;

for i = 1:length(time)

RLS = (L_SEC-L_SEC0)/L_SEC0;

P_twitch_short(i) = P0(0.0258*(exp((stiff*RLS)))-0.0258);

Answer 1

[Voss]

On that line, everything inside the outermost parentheses on the right-hand side is an index into P0:

P_twitch_short(i) = P0(0.0258*(exp((stiff*RLS)))-0.0258);
%                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ index into P0

If that expression, 0.0258*(exp((stiff*RLS)))-0.0258, is not a positive integer, you get the error you got.

Maybe you meant to multiply that by P0 instead of indexing into P0?

P_twitch_short(i) = P0*(0.0258*(exp((stiff*RLS)))-0.0258);
%                     ^ multiplication

[Josh Riesenberg]

Thank you!

[Voss]

You're welcome!

Answer 2

[John D'Errico]

Is P0 a function? (NO. It is a scalar variable.)

You have this:

P_twitch_short(i) = P0(0.0258*(exp((stiff*RLS)))-0.0258);

Did you intend to multiply by P0? Does MATLAB allow implicit multiplication, where you don't need to use an operator?

What will happen then you write

P0(stuff)

instead of

P0*(stuff)

Any guesses? The point is, MATLAB thinks either you must be calling a function named P0 at that point, OR you are trying to index the variable P0. But since it knows that P0 is a variable, it then tries to use what is inside the parens as an index for that variable. And so when that fails, MATLAB gets upset, and returns an error.

[Josh Riesenberg]

Thank you!