Why do I keep getting "Array indices must be positive integers logical value"

a=0.00810

 g=9.81

w=0:0.5:2

 n=length(w)

 for i=1:n

S(i)=((a*(g.^2)/(w(i)).^5))*exp(-0.74*(w(i)*v/g).^(-4

))

[Star Strider]

That runs for me without error in R2020b, (after defining ‘v’ as a scalar, and adding the missing end).

[Jon]

I can't reproduce your error from what code you have included here. For example you don't define your variable v, and there is no end statement for the for loop so it won't even run as is. Please provide code that reproduces the error. Note that you can format the code nicely using the code button in the MATLAB answers toolbar

[Aarushi Ajit]

a=0.00810
g=9.81
v=10
w=0:0.5:2
n=length(w)
for i=1:n
S(i)=((a*(g.^2)/(w(i)).^5))*exp(-0.74*(w(i)*v/g).^(-4))
figure;
plot(w(i),S(i));

This is the entire code that i actually needed help on. I have to plot a graph for w v/s S. Please help. This is code is still not giving me any figure or graph.

[Walter Roberson]

a=0.00810
g=9.81
v=10
w=0:0.5:2
n=length(w)
for i=1:n
  S(i)=((a*(g.^2)/(w(i)).^5))*exp(-0.74*(w(i)*v/g).^(-4));
end
figure;
plot(w,S);

Answer 1

[Ameer Hamza]

What is value of 'v' in your case? If it is scalar, the code works fine. Run the following

a=0.00810;
g=9.81;
w=0:0.5:2;
n=length(w);
v=1;
S = zeros(1, n); % pre-allocation is a good coding practice
for i=1:n
    S(i)=((a*(g.^2)/(w(i)).^5))*exp(-0.74*(w(i)*v/g).^(-4));
end

Answer 2

[Jon]

In general that error comes from trying to index an array with something other than a positive integer or a logical variable. So for example if I have an array x = [1,8,19.3 2] and a variable b = 43.3 and I try to index x using the non-integer value b as in x(b) I will get the error you describe.

I can't reproduce your error from what code you have included here. For example you don't define your variable v, and there is no end statement for the for loop so it won't even run as is. Even if I assign a value to v, and provide the end statement I still don't get the error you indicate. Please provide code that reproduces the error. Note that you can format the code nicely using the code button in the MATLAB answers toolbar.