Subscript indices must either be real positive integers or logicals - quick question on how to fix

Greetings all,

I have the following code:

Response_values_H= 0:0.01:9;
R_h=zeros(size(Response_values_H));

for n=1:length(Response_values_H)
  rValue_H = Response_values_H(n);
    if rValue_H < 9.1
        R_h(n)=sqrt(4*rValue_H.^2+1)-2*rValue_H;
    else
        R_h(n)=0;
    end
end

I have a quick question. When I enter an integer, i.e. R_h(25), it returns output. But when I try and enter a decimal, say R_h(2.5), I get: Subscript indices must either be real positive integers or logicals.

So, I tried using round(), but I want to input a decimal. This may be a simple solution, but how can I input a decimal without getting an error? Is there any way to fix it knowing the input must be a decimal?

Thanks! -J

Answer 1

[Chad Greene]

As I understand your question, you run the code above, then you want to find out values of R_h for some given x value. When you type R_h(25), it'll give you the 25th value of R_h. If you want to find some value of R_h corresponding to an arbitrary value of Response_values, you'll need to interpolate:

Response_values_H= 0:0.01:9;
R_h=zeros(size(Response_values_H));

for n=1:length(Response_values_H)
  rValue_H = Response_values_H(n);
    if rValue_H < 9.1
        R_h(n)=sqrt(4*rValue_H.^2+1)-2*rValue_H;
    else
        R_h(n)=0;
    end
end

plot(Response_values_H,R_h); 
xlabel('Response values') 
ylabel('R_h')

R_h_i = interp1(Response_values_H,R_h,2.5); 

hold on
plot(2.5,R_h_i,'ro','markersize',10)

[Jesse]

Thanks Chad! I knew I was missing something!

-J

[Chad Greene]

By the way, this is a faster and cleaner method of calculating R_h:

Response_values_H= 0:0.01:9;

R_h = sqrt(4*Response_values_H.^2+1)-2*Response_values_H;
R_h(Response_values_H>=9.1) = 0;

No need for preallocating, looping, or if statements.