Index in Position 2 is invalid.

X=[200:2000]
for i = 1:1801
   A = X(1,i)/2
   B(1,i)= A
end
 i=0
while i<1801
    if i==750
         break
     elseif mod(i,2)==1
       C(1,i)=B(1,i)*2
      elseif mod(i,2)==0
       C(1,i)=B(1,i)/2   %Index in Position 2 is invalid.
    end
         i=i+1
end

The programm works only without the second elseif. If I add the second elseif, I get the error: "Index in Position 2 is invalid." Can someone help?

Answer 1

[Ameer Hamza]

In MATLAB, the indexing starts from 1. Therefore you should initialize 'i' with the value of 1

X=[200:2000];
for i = 1:1801
    A = X(1,i)/2;
    B(1,i)= A;
end
i=1 % <==== initialize with 1
while i<1801
    if i==750
        break
    elseif mod(i,2)==1
        C(1,i)=B(1,i)*2;
    elseif mod(i,2)==0
        C(1,i)=B(1,i)/2   %Index in Position 2 is invalid.
    end
    i=i+1;
end

Also, note that I have suppressed the output from printing on the command window using semi-colon (;). Printing an array on command window in each iteration is very inefficient. Suppressing the output will result in a speed increase.

[captainjoker]

Thank you very much sir

[Ameer Hamza]

I am glad to be of help.

[Claudius Simon Appel]

Okay, that makes sense so far.

What I don't get is why this is an error that, according to Bhogal, only occurs while the second elseif

if i==750
        break
    elseif mod(i,2)==1
        C(1,i)=B(1,i)*2;
elseif mod(i,2)==0 %% this line here
        C(1,i)=B(1,i)/2   %Index in Position 2 is invalid.
end

exists. What is it about this line that prompts matlab to run into an error. Or, to be precise, why does it not run into the same error before?

I know that the original question has been answered, but could someone explain this to me?

Thank you.

[Ameer Hamza]

Can you show the code before this if-block? How is B defined?

Answer 2

[Francesco Torricelli]

Hi,

you can easily solve your issue by setting i = 1 just before the while loop (i = 0 leads to the error you encountered in the very first iteration).

[captainjoker]

Thanks for helping