Index exceeds the number of array elements (8).

% Col 1
Q = Q = [12 16 20 25 28 32 36 42];
Asc = 3.3807e+04
i = 1;
while i <= length(Q)
    
    Qa = (3.14/4)*(Q(i).^2);
    Long_col1_bar_req= Asc/Qa;
    col1_bar_provide = round(Long_col1_bar_req)+1;
    Long_col1_Ast_provide = col1_bar_provide*Qa;
    Long_col1_spacing = (Qa/Long_col1_Ast_provide) *x1*1000;
    if Long_col1_spacing > 100
        if Long_col1_spacing > 300
            Long_col1_spacing=300;
            
        end
        spacing_b = Long_col1_spacing;
        break
        
    end
    i = i+1;
end
Long_col1_min_spacing = spacing_b ;
Long_col1_min_q = Q(i);
Long_col1_unit_wt = (Q(i).^2)/162;
Long_col1_bar_provide = col1_bar_provide;
Long_col1_Total_wt = Long_col1_unit_wt*col1_bar_provide;

Answer 1

[DGM]

Your loop condition is

while i <= length(Q)

and the last thing that happens in the loop is

i = i+1;

so the moment the loop exits, i = 9. Then you try to index into Q

Long_col1_min_q = Q(i);

I try to recommend that people avoid while loops unless necessary. If you know how many iterations a loop needs to perform, use a for loop instead.

[Parth Trivedi]

what type of change i should do ? as per your recommendation?

[DGM]

I'm not sure why you're using Q(i) in the lines after the loop, since Q is not changed within the loop. I imagine you intend to do this instead

Long_col1_min_q = Q;
Long_col1_unit_wt = (Q.^2)/162;

Which will return vectors, but I don't know what this script is actually calculating in concept. Maybe you're looking for a specific element of Q instead? The minimum value, maybe?

If the question is "how would I use a for-loop here", then something like this:

Q = [12 16 20 25 28 32 36 42];
Asc = 3.3807e+04
for i=1:numel(Q) % i is constrained to array boundaries
    Qa = (pi/4)*(Q(i).^2);
    Long_col1_bar_req= Asc/Qa;
    col1_bar_provide = round(Long_col1_bar_req)+1;
    Long_col1_Ast_provide = col1_bar_provide*Qa;
    Long_col1_spacing = (Qa/Long_col1_Ast_provide) *x1*1000;
    if Long_col1_spacing > 100
        if Long_col1_spacing > 300
            Long_col1_spacing=300;
        end
        spacing_b = Long_col1_spacing;
        break
    end
end
Long_col1_min_spacing = spacing_b ;
Long_col1_min_q = Q(i); % decide what you want to do with these two lines
Long_col1_unit_wt = (Q(i).^2)/162;
Long_col1_bar_provide = col1_bar_provide;
Long_col1_Total_wt = Long_col1_unit_wt*col1_bar_provide;

But bear in mind that I'm making no recommendations about the correctness of calculations that are being made.