How do I update a variable in a nested loop?

Question

Timothy Chau 2020-12-16

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回答： Walter Roberson 2020-12-16

When I'm running this code, the variable "iter" updates in the outer while loop but it's not updating in the inner while loop. Iter in the outer while loop returns 1, 2, 3, 4, 5 after 5 iterations but the iter in the inner while loop returns 1, 1, 1, 1, 1 after 5 iterations. Is there a way to update iter in the inner while loop?

Here's while loops part of my code:

iter = 1; % Initial iteration
while iter<6 % Outer while loop
    while n*dTm<W
        n = n+1;
        if n==16
            break
        end
        while d>.05*dTm % Inner while loop
            phi = atand((vi+vc)/(w*r*.2*iter));
            alpha = theta - phi;
            Cl = CL(find(ALPHA_CL > alpha, 1));
            dTa = .5*rho*(w*r*.2*iter)^2*c*dr*Cl;
            dTm = 4*pi*r*.2*iter*dr*rho*vi*(vi+vc);
            d = abs(dTm-dTa);
            vi = vi+1;
            if vi==1000
                break
            end
        end
        vi;
        dTm;
    end
    x(iter, 1) = iter;
    x(iter, 2) = vi;
    x(iter, 3) = dTa;
    x(iter, 4) = dTm;
    iter = iter+1;
end

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Answer 1

Jon 2020-12-16

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If you want iter to increment inside of the inner loop you have to move the statement

iter = iter+1;

inside of the inner loop.

However, although I don't know the details of what you are doing here, it seems like you may want iter to stay constant while the inner loop iterates. So maybe your code is already doing what you want.

Also, if you know you want a loop to execute a known number of times, 6, in your case, you should use a for loop instead of a while loop, e.g. for k = 1:6 .... end

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Timothy Chau 2020-12-16

I don't want to put iter inside the inner while loop because I don't want it to increment everytime the inner while loop is done. I want the 2 inner while loops to finish before iter is incremented, hence why it's in my outer while loop, then after iter is incremented, I want the 2 inner while loops to be executed again but with the new value of iter. The problem is, iter is incrementing in the outer while loop, returning 1, 2, 3, 4, 5 after 5 iterations, but not updating in the inner while loop, returning 1, 1, 1, 1, 1 after 5 iterations.

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Answer 2

Walter Roberson 2020-12-16

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在 MATLAB Online 中打开

    while n*dTm<W
        n = n+1;
        if n==16
            break
        end

After the end of the inner loop, you are not changing n, dTm, or W, so if you left the inner loop because n*dTm<W was false, it will still be false.

If you left the inner loop because n == 16 was true, then n will be incremented to 17 because the n=n+1 is before the test, so you will not break because of that. You will, though, then execute the inner loop with n = 17, leaving it when n*dTm<W becomes false, and then the outer iteration after that, you are back to the problem I described in the first paragraph.