Trouble with For loop within If statement.

Question

Mark 2011-11-8

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I'm trying to solve a problem for multiple iterations until the error (err) for some the interior points (ii=2:imax-1) (imax is the length of the vector) in the t vector is below a certain value (errmax).

llmax is an arbitrary large number to allow as many iterations as necessary. I want first for loop to stop when the error at the interior points in the t vector are at or below errmax. I'm trying to accomplish this using an if and break within the first for loop. Within the if statement is another for loop checking the error of the interior points. I'm having trouble getting this to work. The break statement does not seem to associate with the original for loop. Or perhaps something else is wrong.

for ll=1:llmax
    told=t;
    t=A/d;
    err=abs((t(ii)-told(ii))/t(ii))*100;
    if for ii=2:imax-1
            err<=errmax
        end
    break
    end
end

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Answer 1

Drew Weymouth 2011-11-8

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if for is not valid syntax. If you want to break out of the loop when the errors for all points are <= errmax, then the correct code is:

for ll=1:llmax
    told=t;
    t=A/d;
    err=abs((t(ii)-told(ii))/t(ii))*100;
    if max(err) <= errmax
        break
    end
end

If you want to break when just one point has an error <= errmax, then just replace the max with a min.

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Mark 2011-11-8

I only want to break out of the loop when the interior of t (ii=2:imax-1) is <=errmax. The first and last points of the t are unchanging (thus having an error of 0).

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Answer 2

Walter Roberson 2011-11-8

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for ll=1:llmax
  wantbreak = false;
  told=t;
  t=A/d;
  for ii=2:imax-1
    err=abs((t(ii)-told(ii))/t(ii))*100;
    wantbreak = wantbreak || err<=errmax;
    if wantbreak; break; end
  end
  if wantbreak; break; end
end

Notice that "wantbreak" does double duty here: it selects breaking out of the inner "for" loop, and it selects breaking out of the "for" loop that is around that. There is no problem with establishing a variable that tells the outer loop that it should break.