Problem with for loop

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YH
YH 2018-11-23
评论： YH 2018-11-26
Hallo,
I have a large matrix let's say A 150 * 220000 , including columns that are set entirely to zero .
I want to find '' column wise'' the first element that is larger than 5 and the first element smaller than 200 and store them as two vectors
I creat a for loop, but it breaks when it reaches the first zero column in the matrix. so instead of idx_start 1 * 220000 , I get idx_start 1 * 15000
how can i modify my code so the loop continue over the zero columns?
and would be better if I replaced the zero columns with NaN?
I tried something with isempty but it does not work like I want.
wenn the condition is not met, it is enough to be replaced with NaN
[nx,ny] = size(A) ;
for j = 1:ny
idx_start(:,j) = find(A(:,j)> 5 ,1,'first') ;
if (isempty(A(:,j)))
continue
idx_end(:,j) = find (A(:,j) < 200 , 1, 'first');
end
end
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madhan ravi
madhan ravi 2018-11-23
Give a short example of your matrix and your desired output
Jan
Jan 2018-11-23
isempty(A(:,j)) is always false, because it checks the number of elements, not the contents of the elements. See any().

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采纳的回答

Andrei Bobrov
Andrei Bobrov 2018-11-23
编辑：Andrei Bobrov 2018-11-23
s = size(A,2);
[ii,jj] = find(cumsum(cat(3,A > 5,A < 200)) == 1);
out = accumarray([rem(jj-1,s)+1,ceil(jj/s)],ii,[s,2],[],nan);
  1 个评论
YH
YH 2018-11-26
thanks! it is a great code, short, fast and works as I wanted

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更多回答（2 个）

Dennis
Dennis 2018-11-23
The problem is that find might return an empty vector and in that case the assignment fails.
You could catch this error by checking if there are any values > 5.
if any(A(:,j)>5)
idx_start(:,j) = find(A(:,j)> 5 ,1,'first') ;
else
idx_start(:,j)=0 %or NaN or whatever you want to happen
end
I would prefer the use of only one array to store start and end values. Especially since you are always only storing 1 value you could do it like this:
if any(A(:,j)>5) && any(A(:,j)<200)
idx(1,j) = find(A(:,j)> 5 ,1,'first');
idx(2,j) = find (A(:,j) < 200 , 1, 'first');
else
idx(1,j)=0; %or NaN or whatever you want to happen
idx(2,j)=0; %or NaN or whatever you want to happen
end
Jan
Jan 2018-11-23
编辑：Jan 2018-11-23
In for j=1:nx you run a loop over the rows, not columns. Do you mean ny here?
[nx, ny] = size(A);
idx_start = NaN(1, nx); % Pre-allocate!!!
idx_end = NaN(1, nx); % Pre-allocate!!!
for j = 1:ny % Or really nx?
m = find(A(:, j) > 5, 1, 'first');
if ~isempty(m)
idx_start(j) = m;
end
m = find(A(:, j) < 200, 1, 'first');
if ~isempty(m)
idx_end(j) = m;
end
end
Now all elements of the idx_... vectors are NaN, if the corresponding column of A does not contain matching elements. You can skip the search also, if the column contains zeros only:
for j = 1:ny % Or really nx?
col = A(:, j);
if any(col) % Skip if column contains zeros only
m = find(col > 5, 1, 'first');
if ~isempty(m)
idx_start(j) = m;
end
m = find(col < 200, 1, 'first');
if ~isempty(m)
idx_end(j) = m;
end
end
end

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