How to find the index of first and last nonzero elements in each column?

Question

Rabia Zulfiqar 2020-5-22

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/530418-how-to-find-the-index-of-first-and-last-nonzero-elements-in-each-column

评论： Stephen23 2022-3-25

采纳的回答： Image Analyst

Hi,Is there any way to find the index of first and last nonzero element in a matrix?

I have this matrix

A=[0 0 0 0

0 0 0 0

1 0 4 8

2 0 5 9

3 1 6 7

0 2 7 0

0 0 0 0]

as the first non zero element in 1st column is 3 and and the last is 5. In the second column the the 12th element is the first non zero and the last one is 13th etc

I want to store these values in a matrix in which each row represents the index of first non zero element and the 2nd row shows the index of last non zero elements.

the answer matrix should be like this:

B=[3 12 17 24

5 13 21 26]

How can I do this??

Thankyou for time and consideration.I really appreaciate your help.Kindly guide me

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Image Analyst 2020-5-22

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/530418-how-to-find-the-index-of-first-and-last-nonzero-elements-in-each-column#answer_436323

在 MATLAB Online 中打开

You could probably use

A = [0    0     0      0
	0     0     0      0
	1     0     4      8
	2     0     5      9
	3     1     6      7
	0     2     7      0
	0     0     0      0]
[rows, columns] = size(A)
B = zeros(2, columns)
for col = 1 : size(A, 2)
	B(1, col) = find(A(:, col), 1, 'first');
	B(2, col) = find(A(:, col), 1, 'last');
end

This gives

B =

3 5 3 3

5 6 6 5

which makes sense to me but I'm puzzled as to how you get

B=[3 12 17 24

5 13 21 26]

for your example. Can you explain?

4 个评论
显示 2更早的评论隐藏 2更早的评论

Rabia Zulfiqar 2020-5-22

编辑：Rabia Zulfiqar 2020-5-22

Like here I have a matrix of 7x4 but if I have a matrix of 7x4x20 then how can I do this?

Image Analyst 2020-5-23

在 MATLAB Online 中打开

Oh, you wanted the "linear index" rather than the row index within each column (as I had assumed). If you'd said "linear index" (the proper MATLAB lingo) I would have gotten it as :

A = [0    0     0      0
     0     0     0      0
	1     0     4      8
	2     0     5      9
	3     1     6      7
	0     2     7      0
	0     0     0      0]
[rows, columns] = size(A)
B = zeros(2, columns)
for col = 1 : size(A, 2)
	B(1, col) = find(A(:, col), 1, 'first') + rows * (col - 1);
	B(2, col) = find(A(:, col), 1, 'last') + rows * (col - 1);
end
B

but it looks like Stephen figured out, and it even works for higher dimension arrays, so just use his answer.

请先登录，再进行评论。

Answer 2

Stephen23 2020-5-22

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/530418-how-to-find-the-index-of-first-and-last-nonzero-elements-in-each-column#answer_436433

编辑：Stephen23 2020-5-22

在 MATLAB Online 中打开

This works for any array, 2D, 3D, etc., and returns the requested linear indices:

>> A = [0,0,0,0;0,0,0,0;1,0,4,8;2,0,5,9;3,1,6,7;0,2,7,0;0,0,0,0]
A =
   0   0   0   0
   0   0   0   0
   1   0   4   8
   2   0   5   9
   3   1   6   7
   0   2   7   0
   0   0   0   0
>> S = size(A);
>> S(1) = 2;
>> X = A~=0;
>> X = X & (cumsum(X,1)==1 | flipud(cumsum(flipud(X),1))==1);
>> B = reshape(find(X),S)
B =
    3   12   17   24
    5   13   20   26

5 个评论
显示 3更早的评论隐藏 3更早的评论

Stephen23 2020-5-22

在 MATLAB Online 中打开

Lets look at a simple example using one vector (to save space I will use a row vector, but with your data this applies to each column). How can we identify the first non-zero? Start by identifying all non-zeros:

>> V = [0,0,1,2,3,0,0]
V =
  0  0  1  2  3  0  0
>> X = V~=0 % all nonzeros.
X =
  0  0  1  1  1  0  0

Note that only the first non-zero has a cummulative sum of 1:

>> cumsum(X)
ans =
  0  0  1  2  3  3  3
>> X & cumsum(X)==1
ans =
  0  0  1  0  0  0  0

We must include the X & ... because if there is only one non-zero in that vector then all of the trailing zeros will also have a cummulative sum = 1, but we can simply exclude them because they are zero.

To get the last non-zero we do exactly the same, but first flip the columns upside down... and then afterwards flip the result back the right way. That gives the last non-zero in each column.

That gives a simple logical array where each true element is either the first or last non-zero in each column. You can use find on that logical array to trivially return the linear indices of those elements.

Nicole Wan 2022-3-25

What if there are some columns before and after this data set that are columns of zero but the positions in the matrix need to be kept.

Stephen23 2022-3-25

在 MATLAB Online 中打开

A = [0,0,0,0;0,0,0,0;0,0,4,8;0,0,5,9;0,1,6,7;0,2,7,0;0,0,0,0]
A = 7×4
     0     0     0     0
     0     0     0     0
     0     0     4     8
     0     0     5     9
     0     1     6     7
     0     2     7     0
     0     0     0     0
X = A~=0;
X = X & (cumsum(X,1)==1 | flipud(cumsum(flipud(X),1))==1)
X = 7×4 logical array
   0   0   0   0
   0   0   0   0
   0   0   1   1
   0   0   0   0
   0   1   0   1
   0   1   1   0
   0   0   0   0
B = reshape(find(X),2,[])
B = 2×3
    12    17    24
    13    20    26

请先登录，再进行评论。

Answer 3

Matt J 2022-3-25

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/530418-how-to-find-the-index-of-first-and-last-nonzero-elements-in-each-column#answer_926834

编辑：Matt J 2022-3-25

在 MATLAB Online 中打开

A=[0    0     0      0
     0     0     0      0
     1     0     4      0
     2     0     5      0
     3     1      6     0
     0      2      7     0
     0      0      0      0];
a=logical(A);
[~,first]=max(a,[],1,'linear');
[~,last]=min( cummax(a,1,'rev'), [],1,'linear');
B=[first;last-1]
B = 2×4
     3    12    17    22
     5    13    20    21
B(:,~any(A,1))=nan
B = 2×4
     3    12    17   NaN
     5    13    20   NaN

2 个评论
显示无隐藏无

Bruno Luong 2022-3-25

I believe this method fails if a column contains only 0s.

Matt J 2022-3-25

It's easily tweaked to accomodate that case (I've now done so).

请先登录，再进行评论。

Answer 4

Bruno Luong 2022-3-25

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/530418-how-to-find-the-index-of-first-and-last-nonzero-elements-in-each-column#answer_926824

在 MATLAB Online 中打开

Let you do conversion to linear index

A=[0    0     0      0
    0     0     0      0
    1     0     4      8
    2     0     5      9
    3     1      6     7
    0      2      7     0
    0      0      0      0]
A = 7×4
     0     0     0     0
     0     0     0     0
     1     0     4     8
     2     0     5     9
     3     1     6     7
     0     2     7     0
     0     0     0     0
[r,c]=find(A); 
first=accumarray(c(:),r(:),[size(A,2) 1], @min, 0)
first = 4×1
     3
     5
     3
     3
last=accumarray(c(:),r(:),[size(A,2) 1], @max, 0)
last = 4×1
     5
     6
     6
     5