如何在MATLAB中的矩阵中找到列的列？

Question

Essentially I have an image mask and I want to find the width of the image in each column. Is there a way to vectorise this for speed? I tried to figure out a way with arrayfun but haven't hit on anything yet.

r = zeros(1,cols);
for i = 1 : cols
    r(i) = range(find(img(:,i)));
end

Answer 1

The following code does the same as yours in a vectorized manner:

imglog = img~=0; %// convert to 0 and 1 values
[~, i1] = max(imglog); %// i1 is the position of the first 1
[~, i2] = max(flipud(imglog)); %// size(img,1)+1-i2 is the position of the last 1
r = size(img,1)+1-i2 - i1;

It exploits the fact that the second output of max gives the position of the first maximizer (for each column).

Answer 2

我不确定这是否更快，但num2cell + cellfun似乎是对列上的常规函数​​进行矢量化的唯一方法：

r = cellfun(@(x)range(find(x)),num2cell(img,1));

Answer 3

find + unique approach -

[row1,col1]  = find(img);
[~,start1] = unique(col1,'first');
[~,stop1] = unique(col1);
r = row1(stop1) - row1(start1);