submatrix extraction based on a specific criterion

Question

Win co 2014-5-11

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/129209-submatrix-extraction-based-on-a-specific-criterion

评论： Image Analyst 2014-5-12

采纳的回答： Image Analyst

在 MATLAB Online 中打开

Hi everybody, I would like to extract a list of submatrix from an inital matrix, like so:

A=

The 1st column is time in second. I would like to obtain all blocs in which the difference of time between the 1st one and the last one must not be greater than 3s. In this case, we will have:

Block 1:

2 3
1 2
9 1

Block 2:

2 4
1 0
4 8

Block 3:

      9 4 1
     11 2 4

Could you give me some idea to solve this problem?

Regards,

Winn

2 个评论
显示无隐藏无

Image Analyst 2014-5-11

在 MATLAB Online 中打开

What if it doesn't happen, like if

A=[...
2 3
1 2
2 4
1 0
4 8 
4 1
2 4];

Would the first block be

2 3
1 2
2 4

or

    1 2 3
    2 1 2

Win co 2014-5-12

编辑：Win co 2014-5-12

thanks for your very good remark. In fact, time difference must not be above 3s. So in your case, the right one is the second solution. I've edited my post following your remark.

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Image Analyst 2014-5-12

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/129209-submatrix-extraction-based-on-a-specific-criterion#answer_136454

编辑：Image Analyst 2014-5-12

在 MATLAB Online 中打开

How about the brute force approach of a for loop?

clc;
clear 'blocks';
A=[...
    1 2 3
    2 1 2
    4 9 1
    5 2 4
    7 1 0
    8 4 8 
    9 4 1
   11 2 4]
blockStartingRow = 1;
counter = 1;
lastRow = size(A, 1);
for row = 2 : lastRow
  if A(row, 1) > A(blockStartingRow, 1) + 3
    % Start a block
    blocks{counter} = {A(blockStartingRow:(row-1),:)}
    counter = counter + 1;
    blockStartingRow = row; % Move start of block pointer.
  end
end
% Pick up last block if we need to
if blockStartingRow < lastRow
  blocks{counter} = {A(blockStartingRow:end,:)}
end
    
celldisp(blocks)

In the command window:

blocks{1}{1} =
   2     3
   1     2
   9     1
blocks{2}{1} =
   2     4
   1     0
   4     8
blocks{3}{1} =
   4     1
   2     4

If you don't know what a cell array is, read the FAQ.

3 个评论
显示 1更早的评论隐藏 1更早的评论

Win co 2014-5-12

It works perfectly your code. Thank you very much for your help.

Image Analyst 2014-5-12

Don't worry about loops. I did one billion , yes billion , loops in just over 2 seconds on my work computer, so a measly million iterations is just a fraction of a second. Even on my slow old home computer I'm clocking a million iterations at 3 milliseconds. If 3 milliseconds is too slow for you, then you'd better lay off the caffeine.

请先登录，再进行评论。