Find number and location in matrix, make a new matrix with findings.

Question

Håvard 2013-12-7

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/109024-find-number-and-location-in-matrix-make-a-new-matrix-with-findings

编辑： Walter Roberson 2013-12-7

I am trying to make a function which is to find a certain number, or numbers above a threshold number, in a given matrix of unknown size. Preferably without any other function then if,else,while,for.

function search = searchMatrix(matrix)
search = [];
[r c] = size(matrix);
for i = 1:r
    for j = 1:c
        if (matrix(i,j) >= 2000)
              search = matrix(i,j) && i && j;
          end
      end
  end

But I would like the numbers, and the location of the numbers to be put into a new matrix "search" with each new row corresponding to these numbers and location.

E.g:

[ 2300 4 5 ; 4320 8 2 ; 2001 9 4]

Im not sure how to proceed for doing so. Any suggestions?

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

dpb 2013-12-7

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/109024-find-number-and-location-in-matrix-make-a-new-matrix-with-findings#answer_117652

在 MATLAB Online 中打开

Preallocate search for speed improvement in large cases, then populate the array. To cover the worst-case, it would have to be numel(matrix) in size...

srch=zeros(numel(m),1); % preallocate

Then

srch(i,:)=[m(i,j i j]; % each found element

If you keep an additional counter you can truncate unused rows using it to avoid the use of another builtin ( any, all, etc.) or another logical test.

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。

Answer 2

Håvard 2013-12-7

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/109024-find-number-and-location-in-matrix-make-a-new-matrix-with-findings#answer_117660

在 MATLAB Online 中打开

Thank you for your answer!

As of your explanation I tried:

 function search = searchMatrix(matrix)
 search = zeros(numel(matrix),3); 
[r, c] = size(tabell);
 for i = 1:r
    for j = 1:k
        if (tabell(i,j) >= 2000)
            search(i,:) = [tabell(i,j),i,j];
        end
    end
end
end

with input "searchMatrix([ 1990 2001 2002; 2134 1983 1500; 1459 2000 1400])" i received:

        2002           1           3
        2134           2           1
        2000           3           2
           0           0           0
           0           0           0
           0           0           0
           0           0           0
           0           0           0
           0           0           0
a lot of zeros, as you predicted, but it seemed to only be able to get the last number from the row. How can I fix this?

Also, I felt maybe my main concern were not answer, so I'm gonna try with another example. If I make a function which takes in two parameters, and figure out how many times it hit a certain number between the two given parameters, as in this olympics example:

 function years = summerOlympics(firstYear,lastYear)
 years = [];
 for i = firstYear:lastYear
    if mod(i,4) == 0;
        years(i) = i
    end
 end
 end

e.g firstYear = 1999, and lastYear=2012, should give out years = [ 2000 2004 2008 2012] If I here try to do as previously, I´ll get a row of over 2000 zeros. I just want for it to take the true if-condition, and put it neatly into 1,2,3 etc vector space of years.

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

dpb 2013-12-7

在 MATLAB Online 中打开

A) The handling of the values is owing to not incrementing another counter for the location...

n=0;
...
if m(i,j)>setpoint,
  n=n+1;
  s(n,:)=[m(i,j) i j];
  ...

B) As for truncation, as suggested earlier, you have to truncate to the used portion. Now having n, when done thru the loop

s(n+1:end,:)=[]; % clear unused rows

And, of course, while I presume these exercises are pedagogical self-teaching tools, "the Matlab way" would be to use logical addressing and eliminate the loops entirely.

请先登录，再进行评论。