MATLAB Answers

Delete certain rows of a matrix based on specific values

151 views (last 30 days)
Aude Rapet
Aude Rapet on 31 Jan 2017
Commented: Aude Rapet on 2 Feb 2017
Hi,
I am a beginner in Matlab. I have a matrix that has 2 columns and thousands of rows.
I need to delete the rows based on the following conditions:
1. if a value of column 1 is superior at 100 and inferior at 550 => delete this row
2. if a value of column 2 is superior at 100 and inferior at 420 => delete this row
Based on
http://au.mathworks.com/matlabcentral/answers/105768-how-can-i-delete-certain-rows-of-a-matrix-based-on-specific-column-values#answer_226615
I tried :
% Specify conditions
TF1 = (100 < res_select(:,1)) & (res_select(:,1) < 550) ;
TF2 = (100 < res_select(:,2)) & (res_select(:,2) < 420) ;
% combine them
TFall = TF1 & TF2 ;
% remove
res_final = res_select ;
res_final(TFall,:) = [] ;
But it's not working I don't understand why. Should I try to create a loop instead of logical indexing ? I tried this but not working neither :
% Specify conditions
for i = 1:length(res_select(:,1))
TF1 = (100 < res_select(i,1)) & (res_select(i,1) < 550) ;
end
for i = 1:length(res_select(:,2))
TF2 = (100 < res_select(i,2)) & (res_select(i,2) < 420) ;
end
% combine them
TFall = TF1 & TF2 ;
% remove
res_final = res_select ;
res_final(TFall,:) = [] ;
Thank you for your help!

  0 Comments

Sign in to comment.

Accepted Answer

Niels
Niels on 31 Jan 2017
Edited: Niels on 31 Jan 2017
Hi, i would call this a rather bitter solution for a beginner to understand, but it is short and it works, if you want to understand it you should first look what a<100 does when "a" is a matrix (you shouldnt use this for if statements) and then combind it with &
a=randi(900,15,2);
a(a(:,1)>100 & a(:,1)<550,:)=[]; % deletes rows dependent to first condition
a(a(:,2)>100 & a(:,2)<420,:)=[]; % ... second condition
a =
580 176
341 204
731 154
480 205
316 393
846 280
789 832
496 388
561 167
529 815
187 882
272 395
424 101
208 233
760 368
a =
580 176
731 154
846 280
789 832
561 167
760 368
a =
789 832

  4 Comments

Show 1 older comment
Aude Rapet
Aude Rapet on 1 Feb 2017
@Niels What I try to do is to select only the positions x y detected in the corners of my picture dimension 680*512 pixels. But if I do this, I delete everything:
res_final(res_final(:,1)>100 & res_final(:,2)>100,:) =[]; %corner left top
res_final(res_final(:,1)>100 & res_final(:,2)<420,:)=[]; %corner left bottom
res_final(res_final(:,1)<550 & res_final(:,2)>100,:)=[]; %corner right top
res_final(res_final(:,1)<550 & res_final(:,2)<420,:)=[]; %corner right bottom
Do you know how I could this, maybe with a "or" instead of "&" ?
Niels
Niels on 1 Feb 2017
example=randi(100,6,6)
% get a grid/matrix size(Ysize,Xsize) in each corner
% example=imread('replace.png')
[i,j]=size(example);
Xsize=2; % Gridrange #colums
Ysize=3 % # rows
UpRight=example(1:Ysize,j+1-Xsize:j)
UpLeft=example(1:Ysize,1:Xsize)
DownRight=example(i+1-Ysize:i,j+1-Xsize:j)
DownLeft=example(i+1-Ysize:i,1:Xsize)
maybe like this?
Aude Rapet
Aude Rapet on 2 Feb 2017
Thanks. What you told me before works really good to have the four corners :
res_final = res_select ;
res_final(res_final(:,1)>100 & res_final(:,1)<550,:)=[]; % deletes x between 100 and 550
res_final(res_final(:,2)>100 & res_final(:,2)<420,:)=[]; % delete y between 100 and 420
But I was looking for a way to be able to "select" the corner I wanted to keep. For example have bottom right and top left only. With this code I have four of them. I am not sure it is possible though.

Sign in to comment.

More Answers (0)

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by