Delete certain rows of a matrix based on specific values

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!

Answer 1

[Niels]

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

[Aude Rapet]

Thank you very much Niels, it works!

[Aude Rapet]

@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]

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]

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.