omit rows of matrix based on a condition

Question

Pooneh Shah Malekpoor 2022-10-15

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评论： David Hill 2022-10-15

采纳的回答： David Hill

Hello

My matrix B is :

[0.25 0 0;

0.5 -4.91 -4.9;

0.75 -4.94 -4.96;

1 -4.985 -5;

1.25 -5 -5;

1.5 -5 -5;

1.75 -5 -5]

How to write a code to omit all the rows (except from first row and fifth row 1.25,-5,-5) where column 2 and three are equal. I mean keep the first and fifth row where column 2 and 3 are equal but omit other rows with the same situation.

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Answer 1

David Hill 2022-10-15

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编辑：David Hill 2022-10-15

在 MATLAB Online 中打开

Seems like there are more than just two rows.

B=[0.25	0	0;
0.5	-4.91  -4.9;
0.75	-4.94  -4.96;
1	-4.985	-5;
1.25	-5	-5;
1.5	-5	-5;
1.75	-5	-5];
[~,idx]=unique(B(:,2),'stable');
C=B(idx,:);
C(C(:,2)~=C(:,3),:)=[];
C
C = 2×3
    0.2500         0         0
    1.2500   -5.0000   -5.0000

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Pooneh Shah Malekpoor 2022-10-15

Thanks, Then how to get [0.25 0 0;

0.5 -4.91 -4.9;

0.75 -4.94 -4.96;

1 -4.985 -5;

1.25 -5 -5]???

David Hill 2022-10-15

在 MATLAB Online 中打开

B=[0.25	0	0;
5	-4.91  -4.9;
75	-4.94  -4.96;
-4.985	-5;
25	-5	-5;
5	-5	-5;
75	-5	-5];
[~,idx]=unique(B(:,2),'stable');
C=B(idx,:)
C = 5×3
2500         0         0
5000   -4.9100   -4.9000
7500   -4.9400   -4.9600
0000   -4.9850   -5.0000
2500   -5.0000   -5.0000

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Answer 2

Torsten 2022-10-15

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在 MATLAB Online 中打开

B = [0.25	0	0;
5	-4.91  -4.9;
75	-4.94  -4.96;
-4.985	-5;
25	-5	-5;
5	-5	-5;
75	-5	-5];
B = B(union(find(B(:,2)~=B(:,3)),[1 5]),:)
B = 5×3
2500         0         0
5000   -4.9100   -4.9000
7500   -4.9400   -4.9600
0000   -4.9850   -5.0000
2500   -5.0000   -5.0000

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Answer 3

Ghazwan 2022-10-15

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%If you already know that you need to omit the first and last row only
A=A(2:end-1,:);
%If you do not know which rows have the condition
for ii=1:length(A(:,1))
    if A(ii,2)==A(ii,3)  %Your condition
        A(ii,:)=[];  %omitting the row that meets the condition
    end
end

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Pooneh Shah Malekpoor 2022-10-15

Thanks but it keeps the last row while I want it to keep the first row with equal arrays in column 2 and 3 (I mean I want a for loop which keep the fifth row). How?

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