# How can i delete rows with zeros from a matrix and rebuild the matrix?

7 vues (au cours des 30 derniers jours)
Pablo Álvarez Rodríguez le 12 Juil 2017
Commenté : dbmn le 13 Juil 2017
I have this matrix, as an example, because the problem is a nxm one with a more complex program, so i hope this may help.
>> A=rand(9,3)
A =
0.575208595078466 0.73172238565867 0.368484596490336
0.0597795429471558 0.647745963136307 0.62561856072969
0.234779913372406 0.450923706430945 0.780227435151377
0.353158571222071 0.547008892286345 0.0811257688657853
0.821194040197959 0.296320805607773 0.92938597096873
0.0154034376515551 0.744692807074156 0.775712678608402
0.0430238016578078 0.188955015032545 0.486791632403172
0.168990029462704 0.686775433365315 0.435858588580919
0.649115474956452 0.18351115573727 0.446783749429806
>> B=[A;zeros(1,3)]
B =
0.575208595078466 0.73172238565867 0.368484596490336
0.0597795429471558 0.647745963136307 0.62561856072969
0.234779913372406 0.450923706430945 0.780227435151377
0.353158571222071 0.547008892286345 0.0811257688657853
0.821194040197959 0.296320805607773 0.92938597096873
0.0154034376515551 0.744692807074156 0.775712678608402
0.0430238016578078 0.188955015032545 0.486791632403172
0.168990029462704 0.686775433365315 0.435858588580919
0.649115474956452 0.18351115573727 0.446783749429806
0 0 0
>> C=[0:9]
C =
0 1 2 3 4 5 6 7 8 9
>> D=C'
D =
0
1
2
3
4
5
6
7
8
9
>> E=[D,B]
E =
0 0.575208595078466 0.73172238565867 0.368484596490336
1 0.0597795429471558 0.647745963136307 0.62561856072969
2 0.234779913372406 0.450923706430945 0.780227435151377
3 0.353158571222071 0.547008892286345 0.0811257688657853
4 0.821194040197959 0.296320805607773 0.92938597096873
5 0.0154034376515551 0.744692807074156 0.775712678608402
6 0.0430238016578078 0.188955015032545 0.486791632403172
7 0.168990029462704 0.686775433365315 0.435858588580919
8 0.649115474956452 0.18351115573727 0.446783749429806
9 0 0 0
>>
My question is how can transform matrix E into
E =
0 0.575208595078466 0.73172238565867 0.368484596490336
1 0.0597795429471558 0.647745963136307 0.62561856072969
2 0.234779913372406 0.450923706430945 0.780227435151377
3 0.353158571222071 0.547008892286345 0.0811257688657853
4 0.821194040197959 0.296320805607773 0.92938597096873
5 0.0154034376515551 0.744692807074156 0.775712678608402
6 0.0430238016578078 0.188955015032545 0.486791632403172
7 0.168990029462704 0.686775433365315 0.435858588580919
8 0.649115474956452 0.18351115573727 0.446783749429806
in order to print it and to write it in a txt file easily without the zeros. And remember, as this is an nxm matrix it can be also...
E =
0 0.575208595078466 0.73172238565867 0.368484596490336
1 0.0597795429471558 0.647745963136307 0.62561856072969
2 0.234779913372406 0.450923706430945 0.780227435151377
3 0.353158571222071 0.547008892286345 0.0811257688657853
4 0.821194040197959 0.296320805607773 0.92938597096873
5 0.0154034376515551 0.744692807074156 0.775712678608402
6 0.0430238016578078 0.188955015032545 0.486791632403172
7 0.168990029462704 0.686775433365315 0.435858588580919
8 0.649115474956452 0.18351115573727 0.446783749429806
9 0 0 0
10 0 0 0
Also, i would like to transform a matrix like
E =
0 0.575208595078466 0.73172238565867 0.368484596490336
1 0.0597795429471558 0.647745963136307 0.62561856072969
2 0.234779913372406 0.450923706430945 0.780227435151377
3 0.353158571222071 0.547008892286345 0.0811257688657853
4 0.821194040197959 0.296320805607773 0.92938597096873
5 0.0154034376515551 0.744692807074156 0.775712678608402
6 0.0430238016578078 0.188955015032545 0.486791632403172
7 0.168990029462704 0.686775433365315 0.435858588580919
8 0.649115474956452 0.18351115573727 0.446783749429806
0 0 0 0
0 0 0 0
in the same as before.
E =
0 0.575208595078466 0.73172238565867 0.368484596490336
1 0.0597795429471558 0.647745963136307 0.62561856072969
2 0.234779913372406 0.450923706430945 0.780227435151377
3 0.353158571222071 0.547008892286345 0.0811257688657853
4 0.821194040197959 0.296320805607773 0.92938597096873
5 0.0154034376515551 0.744692807074156 0.775712678608402
6 0.0430238016578078 0.188955015032545 0.486791632403172
7 0.168990029462704 0.686775433365315 0.435858588580919
8 0.649115474956452 0.18351115573727 0.446783749429806
Thank you very much.
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### Réponses (2)

dbmn le 12 Juil 2017
A not so easily readable solution would be the following
% Create a zero vector to compare to
a=zeros(1,size(E,2)-1);
% Compare to that vector and check if each element is the same
index = [sum([E(:,2:end)==a]')==(size(E,2)-1)]';
% delete all the rows that are the same
E(index, :)=[];
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Pablo Álvarez Rodríguez le 12 Juil 2017
I will have to try this one for the zero cases, thanks... bit i think it won't help the numbered one
dbmn le 13 Juil 2017
With the numbered one it should work as well, because the first row is not taken into consideration at all with E(:, 2:end). Hence it does not matter what is written in the first column.
Geoff`s solution is more beautiful than mine - so you might want to combine the two

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Geoff Hayes le 12 Juil 2017
Pablo - you could try the following but it assumes that equality with zero will always return true (which isn't necessarily the case unless you are using integers)
A = rand(9,3);
A = [A ; zeros(1,3)];
A(all(A==0,2),:) = [];
In the above, we use all to find all rows of A that have all zeros. all(A==0,2) will return a column array of zeros and ones where a one indicates all zeros in that row. We then use that column array to set all zero rows to the empty array which will remove the row from A.
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Pablo Álvarez Rodríguez le 12 Juil 2017
I am using integers for indexing the rows (first column)
It may help in the case of all rows as zero.

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