Wrong output of ismember command

Question

Hamid 2022-7-13

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1758835-wrong-output-of-ismember-command

评论： Hamid 2022-7-14

Dear all,

In part of my code I am using ismember command and as the array I am using has 21*9 shape and all of these elements are in the first element I expected an array of 21*9 members to be my out put while the output has shape of 22*9! Any idea about the cause? Please find my code in the following. I found the rows having same value in the 9th column in all_com variable and then used idx commands to find the rows corresponding to that.

Thank you in advance.

in1=load('in1.mat')
in1 = struct with fields:
    in1: [265×9 double]
in2=load('in2.mat')
in2 = struct with fields:
    in2: [251×9 double]
out1=load('out1.mat')
out1 = struct with fields:
    out1: [271×9 double]
out2=load('out2.mat')
out2 = struct with fields:
    out2: [256×9 double]
all_common= intersect(intersect(intersect(in1(:,9), in2(:,9)), out1(:,9)), out2(:,9));
Index in position 2 exceeds array bounds. Index must not exceed 1.
idx1 = ismember(in1(:,9),all_common);
in1_com = in1(idx1,:);
idx2 = ismember(in2(:,9),all_common);
in2_com = in2(idx2,:);
idx3 = ismember(out1(:,9),all_common);
out1_com = out1(idx3,:);
idx4 = ismember(out2(:,9),all_common);
out2_com = out2(idx4,:);

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Answer 1

Walter Roberson 2022-7-13

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1758835-wrong-output-of-ismember-command#answer_1006125

在 MATLAB Online 中打开

filedata = load('in1.mat');
in1 = filedata.in1;
filedata = load('in2.mat');
in2 = filedata.in2;
filedata = load('out1.mat');
out1 = filedata.out1;
filedata = load('out2.mat');
out2 = filedata.out2;
all_common= intersect(intersect(intersect(in1(:,9), in2(:,9)), out1(:,9)), out2(:,9));
idx1 = ismember(in1(:,9),all_common);
in1_com = in1(idx1,:);
idx2 = ismember(in2(:,9),all_common);
in2_com = in2(idx2,:);
idx3 = ismember(out1(:,9),all_common);
out1_com = out1(idx3,:);
idx4 = ismember(out2(:,9),all_common);
out2_com = out2(idx4,:);

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Walter Roberson 2022-7-13

out2(69,9) and out2(235,9) are both 13740001 which is a member of all_common. In order to get the 21 results that you expect, you would need to decide between selecting row 69 or row 235; how do you want to decide which of the two to use?

Your logic of expecting the same number of rows as entries in all_common assumes that the entries in column 9 are unique, but they are not.

Hamid 2022-7-14