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Can i find 'mean' of on column , based on second column ?

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Wlmistrzow
Wlmistrzow 2019-11-3
编辑： Thiago Henrique Gomes Lobato 2019-11-3
I have random matrix A 120x6. In last column I have only values 1 or 2. I want to find the min, max, median, and mean of first column for this rows, where in 6 column is 1.
6 8 2 9 5 2
1 8 6 10 5 2
1 10 2 7 2 1
3 3 2 7 2 1
2 10 8 3 1 1
2 1 4 6 4 1
10 9 2 2 9 2
3 8 5 2 5 1
4 3 9 8 6 2
4 8 4 7 7 2
10 1 4 9 7 2
  1 个评论
Wlmistrzow
Wlmistrzow 2019-11-3
i tryied with this
for i=1:120
if A(i,6)==1
d = mean(mean(A(:,1)));
e = median(A(:,1));
end
end
disp(d);

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采纳的回答

Image Analyst
Image Analyst 2019-11-3
Try this:
m = [...
6 8 2 9 5 2
1 8 6 10 5 2
1 10 2 7 2 1
3 3 2 7 2 1
2 10 8 3 1 1
2 1 4 6 4 1
10 9 2 2 9 2
3 8 5 2 5 1
4 3 9 8 6 2
4 8 4 7 7 2
10 1 4 9 7 2]
col6Is1 = m(:, 6) == 1
minOfColumn1 = min(m(col6Is1, 1))
maxOfColumn1 = max(m(col6Is1, 1))
medianOfColumn1 = median(m(col6Is1, 1))
meanOfColumn1 = mean(m(col6Is1, 1))

更多回答（1 个）

Thiago Henrique Gomes Lobato
编辑：Thiago Henrique Gomes Lobato 2019-11-3
You can first find the index where columm 6 is one, save it in a vector and then use it as a mask for your first columm, in this way you also vectorize your code, which runs faster in Matlab :
A = rand(120,6)*2;
A(:,6) = ceil(A(:,6)); % Values here can be only either 1 or 2
IndexToTakeAverageAndEtc = find(A(:,6)==1); % Find all rows where A(:,6)==1
Mean = mean(A(IndexToTakeAverageAndEtc,1)); % Take the mean only for the rows where the last columm is 1
Max = max(A(IndexToTakeAverageAndEtc,1));
...
  2 个评论
Wlmistrzow
Wlmistrzow 2019-11-3
6 8 2 9 5 2
1 8 6 10 5 2
1 10 2 7 2 1
3 3 2 7 2 1
2 10 8 3 1 1
2 1 4 6 4 1
10 9 2 2 9 2
3 8 5 2 5 1
4 3 9 8 6 2
4 8 4 7 7 2
10 1 4 9 7 2
i must to have mean from values from column 1 where in the same row , but i the 6 column is number 1. in this example i must to have mean from (1 ,3 ,2,2,3).
Thiago Henrique Gomes Lobato
Thiago Henrique Gomes Lobato 2019-11-3
编辑：Thiago Henrique Gomes Lobato 2019-11-3
Sorry, I made a typo in my initial code in getting the value from the frist columm, just change the 6 for a 1 (I also edited the answer), this give the result that you want:
A = [6 8 2 9 5 2
1 8 6 10 5 2
1 10 2 7 2 1
3 3 2 7 2 1
2 10 8 3 1 1
2 1 4 6 4 1
10 9 2 2 9 2
3 8 5 2 5 1
4 3 9 8 6 2
4 8 4 7 7 2
10 1 4 9 7 2];
A(:,6) = ceil(A(:,6)); % Values here can be only either 1 or 2
IndexToTakeAverageAndEtc = find(A(:,6)==1); % Find all rows where A(:,6)==1
Mean = mean(A(IndexToTakeAverageAndEtc,1)) % Take the mean only for the rows where the last columm is 1
...
Mean =
2.2000
Which is the mean for (1 ,3 ,2,2,3)

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