How do I Iterate over similar elements in an array

2018 2 28
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更新时间：2018 3 2
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Hi, I have an input text file that looks like what's below (the actual text file is 1000 lines so I'm just providing a sample). Anyway, I need to compute the mean, and variance (to fit data into a Gaussian). The last column of input is "class" I need to compute the values for each unique class. So for each of the 10 classes I need the mean and variance. But the data is not ordered. The output should look like: Class %d Column %d Mean %.2f Variance %.2f. How can I accomplish this goal? Thanks a ton
0.5000 0.4600 0.6400 0.3600 0.5000 0 0.4900 0.2200 1
0.5300 0.5600 0.4900 0.4600 0.5000 0 0.5200 0.2200 1
0.5200 0.5300 0.5800 0.6900 0.5000 0 0.5000 0.2200 1
0.6700 0.6200 0.5400 0.4300 0.5000 0 0.5300 0.2200 1
0.4500 0.5700 0.3000 0.1700 0.5000 0 0.5100 0.2200 3
0.6800 0.5300 0.4400 0.3100 0.5000 0 0.5100 0.2200 1
0.3800 0.4200 0.3100 0.3500 0.5000 0 0.5200 0.2200 3
0.4800 0.4600 0.5800 0.2600 0.5000 0 0.4400 0.2200 2

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jonas
jonas 2018-2-28
Just to clarify, you want one set of values per column and class?
Faisal Alnahhas
Faisal Alnahhas 2018-2-28
Here's the prompt of the question: The output of your code should contain one line for each dimension of each class. Such a line should look like this: Class %d, dimension %d, mean = %.2f, variance = %.2f
jonas
jonas 2018-2-28
Okay, I would suggest using logical indexing to group classes. For example, if A is your matrix then the mean for class 1 would be:
mean(A(A(:,end)==1,:))
Faisal Alnahhas
Faisal Alnahhas 2018-3-1
That worked perfectly! Thank you so much

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Rik
Rik 2018-2-28

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You can use unique to get the list of classes, and then you can loop over the submatrix with logical indexing (which you might need to convert to indices with find).

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Faisal Alnahhas
Faisal Alnahhas 2018-3-1
I did what Jonas suggested mean(A(A(:,end)==1,:)) And it worked great. Thank you though
Rik
Rik 2018-3-2
His suggestion is similar to what I meant. You can still get a list of all classes with unique:
list_of_classes=unique(A(:,end));
for class_index=1:length(list_of_classes);
mean_list(class_index)= mean(A(A(:,end)==list_of_classes(class_index),:));
end
Faisal Alnahhas
Faisal Alnahhas 2018-3-2
Thanks for the clarification. Can you tell me what are the pros and cons of using unique in this case? I am still new to matlab. Also why can't we just say mean(A(:,end) == ..)? I am confused as to why we use mean(A(A...))
Rik
Rik 2018-3-2
There aren't really cons. It just looks at the data you put in (in this case the last column of A), and returns a list of all unique values.
As for your second question, you should look at a small example:
A=[1 1;2 1;5 2];
list_of_classes=unique(A(:,end));
%is equivalent to:
%list_of_classes=unique([1;1;2]);
%A(:,end)==1 will return a logical vector:
pos=A(:,end)==1;%returns [true;true;false]
%you can use this vector for logical indexing:
data=A(pos,:);%returns [1;2]
%and then we can calculate the mean:
mean_list=mean(data);
Now you can see that using your suggested code would result in calculating the mean of the logical position vector, not the data.
Faisal Alnahhas
Faisal Alnahhas 2018-3-2
Thank you for the great explanation. I 100% understand what's going on now.

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