How to find the mean of one column based on the value in another column

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I want to find the mean of the values in the second column only for the rows which have the value 3 in column 10 I have tried mean = mean(data(data(:,10)==3,8))

Accepted Answer

Bruno Luong
Bruno Luong on 30 Oct 2018
Edited: Bruno Luong on 30 Oct 2018
You command is correct (beside using 8th column instead of 2nd) BUT:
Don't name your (output) variable MEAN, you will have conflict later when using function MEAN.
  6 Comments
Swazi Sharma
Swazi Sharma on 13 May 2021
Hey guys! If I want to do the same thing, but change the parameter from 3 to a word (for example: "January"). Can i just change the 3 to January, or do I have to change other bits in code?
DGM
DGM on 13 May 2021
Edited: DGM on 13 May 2021
Comparisons between words don't really work the same way, and the approach depends on the variable type.
For character vectors:
myword = 'apple';
myword == 'orange' % results in error if the words aren't the same size
strcmpi(myword,'orange') % this would be a valid comparison
If you don't need case-insensitivity, use strcmp() instead.
If the variables are actual strings (in double quotes), the behavior is a bit different.
myword = "apple"
myword == "orange" % this actually does work with strings
strcmpi(myword,"orange") % this still works with either
Similarly, the indexing expressions you use to apply this comparison would depend entirely on the type of array in which these variables exist. Practical arraying of strings and character vectors vary, so I can't guess what you have.
If I presume that you're using a cell array (note the case-insensitive behavior of strcmpi()):
A = {12 56 'orange';
64 46 'apple';
32 36 'Peach';
64 16 'apple';
32 26 'peAch';
64 41 'apple';
32 72 'peach';
64 26 'apple';
32 86 'peach';
34 48 'banana'}
% mean of col2 for rows where col3 is 'peach'
mn = mean([A{strcmpi(A(:,3),'peach'),2}])
ans =
55
Otherwise, you'll have to adapt this to whatever array type you're using.

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More Answers (1)

Matt J
Matt J on 30 Oct 2018
mean(data(data(:,10)==3,2))
  5 Comments
Catherine Branter
Catherine Branter on 30 Oct 2018
Edited: Catherine Branter on 30 Oct 2018
this is the error i get:
Index exceeds the number of array elements (0).
Error in ML_Coursework (line 16) output = mean(data(data(:,10)==3,2))

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