Finding the average of every nth row but n is not fixed.

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pavlos
pavlos 2019-7-23
评论： Stephen23 2019-7-24
Hello,
Consider a nx2 matrix M with
M= [
1 10
1 11
1 20
2 4
2 9
2 8
2 7
.
.
.
];
How to find the average of the second column when the first column change values, i.e. "1 rows" have 3
values while "2 rows" have 4, etc. so we need the average of the 3 first values, then the next four, etc.
Thank you.

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Stephen23
Stephen23 2019-7-23
编辑：Stephen23 2019-7-23
>> M = [1,10;1,11;1,20;2,4;2,9;2,8;2,8];
Method one: accumarray:
>> V = accumarray(M(:,1),M(:,2),[],@mean)
V =
13.667
7.25
Method two: splitapply:
>> V = splitapply(@mean,M(:,2),M(:,1))
V =
13.667
7.25
And if you want those values back in the matrix:
>> M(:,3) = V(M(:,1))
M =
1 10 13.667
1 11 13.667
1 20 13.667
2 4 7.25
2 9 7.25
2 8 7.25
2 8 7.25
  2 个评论
pavlos
pavlos 2019-7-23
@ Stephen
Thank you, it works perfectly.
But I didn`t mentioned in my question that the values of the 1st column are repeated.
Actually, the 1st column corresponds to hours in a day within a month so, 0,1,..,23 are repeated:
0 0 1. . .23 23 23 1st day
0 1 1...23 2nd day
etc.
So each day has different number of hourly values.
Pavlos
Stephen23
Stephen23 2019-7-24
@pavlos: Then to use this method you will have to generate a unique index for each group that you want to average. You could use findgroups or discretize or the third output from unique (possibly with the 'rows' option) or something similar.

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