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

arrayMATLABmatrixmatrix manipulationStatistics and Machine Learning Toolbox

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.

Best Answer

>> 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

Related Solutions

MATLAB: Finding the sum of every nth row ( n is not fixed)

You'll have to introduce a day variable...by locating the positions where the hour returns to 0 if is always a 0-hour reading or when the diff() is <0 if not; then fill between those with either an arbitrary day (but sequential) number or an actual date if have it.

Once that's done,

[g,da,hr]=findgroups(M(:),M(:));  % grouping variable by day, hour
tot=splitapply(@sum,M(:,3),g);    % sum by group
res=table(da,hr,tot);             % assemble results in table

the above assumes the day number is prepended to M as the first column.

MATLAB: Change column and row

Hello, You can swap columns and rows quite easily in matlab. For example:

given matrix :

matrix=[1 2 3 4 5;6 7 8 9 10; 11 12 13 14 15; 16 17 18 19 20];

i can swap column 1 and 2 by performing this operation:

matrix(:,[1,2])=matrix(:,[2,1]);

for row 1 and 2, a similar operation is done:

matrix([1,2],:)=matrix([2,1],:);

Please be aware of the sequence in which you perform this operation, since column 10 and row 10 have 1 variable in common, if you swap columns and rows sequentialy, you will mix up that one variable.

Another (manual) approach to retain information is to make a copy of the row and column you want to swap in another variable such as:

copy_matrix=matrix(:,:);
matrix(2,:)=copy_matrix(1,:);
matrix(1,:)=copy_matrix(2,:);

Hope this helps,

Dennie

Best Answer

Related Solutions

MATLAB: Finding the sum of every nth row ( n is not fixed)

MATLAB: Change column and row

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