Yang Yang - 3 months ago 30

R Question

I have a matrix which contains 100 rows and 120 columns, I wonder how I can find the mean value for every 12 columns for each row. so I can have the annual mean. Thanks.

`set.seed(1234)`

data=rnorm(100*120)

data=matrix(data,nrow = 100,ncol = 120)

Answer

For each row `r`

, apply `tapply(r, gl(10, 12), mean)`

to it:

```
t(apply(data, 1, tapply, gl(10, 12), mean))
```

Note that the above does not use any properties of `mean`

so it generalizes to other aggregate functions by replacing `mean`

with some other function.

Another possibility which also generalizes is to reshape the matrix into a 3d array and take the mean over the indicated dimensions:

```
apply(array(data, c(100, 12, 10)), c(1, 3), mean)
```