Nicolai Jørgensen - 1 year ago 63

R Question

I have an out from a lower.tri output and wnat to do the mean on the first column.

How to do that?

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Answer Source

An easy way to do it (and also get any column mean) would be to (instead of using `lower.tri`

directly on the matrix) do the following:

```
#Assume a 5x5 matrix
a <- matrix(1:25, nrow=5)
#set upper.tri and the diag to NA
#so what you are left with is just the lower.tri values in a matrix
a[upper.tri(a)] <- diag(a) <- NA
```

Then it is easy to do:

```
colMeans(a, na.rm=TRUE)
[1] 3.5 9.0 14.5 20.0 NaN
```

And get all of them. `colMeans(a, na.rm=TRUE)[1]`

will just give you the first.

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