different usages of tapply

using tapply to run a command, in this case calculating the variance of the variable met within each of the categories of the variable coffee consumption (3 categories)

```
tapply(coffee.exercise$met, coffee.exercise$coffee.consumption, var)

```
rowMeans, rowSums, colMeans, colSums, can do similar functions as apply. These are pretty similar to Stata's egen rowmean and rowtotal functions

Below are some examples:
```
colSums(ham, na.rm = T)
colMeans(ham, na.rm = T)
```