```r
data_means <- list(
  a = mean(loop_data$a),
  b = mean(loop_data$b),
  c = mean(loop_data$c),
  d = mean(loop_data$d)
)
str(data_means)
```

```
## List of 4
##  $ a: num -0.383
##  $ b: num 0.417
##  $ c: num 0.855
##  $ d: num -20.9
```