Day 13 - Operating on Lists

Skills: None

Last time we saw operations on lists of numbers from math and statistics. Today we are going to dive into many more built in operations on lists that come from lists.

Imagine a list of discount codes used by a magazine:

discount-codes = [list: "NEWYEAR", "student", "NONE", "student", "VIP", "none"]

Which may have come from a column of a table. As part of data cleaning, one thing you might want to do first is figure out what are the distinct codes used.
```
import lists as L

unique-codes = L.distinct(discount-codes)
```
We also have operations on lists that are similar to operations you saw previously on tables. For example, we can L.filter to remove codes that represent no discount (in this case, normalized "none").
```
fun is-real-code(code :: String) -> Boolean:
  not(string-to-lower(code) == "none")
end

real-codes = L.filter(is-real-code, unique-codes)
```
Like how tables have row-n(), lists have a way of getting an element by position:
```
first-code = real-codes.get(0)
```
For tables, we had a few ways of transforming the existing data -- adding a new column with build-column, or transforming a single column with transform-column. Since lists have only a single value, there is only one version, called L.map.
We can use this to, e.g., get lowercase versions of each discount code:
```
lower-codes = L.map(string-to-lower, codes-real)
```

FIXME / TODO

Lists have many built-in operations -- we showed a few, but there are more at https://pyret.org/docs/latest/lists.html.