Almost all data in R is stored in a vector, or even a vector of vectors. A list is a recursive vector : a vector that can contain another vector or list in each of its elements. Lists are one of the most flexible data structures in R. As a result, they are used as a general purpose glue to hold objects together. You will find lists disguised as model objects, data frames, list-columns within data frames, and more.
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Data frames are a sub-type of list. Each column of a data frame is an element of the list that the data frame is built around. More than any other part of R, lists demonstrate how a programming language can appear different to beginners than to experts. Seasoned R programmers do not distinguish between lists and vectors because the two are equivalent: a list is a type of vector.
However, a beginner who uses R for data science will quickly see that lists behave differently than other types of vectors. First, many R functions will not accept lists as input, even though they accept other types of vectors. Second, when you subset a list with [ ] to extract the value of one of its elements, R will give you a new list of length one that contains the value as its first element.
This poses a problem if you want to pass that value to a function that does not accept lists as input solve that problem with this recipe. To respect this difference, I try to be clear when talking about vectors that are not lists. This introduces a new problem: when you speak about R, it is difficult to distinguish between vectors that are lists and vectors that are not lists.
Data vectors come in six atomic types : double, integer logical, character, complex, and raw. Jk jk, we will continue to select different cookbooks every month. Pre-order your copy. Ask your local bookstore to stock it. Schedule your own cookbook club night.
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We can perform this operation as follows:. You can limit the number of documents returned by your queries with the limit command. There are multiple ways to paginate results in RethinkDB.
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The slice command returns a range from a given start value through but not including a given end value. To retrieve posts from the database using slice :. This is extremely efficient, but it requires starting each fetch by looking up a record by actual index value. That is, instead of fetching the 11th record with the number 15 , you need to fetch it by the value it has in the indexed field. Suppose you wanted to paginate through a set of users, 25 at a time.
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You could get the first 25 records efficiently just with limit. You can count the number of documents with a count command:. You can compute the average value of a field with the avg command. We could do this using a subquery:.
Suppose the table marks stores the marks of every students per course:. Note: A nicer syntax will eventually be added.
See the Github issue to track progress. We could do:.
ReQL has commands for extracting parts of dates and times , including year , month , day , dayOfWeek and more. You can use these with group to group by various intervals. Suppose you had a table of invoices and wanted to retrieve them in groups ordered by year and month:.
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Currently, ReQL has a default limit of , elements in an array, and the implementation of group requires the total number of documents grouped to fit within that boundary, so you are limited to , invoices. Also note that ungroup always returns an array, although this may change in a future version.
Follow issue for progress on this. You can also use this approach with a compound index on the intervals you want to group:. Then you can use that index in the group function. This query would return the highest-value invoice for each day.