RPR-OBJECTS-Lists
R "Lists"
(R Lists)
Abstract:
Introduction to R list data types: properties, how to create, and modify them and how to retrieve data.
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Objectives:
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Outcomes:
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Deliverables:
- Time management: Before you begin, estimate how long it will take you to complete this unit. Then, record in your course journal: the number of hours you estimated, the number of hours you worked on the unit, and the amount of time that passed between start and completion of this unit.
- Journal: Document your progress in your Course Journal. Some tasks may ask you to include specific items in your journal. Don't overlook these.
- Insights: If you find something particularly noteworthy about this unit, make a note in your insights! page.
Prerequisites:
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This unit builds on material covered in the following prerequisite units:
Evaluation
Evaluation: NA
Contents
Task:
- Load the
R-Exercise_BasicSetupproject in RStudio if you don't already have it open. - Type
init()as instructed after the project has loaded. - Continue below.
Lists
The elements of matrices and arrays all have to be of the same type. Data frames allow us to store elements of different types in columns, but all columns have to have the same length and the elements have to be "atomic" - i.e. you can't put vectors into dataframe columns. But R's lists are much more versatile. They are simply ordered collections of components. These components can have different type - all kinds of R objects can go into a list: characters, booleans, any kind of numeric data, even functions - AND they can have different size.
Lists are created with the list() function, which works similar to the c() function for vectors. Components are accessed through their index in double square brackets, or through their name, using the "$" operator, if the name has been defined. Here is an example:
pUC19 <- list(size=2686, marker="ampicillin", ori="ColE1", accession="L01397", BanI=c(235, 408, 550, 1647) ) objectInfo(pUC19) pUC19[[1]] pUC19[[2]] pUC19$ori pUC19$BanI[2]
Note that in our data.frame() example, we stored multiple restriction enzymes in one string, separated by commas. While we can take such strings apart again, by using the strsplit() function, the string itself still has to be one single element in the data frame's column. Lists have no such restriction. In our example above, we assigned a vector of restriction site positions to the element "BanI".
You can easily imagine that we could now create a list of lists, and that list of lists could hold an entire plasmid database in a most versatile way. Let's do this!
Task:
- Create a list like the one above with data for pACYC184 following the structure for the pUC19 example but using only size, marker and ori data:
- size: 4245
- marker: Tet, Cam
- ori: p15A
- Confirm that your new list's structure looks the pUC19 one (minus "accession", and the "BanI"" element).
- Make a new list, call it plasmidDB and assign to it the puc19 list:
plasmidDB <- list() plasmidDB[["pUC19"]] <- pUC19
- Add your pACYC184 list
- Add a third element to plasmidDB, "pBR322" using the pBR322 data:
- size: 4361
- marker: Amp, Tet
- ori: ColE1
Then:
- retrieve the entire pACYC184 list
Whereas data frames allow you to get all data from a column directly, this is not possible for lists. You need a function that iterates over all list elements instead. Such a function is lapply(), one of several "apply" functions. For example, to get all "ori" elements, try:
lapply(plasmidDB, function(x) { return(x$ori) })
- Retrieve all sizes from the list. Use
unlist()to flatten the result. Then usemin()to find the size of the smallest one.
Lost? Solutions here (but don't peek).
pUC19 <- list(size=2686, marker="ampicillin", ori="ColE1", accession="L01397", BanI=c(235, 408, 550, 1647) )
pACYC184 <- list(size=4245, marker="Tet, Cam", ori="p15A" )
plasmidDB <- list()
plasmidDB[["pUC19"]] <- pUC19
plasmidDB[["pACYC184"]] <- pACYC184
plasmidDB[["pBR322"]] <- list(size=4361, marker="Amp, Tet", ori="ColE1" )
plasmidDB[["pACYC184"]] # retrieve the entire pACYC184 list
lapply(plasmidDB, function(x) { return(x$ori) })
x <- unlist(lapply(plasmidDB, function(x) { return(x$size) }))
min(x)
# or, to get the name too ...
x[x == min(x)]
Self-evaluation
Question 1
Execute: x <- strsplit(plasmidData$Sites, ", ") and analyze the result.
- (A) What is
plasmidData$Sites? - (B) What is
x? - (C) Why does
strsplit()have a list as return value, not a vector or a data frame?
Answer ...
- (A) A vector of strings (character elements).
- (B) A list of vectors.
- (C) Because
strsplit()is vectorized and operates on all elements of the vectorplasmidData$Sites. But the function doesn't know in advance how many elements the result is going to have for each element. So it needs to output vectors of different lengths. And the only way to combine them into a single value (note: all R functions return only a single value) is by collecting them in a list.
About ...
Author:
- Boris Steipe <boris.steipe@utoronto.ca>
Created:
- 2017-08-05
Modified:
- 2020-09-17
Version:
- 1.0.3
Version history:
- 1.0.3 Maintenace
- 1.0.2 Maintenace
- 1.0.1 Fixed error in list example
- 1.0 Completed to first live version
- 0.1 Material collected from previous tutorial
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