Chapter 6 Data Wrangling with Tidy Data, Part 2
Today, we will continue learning about common functions from the Tidyverse that is useful for Tidy data manipulations.
6.1 Modifying and creating new columns in dataframes
The mutate() function takes in the following arguments: the first argument is the dataframe of interest, and the second argument is a new or existing data variable that is defined in terms of other data variables.
We create a new column olderAge that is 10 years older than the original Age column.
## [1] 60 36 72 30 30 64 63 56 72 53## [1] 70 46 82 40 40 74 73 66 82 63Here, we used an operation on a column of metadata. Here’s another example with a function:
## [1] 4.634012 4.638653 4.032101 5.503031 3.713696 3.972693 3.235727 4.135042
## [9] 9.017365 3.940167## [1] 1.533423 1.534424 1.394288 1.705299 1.312028 1.379444 1.174254 1.419498
## [9] 2.199152 1.3712236.2 Merging two dataframes together
Suppose we have the following dataframes:
expression
| ModelID | PIK3CA_Exp | log_PIK3CA_Exp |
|---|---|---|
| “ACH-001113” | 5.138733 | 1.636806 |
| “ACH-001289” | 3.184280 | 1.158226 |
| “ACH-001339” | 3.165108 | 1.152187 |
metadata
| ModelID | OncotreeLineage | Age |
|---|---|---|
| “ACH-001113” | “Lung” | 69 |
| “ACH-001289” | “CNS/Brain” | NA |
| “ACH-001339” | “Skin” | 14 |
Suppose that I want to compare the relationship between OncotreeLineage and PIK3CA_Exp, but they are columns in different dataframes. We want a new dataframe that looks like this:
| ModelID | PIK3CA_Exp | log_PIK3CA_Exp | OncotreeLineage | Age |
|---|---|---|---|---|
| “ACH-001113” | 5.138733 | 1.636806 | “Lung” | 69 |
| “ACH-001289” | 3.184280 | 1.158226 | “CNS/Brain” | NA |
| “ACH-001339” | 3.165108 | 1.152187 | “Skin” | 14 |
We see that in both dataframes, the rows (observations) represent cell lines with a common column ModelID, so let’s merge these two dataframes together, using full_join():
The number of rows and columns of metadata:
## [1] 1864 30The number of rows and columns of expression:
## [1] 1450 536The number of rows and columns of merged:
## [1] 1864 565We see that the number of columns in merged combines the number of columns in metadata and expression, while the number of rows in merged is the larger of the number of rows in metadata and expression : full_join() keeps all observations common to both dataframes based on the common column defined via the by argument.
Therefore, we expect to see NA values in merged, as there are some cell lines that are not in expression dataframe.
There are variations of this function depending on your application:
Given xxx_join(x, y, by = "common_col"),
full_join()keeps all observations.left_join()keeps all observations inx.right_join()keeps all observations iny.inner_join()keeps observations common to bothxandy.
6.3 Grouping and summarizing dataframes
In a dataset, there may be multiple levels of observations, and which level of observation we examine depends on our scientific question. For instance, in metadata, the observation is cell lines. However, perhaps we want to understand properties of metadata in which the observation is the cancer type, OncotreeLineage. Suppose we want the mean age of each cancer type, and the number of cell lines that we have for each cancer type.
This is a scenario in which the desired rows are described by a column, OncotreeLineage, and the columns, such as mean age, need to be summarized from other columns.
As an example, this dataframe is transformed from:
| ModelID | OncotreeLineage | Age |
|---|---|---|
| “ACH-001113” | “Lung” | 69 |
| “ACH-001289” | “Lung” | 23 |
| “ACH-001339” | “Skin” | 14 |
| “ACH-002342” | “Brain” | 23 |
| “ACH-004854” | “Brain” | 56 |
| “ACH-002921” | “Brain” | 67 |
into:
| OncotreeLineage | MeanAge | Count |
|---|---|---|
| “Lung” | 46 | 2 |
| “Skin” | 14 | 1 |
| “Brain” | 48.67 | 3 |
We use the functions group_by() and summarise() :
metadata_by_type = metadata %>%
group_by(OncotreeLineage) %>%
summarise(MeanAge = mean(Age, rm.na=TRUE), Count = n())Or, without pipes:
metadata_by_type_temp = group_by(metadata, OncotreeLineage)
metadata_by_type = summarise(metadata_by_type_temp, MeanAge = mean(Age, rm.na=TRUE), Count = n())The group_by() function returns the identical input dataframe but remembers which variable(s) have been marked as grouped:
## # A tibble: 6 × 30
## # Groups: OncotreeLineage [3]
## ModelID PatientID CellLineName StrippedCellLineName Age SourceType
## <chr> <chr> <chr> <chr> <dbl> <chr>
## 1 ACH-000001 PT-gj46wT NIH:OVCAR-3 NIHOVCAR3 60 Commercial
## 2 ACH-000002 PT-5qa3uk HL-60 HL60 36 Commercial
## 3 ACH-000003 PT-puKIyc CACO2 CACO2 72 Commercial
## 4 ACH-000004 PT-q4K2cp HEL HEL 30 Commercial
## 5 ACH-000005 PT-q4K2cp HEL 92.1.7 HEL9217 30 Commercial
## 6 ACH-000006 PT-ej13Dz MONO-MAC-6 MONOMAC6 64 Commercial
## # ℹ 24 more variables: SangerModelID <chr>, RRID <chr>, DepmapModelType <chr>,
## # AgeCategory <chr>, GrowthPattern <chr>, LegacyMolecularSubtype <chr>,
## # PrimaryOrMetastasis <chr>, SampleCollectionSite <chr>, Sex <chr>,
## # SourceDetail <chr>, LegacySubSubtype <chr>, CatalogNumber <chr>,
## # CCLEName <chr>, COSMICID <dbl>, PublicComments <chr>,
## # WTSIMasterCellID <dbl>, EngineeredModel <chr>, TreatmentStatus <chr>,
## # OnboardedMedia <chr>, PlateCoating <chr>, OncotreeCode <chr>, …The summarise() returns one row for each combination of grouping variables, and one column for each of the summary statistics that you have specified.
Functions you can use for summarise() must take in a vector and return a simple data type, such as any of our summary statistics functions: mean(), median(), min(), max(), etc.
The exception is n(), which returns the number of entries for each grouping variable’s value.
You can combine group_by() with other functions. See this guide.
6.4 Appendix: How functions are built
As you become more independent R programmers, you will spend time learning about new functions on your own. We have gone over the basic anatomy of a function call back in the first lesson, but now let’s go a bit deeper to understand how a function is built and how to call them.
Recall that a function has a function name, input arguments, and a return value.
Function definition consists of assigning a function name with a “function” statement that has a comma-separated list of named function arguments, and a return expression. The function name is stored as a variable in the global environment.
In order to use the function, one defines or import it, then one calls it.
Example:
addFunction = function(num1, num2) {
result = num1 + num2
return(result)
}
result = addFunction(3, 4)With function definitions, not all code runs from top to bottom. The first four lines defines the function, but the function is never run. It is called on line 5, and the lines within the function are executed.
When the function is called in line 5, the variables for the arguments are reassigned to function arguments to be used within the function and helps with the modular form.
To see why we need the variables of the arguments to be reassigned, consider the following function that is not modular:
x = 3
y = 4
addFunction = function(num1, num2) {
result = x + y
return(result)
}
result = addFunction(10, -10)Some syntax equivalents on calling the function:
addFunction(3, 4)
addFunction(num1 = 3, num2 = 4)
addFunction(num2 = 4, num1 = 3)but this could be different:
addFunction(4, 3)With a deeper knowledge of how functions are built, when you encounter a foreign function, you can look up its help page to understand how to use it. For example, let’s look at mean():
?mean
Arithmetic Mean
Description:
Generic function for the (trimmed) arithmetic mean.
Usage:
mean(x, ...)
## Default S3 method:
mean(x, trim = 0, na.rm = FALSE, ...)
Arguments:
x: An R object. Currently there are methods for numeric/logical
vectors and date, date-time and time interval objects.
Complex vectors are allowed for ‘trim = 0’, only.
trim: the fraction (0 to 0.5) of observations to be trimmed from
each end of ‘x’ before the mean is computed. Values of trim
outside that range are taken as the nearest endpoint.
na.rm: a logical evaluating to ‘TRUE’ or ‘FALSE’ indicating whether
‘NA’ values should be stripped before the computation
proceeds.
...: further arguments passed to or from other methods.Notice that the arguments trim = 0, na.rm = FALSE have default values. This means that these arguments are optional - you should provide it only if you want to. With this understanding, you can use mean() in a new way:
## [1] 2.3333336.5 Exercises
You can find exercises and solutions on Posit Cloud, or on GitHub.
