Lesson 4 Filtering Data
We often want to “subset” our data. That is, we only want to look at data for a certain year, or from a certain class products or customers. We generally call this process “filtering” in Excel or “selection” in SQL. The key idea is that we use some criteria to extract a subset of rows from our data and use only those rows in subsequent analysis.
There are two ways to subset data in R:
- Use R’s built in data manipulation tools. These are easily identified by their square bracket
[]syntax. - Use the dplyr library. Think of dplyr as “data pliers” (where pliers are very useful tools around the house).
I personally find dplyr much easier to use than the square bracket notation, so that is what we will use.
4.1 Preliminaries
4.1.1 Import data
Import the Bank data in the normal way in R Studio. You can either use Tools –> Import Dataset from within R Studio or run the command line version of the import functions from the tidyverse. I typically use the menu the first time but then save the command line version created by R Studio.
Click on the Bank tibble in the panel at the top right of R Studio to inspect the contents of the imported file.
4.2 Filters
4.2.1 Using a logical critereon
The easiest way to filter is to call dplyr’s filter function to create a new, smaller tibble:
<new tibble> <- filter(<tibble>, <critereon>)
For example:
FemaleEmployees <- dplyr::filter(Bank, Gender=="Female") ## "dplyr::" not required
View(FemaleEmployees)The new tibble is called FemaleEmployees (although you can call it anything). The source tibble is, of course, the Bank tibble. The logical criterion is Gender=="Female". A few things to note about the logical criterion:
- Gender is the name of a column in the Bank tibble.
- The logical comparison operator for equals is
==, not=. This is the convention in many computer programming languages in which the single equals sign is the assignment operator. In R,<-is the assignment operator and==is the equals comparison operator. If you make a mistake in filtering, it is almost always because you use=instead of==. - “Female” is a literal string. It means: Only keep rows in which the value of Gender is exactly equal to “Female”. The string “female” is not close enough. A literal string is a literal string.
4.2.2 Filtering Using a List
One very powerful trick in R is to extract rows that match a list of values. For example, say we wanted to extract a list of managers. In this dataset, managers have a value of JobGrade >= 4, so we could use a logical criterion:
## # A tibble: 63 x 9
## Employee EducLev JobGrade YrHired YrBorn Gender YrsPrior PCJob Salary
## <dbl> <dbl> <dbl> <dbl> <dbl> <chr> <dbl> <chr> <dbl>
## 1 146 5 4 90 62 Male 3 No 44.5
## 2 147 5 4 91 65 Male 1 No 41
## 3 148 5 4 89 58 Male 3 No 44
## 4 149 5 4 89 65 Male 0 No 44
## 5 150 5 4 90 63 Female 4 No 42.5
## 6 151 5 4 88 58 Female 3 No 40.3
## 7 152 5 4 90 66 Male 1 No 44.5
## 8 153 1 4 82 45 Female 9 No 35.5
## 9 154 5 4 89 66 Male 0 No 42.5
## 10 155 5 4 88 63 Female 0 No 44
## # ... with 53 more rowsNote that there is no assignment operator here, so I have not created a new tibble. R simply summarizes the results in the console window.
The problem with this approach is that it requires job grades to be numeric (and thus ordinal). I could accomplish the same thing in a more general way using a list of the job grades I want to include:
- Create a new vector of managerial job grades using the “combine” function,
c(). I call the resulting vector “Mgmt”. - Use the
is.element()function to test membership in the list for each employee. The full syntax is:is.element(x, y). The function returns TRUE ifxis a member ofyand FALSE otherwise.
## # A tibble: 63 x 9
## Employee EducLev JobGrade YrHired YrBorn Gender YrsPrior PCJob Salary
## <dbl> <dbl> <dbl> <dbl> <dbl> <chr> <dbl> <chr> <dbl>
## 1 146 5 4 90 62 Male 3 No 44.5
## 2 147 5 4 91 65 Male 1 No 41
## 3 148 5 4 89 58 Male 3 No 44
## 4 149 5 4 89 65 Male 0 No 44
## 5 150 5 4 90 63 Female 4 No 42.5
## 6 151 5 4 88 58 Female 3 No 40.3
## 7 152 5 4 90 66 Male 1 No 44.5
## 8 153 1 4 82 45 Female 9 No 35.5
## 9 154 5 4 89 66 Male 0 No 42.5
## 10 155 5 4 88 63 Female 0 No 44
## # ... with 53 more rowsI did not have to put the members of Mgmt in quotation marks because JobGrade is an integer. If my list contains text I have to use quotation marks:
Animals <- c("cat", "dog", "horse", "pig")
4.3 Syntatic sugar
Many computer languages offer “syntactic sugar”: shortcuts that make long or complex commands a bit easier to type. The tidyverse packages offers a couple of sweeteners. The important thing to remember about these shortcuts is that they (generally) only work in tidyverse packages.
4.3.1 Membership
Instead of remembering the syntax of is.element(x, y), you can use the alternative %in%. This makes the filter syntax a bit more readable. As you see from the output, the results are identical to the un-sweetened version.
## # A tibble: 63 x 9
## Employee EducLev JobGrade YrHired YrBorn Gender YrsPrior PCJob Salary
## <dbl> <dbl> <dbl> <dbl> <dbl> <chr> <dbl> <chr> <dbl>
## 1 146 5 4 90 62 Male 3 No 44.5
## 2 147 5 4 91 65 Male 1 No 41
## 3 148 5 4 89 58 Male 3 No 44
## 4 149 5 4 89 65 Male 0 No 44
## 5 150 5 4 90 63 Female 4 No 42.5
## 6 151 5 4 88 58 Female 3 No 40.3
## 7 152 5 4 90 66 Male 1 No 44.5
## 8 153 1 4 82 45 Female 9 No 35.5
## 9 154 5 4 89 66 Male 0 No 42.5
## 10 155 5 4 88 63 Female 0 No 44
## # ... with 53 more rows4.3.2 Pipes
Pipes are use to solve the problem of nested function calls. A nested function occurs whenever the argument of f() is itself a function g(). As you have probably discovered, it is hard to keep the parentheses straight when you write long statements of the form: f(g(x)).
A pipe takes the result of the interior function then pass it along to the exterior function. So f(g(x)) can be rewritten using a pipe: g(x) %>% f. This can be helpful for very long, multi-line statements in R. Just read the pipe operator %>% as “THEN”.
To illustrate, start with the tibble Bank THEN filter it THEN view it: