Subset a Data Frame

In lessR: Less Code, More Results

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Subset a Data Frame with Base R Extract[]

The most general way to subset a data frame by rows and/or columns is the base R Extract[] function, indicated by matched square brackets instead of the usual matched parentheses. For a data frame named d the general format is d[rows, columms].

For the rows parameter, pass either

• the row names of the selected rows, the indices such as 1, 2, etc., or actual row names
• a logical statement that, when evaluated, reduces to the row indices

To specify a logical expression for the rows parameter, use the standard R operators.

operator | meaning -------- | -------- & | and \| | or ! | not == | is equal to != | is not equal to %in% | is in a vector

For example, to obtain a subset of the data frame that consists of only those who report a value of the Gender variable as Female, specify a logical condition such as:

Gender=="Female"

For the cols parameter, pass either

• the column indices of the selected columns
• a list of variable names that reduces to the column indices

If subsetting is done by only rows or only columns, then leave the other value blank. For example, to subset the d data frame only by rows, the general form reduces to d[rows,]. Similarly, to subset only by columns, d[,cols].

Annoying Features of Base R Extract

When Extract[] evaluates the row or column specifications to obtain the indices, there are several annoying properties.

1. rows: Any reference to the variables in the data frame for this specification must contain the name of the data frame followed by a \$. But this name has already been specified in the call to Extract[] by listing the data frame name in front of the square brackets, so now is redundant, repeated for every variable reference.
2. rows: When specifying a value of a variable for a row selection, any missing values for the variable are also provided even though the missing data values are not the requested value of the variable.
3. cols: Usually specified with a vector of variable names but all variable names in in the provided vector of names must be quoted.
4. cols: No variable ranges specified with a colon : such as m01:m10 to specify 10 variables: m01, m02, ..., up to m10.
5. No character strings that store the values passed to rows and cols. Instead directly enter the conditions for both rows and columns, which can make the entire expression quite large.

More Flexible Use of Extract[]

To address the first two deficiencies, one possibility is the base R subset() function. To address these deficiencies and still use Extract[] directly, lessR provides the function .() for obtaining the indices of selected rows and of selected columns. This function is only callable within the base R Extract[] function, with what R refers to as non-standard evaluation. That basically means that the annoying restrictions are removed, though in some advanced programming uses the .() may not apply.

The general form of the subsetting with the .() function follows.

d[.(rows), .(columns)]

That is, call the same Extract[] function with reference to rows and cols, but now wrap the row and column expressions with the lessR function call .().

To illustrate, use the Employee data set contained in lessR, here read into the d data frame.

d <- Read("Employee")

Subset the data frame by only listing observations with a Gender of "M" with scores on Post larger than 90. Only list columns for the variables in the range from Years to Salary, and Post. Referring back to the output of Read(), the variable range includes Years, Gender, Dept, and Salary.

d[.(Gender=="M" & Post>90), .(Years:Salary, Post)]

Following is the traditional R call to Extract[] to obtain the same subsetting.

d[d$Gender=="M" & d$Post>90, c("Years", "Gender", "Dept", "Salary", "Post")]

A row selection is a logical condition. To negate a row selection, add a ! to the beginning of the condition passed to .(), within the call to .(). To exclude the specified variables, place a -, in front of the call to .().

d[.(!(Gender=="M" & Post>90)), -.(Dept:Plan, Pre)]

Can still provide the indices directly for one or both of the expressions as the base R Extract[] function is unmodified with the use of .(). The purpose of .() is simply to return the row or column row indices to identify specific rows or columns of the specified data frame. You can either specify the indices directory for the rows or columns, or let .() identify them for you.

d[1:3, .(Years:Salary, Post)]
d[.(Gender=="M" & Post>90), 1:3]

To enhance readability, store the specified row or column conditions as character strings. Each string must be named either rows or cols. Because the entire expression for rows or cols is a character string, differentiate between single and double quotes as needed. For example, use single quotes within the string and double quotes to define the entire string, illustrated here.

rows <- "Gender=='M' & Post>93"
cols <- "Gender:Salary, Post"

To subset, pass the respective character strings, rows and cols, to .(), respectively.

d[.(rows), .(cols)]

To negate, as with the literal expressions, use ! for the logical expression that defines the rows and - for the columns. Notice their placement, where the ! is inside the call to .(), and the - is outside the call.

d[.(!rows), -.(cols)]

Missing Data

The variable Dept is missing for the fourth row of data.

d[1:5,]

Here with the traditional use of Extract[], specify rows of data only when the value of Dept is ADMN.

d[d$Dept=="ADMN", c('Gender', 'Dept', 'Salary')]

The result provides what is requested, and also when Dept is <NA>, which is not requested. The requested value of ADMN is not the same as <NA>.

Use the .() function to obtain what is requested, rows of data in which the value of Dept is ADMN.

d[.(Dept=="ADMN"), .(Gender:Salary)]

Random Selection of Rows

The function .() also provides for random selection of rows. To randomly select the specified number of rows from the data frame to subset, specify the random() function for the logical criterion of the rows. The value passed to random() can either be the actual number of rows to select, or the proportion of rows to select.

Here randomly select five rows of data from the d data frame.

d[.(random(5)), .(Years:Salary)]

Here specify a proportion of rows to select.

d[.(random(0.1)), .(Years:Salary)]

lessR documentation built on Nov. 12, 2023, 1:08 a.m.