# Subset a Data Frame

#### David Gerbing

    library("lessR")

## 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, called by `d[rows, columms]`, where d is the data frame. To use this function, for the rows parameter, pass the row names of the selected rows, the indices or actual names, or pass a logical statement that, when evaluated, results in these names. For the cols parameter, pass the column indices of the selected columns, or pass a list of variable names that reduces to these indices.

In the logical expression of rows, use the standard R operators.

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

For the column specification, specify a variable range of contiguous variables with a colon, `:`.

## Annoying Features of Base R `Extract`

When `Extract` evaluates the row or column specifications to obtain the indices, there are several limitations.

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 function call, and now is redundant, repeated for every variable reference.
2. `cols`: Usually specified with a vector of variable names.

- All variable names in in the provided vector of names must be quoted.
- No variable ranges.

3. 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 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 uses the `.()` may not apply.

The general form of the subsetting follows.

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

That is, call the `Extract` function as before, but now wrap the row and column expressions with `.()`.

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

    d <- Read("Employee")

    ##
    ## >>> Suggestions
    ## Details about your data, Enter:  details()  for d, or  details(name)
    ##
    ## Data Types
    ## ------------------------------------------------------------
    ## character: Non-numeric data values
    ## integer: Numeric data values, integers only
    ## double: Numeric data values with decimal digits
    ## ------------------------------------------------------------
    ##
    ##     Variable                  Missing  Unique
    ##         Name     Type  Values  Values  Values   First and last values
    ## ------------------------------------------------------------------------------------------
    ##  1     Years   integer     36       1      16   7  NA  15 ... 1  2  10
    ##  2    Gender character     37       0       2   M  M  M ... F  F  M
    ##  3      Dept character     36       1       5   ADMN  SALE  SALE ... MKTG  SALE  FINC
    ##  4    Salary    double     37       0      37   53788.26  94494.58 ... 56508.32  57562.36
    ##  5    JobSat character     35       2       3   med  low  low ... high  low  high
    ##  6      Plan   integer     37       0       3   1  1  3 ... 2  2  1
    ##  7       Pre   integer     37       0      27   82  62  96 ... 83  59  80
    ##  8      Post   integer     37       0      22   92  74  97 ... 90  71  87
    ## ------------------------------------------------------------------------------------------

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)]

    ##                   Years Gender Dept    Salary Post
    ## Ritchie, Darnell      7      M ADMN  53788.26   92
    ## Hoang, Binh          15      M SALE 111074.86   97
    ## Pham, Scott          13      M SALE  81871.05   94
    ## Correll, Trevon      21      M SALE 134419.23   94
    ## Langston, Matthew     5      M SALE  49188.96   93
    ## Anderson, David       9      M ACCT  69547.60   91

Following is the traditional R call to `Extract` for subsetting.

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

    ##                   Years Gender Dept    Salary Post
    ## Ritchie, Darnell      7      M ADMN  53788.26   92
    ## Hoang, Binh          15      M SALE 111074.86   97
    ## Pham, Scott          13      M SALE  81871.05   94
    ## Correll, Trevon      21      M SALE 134419.23   94
    ## Langston, Matthew     5      M SALE  49188.96   93
    ## Anderson, David       9      M ACCT  69547.60   91

To negate a row selection, add a `!` to the beginning of the logical 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)]

    ##                     Years Gender Post
    ## Wu, James              NA      M   74
    ## Jones, Alissa           5      F   62
    ## Downs, Deborah          7      F   86
    ## Afshari, Anbar          6      F  100
    ## Knox, Michael          18      M   84
    ## Campagna, Justin        8      M   84
    ## Kimball, Claire         8      F   92
    ## Cooper, Lindsay         4      F   91
    ## Saechao, Suzanne        8      F  100
    ## Tian, Fang              9      F   61
    ## Bellingar, Samantha    10      F   72
    ## Sheppard, Cory         14      M   73
    ## Kralik, Laura          10      F   71
    ## Skrotzki, Sara         18      F   61
    ## James, Leslie          18      F   70
    ## Osterman, Pascal        5      M   70
    ## Adib, Hassan           14      M   69
    ## Gvakharia, Kimberly     3      F   79
    ## Stanley, Grayson        9      M   73
    ## Link, Thomas           10      M   83
    ## Portlock, Ryan         13      M   73
    ## Stanley, Emma           3      F   84
    ## Singh, Niral            2      F   59
    ## Fulton, Scott          13      M   73
    ## Korhalkar, Jessica      2      F   87
    ## LaRoe, Maria           10      F   86
    ## Billing, Susan          4      F   90
    ## Capelle, Adam          24      M   81
    ## Hamide, Bita            1      F   90
    ## Anastasiou, Crystal     2      F   71
    ## Cassinelli, Anastis    10      M   87

Can still provide the indices directly for one or both of the expressions as the base R `Extract` function is unmodified.

    d[1:3, .(Years:Salary, Post)]

    ##                  Years Gender Dept    Salary Post
    ## Ritchie, Darnell     7      M ADMN  53788.26   92
    ## Wu, James           NA      M SALE  94494.58   74
    ## Hoang, Binh         15      M SALE 111074.86   97

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

    ##                   Years Gender Dept
    ## Ritchie, Darnell      7      M ADMN
    ## Hoang, Binh          15      M SALE
    ## Pham, Scott          13      M SALE
    ## Correll, Trevon      21      M SALE
    ## Langston, Matthew     5      M SALE
    ## Anderson, David       9      M ACCT

To enhance readability, store the specified row or column conditions as character strings. Each string must be named `rows` or `cols`. Because the entire expression 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, as illustrated next.

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

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

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

    ##                 Gender Dept    Salary Post
    ## Hoang, Binh          M SALE 111074.86   97
    ## Pham, Scott          M SALE  81871.05   94
    ## Correll, Trevon      M SALE 134419.23   94

To negate, as with the literal expressions, use `!` for the rows and `-` for the columns. Notice their placement.

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

    ##                     Years JobSat Plan Pre
    ## Ritchie, Darnell        7    med    1  82
    ## Wu, James              NA    low    1  62
    ## Jones, Alissa           5   <NA>    1  65
    ## Downs, Deborah          7   high    2  90
    ## Afshari, Anbar          6   high    2 100
    ## Knox, Michael          18    med    3  81
    ## Campagna, Justin        8    low    1  76
    ## Kimball, Claire         8   high    2  93
    ## Cooper, Lindsay         4   high    1  78
    ## Saechao, Suzanne        8    med    1  98
    ## Tian, Fang              9    med    2  60
    ## Bellingar, Samantha    10    med    1  67
    ## Sheppard, Cory         14    low    3  66
    ## Kralik, Laura          10    med    2  74
    ## Skrotzki, Sara         18    med    2  63
    ## James, Leslie          18    low    3  70
    ## Osterman, Pascal        5   high    3  69
    ## Adib, Hassan           14    med    2  71
    ## Gvakharia, Kimberly     3    med    2  83
    ## Stanley, Grayson        9    low    1  74
    ## Link, Thomas           10    low    1  83
    ## Portlock, Ryan         13    low    1  72
    ## Langston, Matthew       5    low    3  94
    ## Stanley, Emma           3   high    2  86
    ## Singh, Niral            2   high    2  59
    ## Anderson, David         9    low    1  94
    ## Fulton, Scott          13    low    1  72
    ## Korhalkar, Jessica      2   <NA>    2  74
    ## LaRoe, Maria           10   high    2  80
    ## Billing, Susan          4    med    2  91
    ## Capelle, Adam          24    med    2  83
    ## Hamide, Bita            1   high    2  83
    ## Anastasiou, Crystal     2    low    2  59
    ## Cassinelli, Anastis    10   high    1  80

## Missing Data

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

    d[1:5,]

    ##                  Years Gender Dept    Salary JobSat Plan Pre Post
    ## Ritchie, Darnell     7      M ADMN  53788.26    med    1  82   92
    ## Wu, James           NA      M SALE  94494.58    low    1  62   74
    ## Hoang, Binh         15      M SALE 111074.86    low    3  96   97
    ## Jones, Alissa        5      F <NA>  53772.58   <NA>    1  65   62
    ## Downs, Deborah       7      F FINC  57139.90   high    2  90   86

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')]

    ##                  Gender Dept    Salary
    ## Ritchie, Darnell      M ADMN  53788.26
    ## NA                 <NA> <NA>        NA
    ## Afshari, Anbar        F ADMN  69441.93
    ## James, Leslie         F ADMN 122563.38
    ## Singh, Niral          F ADMN  61055.44
    ## Billing, Susan        F ADMN  72675.26
    ## Capelle, Adam         M ADMN 108138.43

The result provides what was 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)]

    ##                  Gender Dept    Salary
    ## Ritchie, Darnell      M ADMN  53788.26
    ## Afshari, Anbar        F ADMN  69441.93
    ## James, Leslie         F ADMN 122563.38
    ## Singh, Niral          F ADMN  61055.44
    ## Billing, Susan        F ADMN  72675.26
    ## Capelle, Adam         M ADMN 108138.43

If rows with the value of the variable missing are desired, then `.()` provides that information only when requested, such as with the base R function `is.na()`.

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

    ##                  Gender Dept    Salary
    ## Ritchie, Darnell      M ADMN  53788.26
    ## Jones, Alissa         F <NA>  53772.58
    ## Afshari, Anbar        F ADMN  69441.93
    ## James, Leslie         F ADMN 122563.38
    ## Singh, Niral          F ADMN  61055.44
    ## Billing, Susan        F ADMN  72675.26
    ## Capelle, Adam         M ADMN 108138.43

## 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)]

    ##                  Years Gender Dept    Salary
    ## Stanley, Grayson     9      M SALE  69624.87
    ## Fulton, Scott       13      M SALE  87785.51
    ## Hoang, Binh         15      M SALE 111074.86
    ## Anderson, David      9      M ACCT  69547.60
    ## Ritchie, Darnell     7      M ADMN  53788.26

Here specify a proportion of rows to select.

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

    ##                  Years Gender Dept   Salary
    ## Osterman, Pascal     5      M ACCT 49704.79
    ## Jones, Alissa        5      F <NA> 53772.58
    ## Sheppard, Cory      14      M FINC 95027.55
    ## LaRoe, Maria        10      F MKTG 61961.29