Normally, R processes data column-by-column. The data step allows you to process data row-by-row. Row-by-row processing of data is useful when you have related columns, and wish to perform conditional logic on those columns. The datastep() function allows you to realize this style of data processing. It is particularly advantageous when you wish to perform deeply nested conditional logic. It is also very useful for by-group processing.
Here is an example of a simple data step:
library(libr)
# Add some columns to mtcars using data step logic
df <- datastep(mtcars[1:10, 1:3], {
if (mpg >= 20)
mpgcat <- "High"
else
mpgcat <- "Low"
recdt <- as.Date("1974-06-10")
if (cyl == 8)
is8cyl <- TRUE
})
# View results
df
# mpg cyl disp recdt mpgcat is8cyl
# Mazda RX4 21.0 6 160.0 1974-06-10 High NA
# Mazda RX4 Wag 21.0 6 160.0 1974-06-10 High NA
# Datsun 710 22.8 4 108.0 1974-06-10 High NA
# Hornet 4 Drive 21.4 6 258.0 1974-06-10 High NA
# Hornet Sportabout 18.7 8 360.0 1974-06-10 Low TRUE
# Valiant 18.1 6 225.0 1974-06-10 Low NA
# Duster 360 14.3 8 360.0 1974-06-10 Low TRUE
# Merc 240D 24.4 4 146.7 1974-06-10 High NA
# Merc 230 22.8 4 140.8 1974-06-10 High NA
# Merc 280 19.2 6 167.6 1974-06-10 Low NAThe data step has parameters to perform basic shaping of the resulting data frame. These parameters are ‘keep’, ‘drop’, and ‘rename’. For example, the above data step could have been performed by sending all columns into the data step, and keeping only the desired columns. Using the keep parameter also allows you to order the resulting columns.
library(libr)
# Keep and order output columns
df <- datastep(mtcars[1:10,],
keep = c("mpg", "cyl", "disp", "mpgcat", "recdt"), {
if (mpg >= 20)
mpgcat <- "High"
else
mpgcat <- "Low"
recdt <- as.Date("1974-06-10")
if (cyl == 8)
is8cyl <- TRUE
})
df
# mpg cyl disp mpgcat recdt
# Mazda RX4 21.0 6 160.0 High 1974-06-10
# Mazda RX4 Wag 21.0 6 160.0 High 1974-06-10
# Datsun 710 22.8 4 108.0 High 1974-06-10
# Hornet 4 Drive 21.4 6 258.0 High 1974-06-10
# Hornet Sportabout 18.7 8 360.0 Low 1974-06-10
# Valiant 18.1 6 225.0 Low 1974-06-10
# Duster 360 14.3 8 360.0 Low 1974-06-10
# Merc 240D 24.4 4 146.7 High 1974-06-10
# Merc 230 22.8 4 140.8 High 1974-06-10
# Merc 280 19.2 6 167.6 Low 1974-06-10The retain parameter allows you to define variables that will be seeded with the value from the previous step. The retain option is useful for creating cumulative values or for performing conditions based on the value of the previous row.
library(libr)
df <- datastep(mtcars[1:10, ],
drop = c("disp", "hp", "drat", "qsec",
"vs", "am", "gear", "carb"),
retain = list(cumwt = 0 ),
rename = c(mpg = "MPG", cyl = "Cylinders", wt = "Wgt",
cumwt = "Cumulative Wgt"), {
cumwt <- cumwt + wt
})
df
# MPG Cylinders Wgt Cumulative Wgt
# Mazda RX4 21.0 6 2.620 2.620
# Mazda RX4 Wag 21.0 6 2.875 5.495
# Datsun 710 22.8 4 2.320 7.815
# Hornet 4 Drive 21.4 6 3.215 11.030
# Hornet Sportabout 18.7 8 3.440 14.470
# Valiant 18.1 6 3.460 17.930
# Duster 360 14.3 8 3.570 21.500
# Merc 240D 24.4 4 3.190 24.690
# Merc 230 22.8 4 3.150 27.840
# Merc 280 19.2 6 3.440 31.280The datastep() function also has the capabilities of performing by-group processing. A by-group is accomplished using the by parameter, and passing a vector of column names that define the group. Once a by-group is defined, the first. and last. automatic variables become active, which allow you to identify the boundaries between groups. Note that, by default, your data must be sorted properly before sending it into the data step. To turn the sort check off, set the sort_check parameter to FALSE.
library(libr)
# Identify start and end of by-groups
df <- datastep(mtcars[1:10,],
keep = c("mpg", "cyl", "gear", "grp"),
by = c("gear"), sort_check = FALSE, {
if (first. & last.)
grp <- "Start - End"
else if (first.)
grp <- "Start"
else if (last.)
grp <- "End"
else
grp <- "-"
})
df
# mpg cyl gear grp
# Mazda RX4 21.0 6 4 Start
# Mazda RX4 Wag 21.0 6 4 -
# Datsun 710 22.8 4 4 End
# Hornet 4 Drive 21.4 6 3 Start
# Hornet Sportabout 18.7 8 3 -
# Valiant 18.1 6 3 -
# Duster 360 14.3 8 3 End
# Merc 240D 24.4 4 4 Start
# Merc 230 22.8 4 4 -
# Merc 280 19.2 6 4 EndThere may be times when you want to combine row-by-row conditional processing with column-by-column vector operations. For example, let’s say you want to calculate a mean and then perform conditional processing on that mean. This situation can be handled using the calculate parameter on the datastep() function. The function will execute the calculate block first, add any assigned variables to the data frame, and then execute the data step. Below is an example of such a scenario:
library(libr)
# Categorize mpg as above or below the mean
df <- datastep(mtcars,
keep = c("mpg", "cyl", "mean_mpg", "mpgcat"),
calculate = { mean_mpg = mean(mpg) },
{
if (mpg >= mean_mpg)
mpgcat <- "High"
else
mpgcat <- "Low"
})
df[1:10,]
# mpg cyl mean_mpg mpgcat
# Mazda RX4 21.0 6 20.09062 High
# Mazda RX4 Wag 21.0 6 20.09062 High
# Datsun 710 22.8 4 20.09062 High
# Hornet 4 Drive 21.4 6 20.09062 High
# Hornet Sportabout 18.7 8 20.09062 Low
# Valiant 18.1 6 20.09062 Low
# Duster 360 14.3 8 20.09062 Low
# Merc 240D 24.4 4 20.09062 High
# Merc 230 22.8 4 20.09062 High
# Merc 280 19.2 6 20.09062 Lowdplyr
Note that the datastep() function is pipe-friendly, and can be combined with dplyr functions in a data pipeline. Also note that the datastep() function will recognize any group attributes added by the group_by() function. Therefore, within a dplyr pipeline, it is not necessary to use any datastep parameters. The following example recreates the above data frame from Example 5, but with a dplyr pipeline.
library(libr)
library(dplyr)
library(magrittr)
# Add datastep to dplyr pipeline
df <- mtcars %>%
select(mpg, cyl, gear) %>%
mutate(mean_mpg = mean(mpg)) %>%
datastep({
if (mpg >= mean_mpg)
mpgcat <- "High"
else
mpgcat <- "Low"
}) %>%
filter(row_number() <= 10)
df
# mpg cyl gear mean_mpg mpgcat
# 1 21.0 6 4 20.09062 High
# 2 21.0 6 4 20.09062 High
# 3 22.8 4 4 20.09062 High
# 4 21.4 6 3 20.09062 High
# 5 18.7 8 3 20.09062 Low
# 6 18.1 6 3 20.09062 Low
# 7 14.3 8 3 20.09062 Low
# 8 24.4 4 4 20.09062 High
# 9 22.8 4 4 20.09062 High
# 10 19.2 6 4 20.09062 LowThe libr package recognizes several useful data attributes that are not normally recognized by other R functions. For example, it is very convenient to assign label and description attributes to your columns, so other people can understand what data the columns contain.
For this reason, the datastep() function provides an attrib parameter that allows you to supply such attributes as part of a data step. Attributes are assigned with a named list and the dsattr() object.
library(libr)
# Assign label attributes to all columns
df <- datastep(mtcars[1:10, ],
keep = c("mpg", "cyl", "mpgcat"),
calculate = { mean_mpg = mean(mpg) },
attrib = list(mpg = dsattr(label = "Miles Per Gallon"),
cyl = dsattr(label = "Cylinders"),
mpgcat = dsattr(label = "Mileage Category")), {
if (mpg >= mean_mpg)
mpgcat <- "High"
else
mpgcat <- "Low"
})
# View attributes in dictionary
dictionary(df)
# # A tibble: 3 x 10
# Name Column Class Label Description Format Width Justify Rows NAs
# <chr> <chr> <chr> <chr> <chr> <lgl> <int> <chr> <int> <int>
# 1 df mpg numeric Miles Per Gallon NA NA NA NA 10 0
# 2 df cyl numeric Cylinders NA NA NA NA 10 0
# 3 df mpgcat character Mileage Category NA NA 4 NA 10 0As mentioned previously, R typically operates in a column-wise manner. That is, R processes data column-by-column. But what if you need to get a sum or mean across a row?
This situation is what led to the development of the data step array. The data step array allows you to define a list of columns and
iterate over the list inside a data step. Data step arrays are defined with the arrays parameter, which accepts a named list of dsarray() objects.
To see the array in action, we’ll use the AirPassengers sample data. This data shows international airline passengers by month between 1949 and 1960. The data looks like this:
AirPassengers
# Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
# 1949 112 118 132 129 121 135 148 148 136 119 104 118
# 1950 115 126 141 135 125 149 170 170 158 133 114 140
# 1951 145 150 178 163 172 178 199 199 184 162 146 166
# 1952 171 180 193 181 183 218 230 242 209 191 172 194
# 1953 196 196 236 235 229 243 264 272 237 211 180 201
# 1954 204 188 235 227 234 264 302 293 259 229 203 229
# 1955 242 233 267 269 270 315 364 347 312 274 237 278
# 1956 284 277 317 313 318 374 413 405 355 306 271 306
# 1957 315 301 356 348 355 422 465 467 404 347 305 336
# 1958 340 318 362 348 363 435 491 505 404 359 310 337
# 1959 360 342 406 396 420 472 548 559 463 407 362 405
# 1960 417 391 419 461 472 535 622 606 508 461 390 432This example illustrates how to create row totals, row means, and find the top month using a data step array. The array has an indexer to extract values. You can use the indexer to extract a single value or a subset of values. An empty indexer will return all the values in the array.
library(libr)
# Create AirPassengers Data Frame
df <- as.data.frame(t(matrix(AirPassengers, 12,
dimnames = list(month.abb, seq(1949, 1960)))),
stringsAsFactors = FALSE)
# Use datastep array to get year tot, mean, and top month
dat <- datastep(df,
arrays = list(months = dsarray(names(df))),
attrib = list(Tot = 0, Mean = 0, Top = ""),
drop = "mth",
{
Tot <- sum(months[])
Mean <- mean(months[])
for (mth in months) {
if (months[mth] == max(months[])) {
Top <- mth
}
}
})
dat
# Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Tot Mean Top
# 1949 112 118 132 129 121 135 148 148 136 119 104 118 1520 126.6667 Aug
# 1950 115 126 141 135 125 149 170 170 158 133 114 140 1676 139.6667 Aug
# 1951 145 150 178 163 172 178 199 199 184 162 146 166 2042 170.1667 Aug
# 1952 171 180 193 181 183 218 230 242 209 191 172 194 2364 197.0000 Aug
# 1953 196 196 236 235 229 243 264 272 237 211 180 201 2700 225.0000 Aug
# 1954 204 188 235 227 234 264 302 293 259 229 203 229 2867 238.9167 Jul
# 1955 242 233 267 269 270 315 364 347 312 274 237 278 3408 284.0000 Jul
# 1956 284 277 317 313 318 374 413 405 355 306 271 306 3939 328.2500 Jul
# 1957 315 301 356 348 355 422 465 467 404 347 305 336 4421 368.4167 Aug
# 1958 340 318 362 348 363 435 491 505 404 359 310 337 4572 381.0000 Aug
# 1959 360 342 406 396 420 472 548 559 463 407 362 405 5140 428.3333 Aug
# 1960 417 391 419 461 472 535 622 606 508 461 390 432 5714 476.1667 JulNext: Enhanced Equality