A data frame is the most common way of storing data in R and, generally, is the data structure most often used for data analyses. Under the hood, a data frame is a list of equal-length vectors. Each element of the list can be thought of as a column and the length of each element of the list is the number of rows. As a result, data frames can store different classes of objects in each column (i.e. numeric, character, factor). In essence, the easiest way to think of a data frame is as an Excel worksheet that contains columns of different types of data but are all of equal length rows. In this section you will learn how to perform basic operations with a data frame to include:
Data frames are usually created by reading in a dataset using the read.table() or read.csv(); this will be covered in the importing and scraping data tutorials. However, data frames can also be created explicitly with the data.frame() function or they can be coerced from other types of objects like lists. In this case I’ll create a simple data frame df and assess its basic structure:
df <- data.frame(col1 = 1:3,
col2 = c("this", "is", "text"),
col3 = c(TRUE, FALSE, TRUE),
col4 = c(2.5, 4.2, pi))
# assess the structure of a data frame
str(df)
## 'data.frame': 3 obs. of 4 variables:
## $ col1: int 1 2 3
## $ col2: Factor w/ 3 levels "is","text","this": 3 1 2
## $ col3: logi TRUE FALSE TRUE
## $ col4: num 2.5 4.2 3.14
# number of rows
nrow(df)
## [1] 3
# number of columns
ncol(df)
## [1] 4
Note how col2 in df was converted to a column of factors. This is because there is a default setting in data.frame() that converts character columns to factors. We can turn this off by setting the stringsAsFactors = FALSE argument:
df <- data.frame(col1 = 1:3,
col2 = c("this", "is", "text"),
col3 = c(TRUE, FALSE, TRUE),
col4 = c(2.5, 4.2, pi),
stringsAsFactors = FALSE)
# note how col2 now is of a character class
str(df)
## 'data.frame': 3 obs. of 4 variables:
## $ col1: int 1 2 3
## $ col2: chr "this" "is" "text"
## $ col3: logi TRUE FALSE TRUE
## $ col4: num 2.5 4.2 3.14
We can also convert pre-existing structures to a data frame. The following illustrates how we can turn multiple vectors, a list, or a matrix into a data frame:
v1 <- 1:3
v2 <-c("this", "is", "text")
v3 <- c(TRUE, FALSE, TRUE)
# convert same length vectors to a data frame using data.frame()
data.frame(col1 = v1, col2 = v2, col3 = v3)
## col1 col2 col3
## 1 1 this TRUE
## 2 2 is FALSE
## 3 3 text TRUE
# convert a list to a data frame using as.data.frame()
l <- list(item1 = 1:3, item2 = c("this", "is", "text"), item3 = c(2.5, 4.2, 5.1))
l
## $item1
## [1] 1 2 3
##
## $item2
## [1] "this" "is" "text"
##
## $item3
## [1] 2.5 4.2 5.1
as.data.frame(l)
## item1 item2 item3
## 1 1 this 2.5
## 2 2 is 4.2
## 3 3 text 5.1
# convert a matrix to a data frame using as.data.frame()
m1 <- matrix(1:12, nrow = 4, ncol = 3)
m1
## [,1] [,2] [,3]
## [1,] 1 5 9
## [2,] 2 6 10
## [3,] 3 7 11
## [4,] 4 8 12
as.data.frame(m1)
## V1 V2 V3
## 1 1 5 9
## 2 2 6 10
## 3 3 7 11
## 4 4 8 12
We can leverage the cbind() function for adding columns to a data frame. Note that one of the objects being combined must already be a data frame otherwise cbind() could produce a matrix.
df
## col1 col2 col3 col4
## 1 1 this TRUE 2.500000
## 2 2 is FALSE 4.200000
## 3 3 text TRUE 3.141593
# add a new column
v4 <- c("A", "B", "C")
cbind(df, v4)
## col1 col2 col3 col4 v4
## 1 1 this TRUE 2.500000 A
## 2 2 is FALSE 4.200000 B
## 3 3 text TRUE 3.141593 C
We can also use the rbind() function to add data frame rows together. However, severe caution should be taken because this can cause changes in the classes of the columns. For instance, our data frame df currently consists of an integer, character, logical, and numeric variables.
df
## col1 col2 col3 col4
## 1 1 this TRUE 2.500000
## 2 2 is FALSE 4.200000
## 3 3 text TRUE 3.141593
str(df)
## 'data.frame': 3 obs. of 4 variables:
## $ col1: int 1 2 3
## $ col2: chr "this" "is" "text"
## $ col3: logi TRUE FALSE TRUE
## $ col4: num 2.5 4.2 3.14
If we attempt to add a row using rbind() and c() it converts all columns to a character class. This is because all elements in the vector created by c() must be of the same class so they are all coerced to the character class which coerces all the variables in the data frame to the character class.
df2 <- rbind(df, c(4, "R", F, 1.1))
df2
## col1 col2 col3 col4
## 1 1 this TRUE 2.5
## 2 2 is FALSE 4.2
## 3 3 text TRUE 3.14159265358979
## 4 4 R FALSE 1.1
str(df2)
## 'data.frame': 4 obs. of 4 variables:
## $ col1: chr "1" "2" "3" "4"
## $ col2: chr "this" "is" "text" "R"
## $ col3: chr "TRUE" "FALSE" "TRUE" "FALSE"
## $ col4: chr "2.5" "4.2" "3.14159265358979" "1.1"
To add rows appropriately, we need to convert the items being added to a data frame and make sure the columns are the same class as the original data frame.
adding_df <- data.frame(col1 = 4, col2 = "R", col3 = FALSE, col4 = 1.1,
stringsAsFactors = FALSE)
df3 <- rbind(df, adding_df)
df3
## col1 col2 col3 col4
## 1 1 this TRUE 2.500000
## 2 2 is FALSE 4.200000
## 3 3 text TRUE 3.141593
## 4 4 R FALSE 1.100000
str(df3)
## 'data.frame': 4 obs. of 4 variables:
## $ col1: num 1 2 3 4
## $ col2: chr "this" "is" "text" "R"
## $ col3: logi TRUE FALSE TRUE FALSE
## $ col4: num 2.5 4.2 3.14 1.1
There are better ways to join data frames together than to use cbind() and rbind(). These are covered later on in the transforming your data with dplyr tutorial.
Similar to matrices, data frames will have a dimension attribute. In addition, data frames can also have additional attributes such as row names, column names, and comments. We can illustrate with data frame df.
# basic matrix
df
## col1 col2 col3 col4
## 1 1 this TRUE 2.500000
## 2 2 is FALSE 4.200000
## 3 3 text TRUE 3.141593
dim(df)
## [1] 3 4
attributes(df)
## $names
## [1] "col1" "col2" "col3" "col4"
##
## $row.names
## [1] 1 2 3
##
## $class
## [1] "data.frame"
Currently df does not have row names but we can add them with rownames():
# add row names
rownames(df) <- c("row1", "row2", "row3")
df
## col1 col2 col3 col4
## row1 1 this TRUE 2.500000
## row2 2 is FALSE 4.200000
## row3 3 text TRUE 3.141593
attributes(df)
## $names
## [1] "col1" "col2" "col3" "col4"
##
## $row.names
## [1] "row1" "row2" "row3"
##
## $class
## [1] "data.frame"
We can also also change the existing column names by using colnames() or names():
# add/change column names with colnames()
colnames(df) <- c("col_1", "col_2", "col_3", "col_4")
df
## col_1 col_2 col_3 col_4
## row1 1 this TRUE 2.500000
## row2 2 is FALSE 4.200000
## row3 3 text TRUE 3.141593
attributes(df)
## $names
## [1] "col_1" "col_2" "col_3" "col_4"
##
## $row.names
## [1] "row1" "row2" "row3"
##
## $class
## [1] "data.frame"
# add/change column names with names()
names(df) <- c("col.1", "col.2", "col.3", "col.4")
df
## col.1 col.2 col.3 col.4
## row1 1 this TRUE 2.500000
## row2 2 is FALSE 4.200000
## row3 3 text TRUE 3.141593
attributes(df)
## $names
## [1] "col.1" "col.2" "col.3" "col.4"
##
## $row.names
## [1] "row1" "row2" "row3"
##
## $class
## [1] "data.frame"
Lastly, just like vectors, lists, and matrices, we can add a comment to a data frame without affecting how it operates.
# adding a comment attribute
comment(df) <- "adding a comment to a data frame"
attributes(df)
## $names
## [1] "col.1" "col.2" "col.3" "col.4"
##
## $row.names
## [1] "row1" "row2" "row3"
##
## $class
## [1] "data.frame"
##
## $comment
## [1] "adding a comment to a data frame"
Data frames possess the characteristics of both lists and matrices: if you subset with a single vector, they behave like lists and will return the selected columns with all rows; if you subset with two vectors, they behave like matrices and can be subset by row and column:
df
## col.1 col.2 col.3 col.4
## row1 1 this TRUE 2.500000
## row2 2 is FALSE 4.200000
## row3 3 text TRUE 3.141593
# subsetting by row numbers
df[2:3, ]
## col.1 col.2 col.3 col.4
## row2 2 is FALSE 4.200000
## row3 3 text TRUE 3.141593
# subsetting by row names
df[c("row2", "row3"), ]
## col.1 col.2 col.3 col.4
## row2 2 is FALSE 4.200000
## row3 3 text TRUE 3.141593
# subsetting columns like a list
df[c("col.2", "col.4")]
## col.2 col.4
## row1 this 2.500000
## row2 is 4.200000
## row3 text 3.141593
# subsetting columns like a matrix
df[ , c("col.2", "col.4")]
## col.2 col.4
## row1 this 2.500000
## row2 is 4.200000
## row3 text 3.141593
# subset for both rows and columns
df[1:2, c(1, 3)]
## col.1 col.3
## row1 1 TRUE
## row2 2 FALSE
# use a vector to subset
v <- c(1, 2, 4)
df[ , v]
## col.1 col.2 col.4
## row1 1 this 2.500000
## row2 2 is 4.200000
## row3 3 text 3.141593
Note that subsetting data frames with the [ operator will simplify1 the results to the lowest possible dimension. To avoid this you can introduce the drop = FALSE argument:
# simplifying results in a named vector
df[, 2]
## [1] "this" "is" "text"
# preserving results in a 3x1 data frame
df[, 2, drop = FALSE]
## col.2
## row1 this
## row2 is
## row3 text
You can also subset data frames based on conditional statements. To illustrate we’ll use the built in mtcars data frame:
head(mtcars)
## mpg cyl disp hp drat wt qsec vs am gear carb
## Mazda RX4 21.0 6 160 110 3.90 2.620 16.46 0 1 4 4
## Mazda RX4 Wag 21.0 6 160 110 3.90 2.875 17.02 0 1 4 4
## Datsun 710 22.8 4 108 93 3.85 2.320 18.61 1 1 4 1
## Hornet 4 Drive 21.4 6 258 110 3.08 3.215 19.44 1 0 3 1
## Hornet Sportabout 18.7 8 360 175 3.15 3.440 17.02 0 0 3 2
## Valiant 18.1 6 225 105 2.76 3.460 20.22 1 0 3 1
If we want to subset mtcars for all rows where mpg is greater than 20 we can perform this in two ways:
# using brackets
mtcars[mtcars$mpg > 20, ]
## mpg cyl disp hp drat wt qsec vs am gear carb
## Mazda RX4 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4
## Mazda RX4 Wag 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4
## Datsun 710 22.8 4 108.0 93 3.85 2.320 18.61 1 1 4 1
## Hornet 4 Drive 21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1
## Merc 240D 24.4 4 146.7 62 3.69 3.190 20.00 1 0 4 2
## Merc 230 22.8 4 140.8 95 3.92 3.150 22.90 1 0 4 2
## Fiat 128 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1
## Honda Civic 30.4 4 75.7 52 4.93 1.615 18.52 1 1 4 2
## Toyota Corolla 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1
## Toyota Corona 21.5 4 120.1 97 3.70 2.465 20.01 1 0 3 1
## Fiat X1-9 27.3 4 79.0 66 4.08 1.935 18.90 1 1 4 1
## Porsche 914-2 26.0 4 120.3 91 4.43 2.140 16.70 0 1 5 2
## Lotus Europa 30.4 4 95.1 113 3.77 1.513 16.90 1 1 5 2
## Volvo 142E 21.4 4 121.0 109 4.11 2.780 18.60 1 1 4 2
# using the simplified subset function
subset(mtcars, mpg > 20)
## mpg cyl disp hp drat wt qsec vs am gear carb
## Mazda RX4 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4
## Mazda RX4 Wag 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4
## Datsun 710 22.8 4 108.0 93 3.85 2.320 18.61 1 1 4 1
## Hornet 4 Drive 21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1
## Merc 240D 24.4 4 146.7 62 3.69 3.190 20.00 1 0 4 2
## Merc 230 22.8 4 140.8 95 3.92 3.150 22.90 1 0 4 2
## Fiat 128 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1
## Honda Civic 30.4 4 75.7 52 4.93 1.615 18.52 1 1 4 2
## Toyota Corolla 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1
## Toyota Corona 21.5 4 120.1 97 3.70 2.465 20.01 1 0 3 1
## Fiat X1-9 27.3 4 79.0 66 4.08 1.935 18.90 1 1 4 1
## Porsche 914-2 26.0 4 120.3 91 4.43 2.140 16.70 0 1 5 2
## Lotus Europa 30.4 4 95.1 113 3.77 1.513 16.90 1 1 5 2
## Volvo 142E 21.4 4 121.0 109 4.11 2.780 18.60 1 1 4 2
We can add on to the conditional statement if we want to filter for multiple conditions. You can see how the subset() function helps to simplify the process by only requiring you to state the data frame once and then directly call the variables to perform the condition on.
# using brackets
mtcars[mtcars$mpg > 20 & mtcars$cyl == 6, ]
## mpg cyl disp hp drat wt qsec vs am gear carb
## Mazda RX4 21.0 6 160 110 3.90 2.620 16.46 0 1 4 4
## Mazda RX4 Wag 21.0 6 160 110 3.90 2.875 17.02 0 1 4 4
## Hornet 4 Drive 21.4 6 258 110 3.08 3.215 19.44 1 0 3 1
# using the simplified subset function
subset(mtcars, mpg > 20 & cyl == 6)
## mpg cyl disp hp drat wt qsec vs am gear carb
## Mazda RX4 21.0 6 160 110 3.90 2.620 16.46 0 1 4 4
## Mazda RX4 Wag 21.0 6 160 110 3.90 2.875 17.02 0 1 4 4
## Hornet 4 Drive 21.4 6 258 110 3.08 3.215 19.44 1 0 3 1
And if we want to perform this filtering along with return only specified columns we simply state the columns we want to return.
# using brackets
mtcars[mtcars$mpg > 20 & mtcars$cyl == 6, c("mpg", "cyl", "wt")]
## mpg cyl wt
## Mazda RX4 21.0 6 2.620
## Mazda RX4 Wag 21.0 6 2.875
## Hornet 4 Drive 21.4 6 3.215
# using the simplified subset function
subset(mtcars, mpg > 20 & cyl == 6, c("mpg", "cyl", "wt"))
## mpg cyl wt
## Mazda RX4 21.0 6 2.620
## Mazda RX4 Wag 21.0 6 2.875
## Hornet 4 Drive 21.4 6 3.215
Its important to understand the difference between simplifying and preserving subsetting. Simplifying subsets returns the simplest possible data structure that can represent the output. Preserving subsets keeps the structure of the output the same as the input. See Hadley Wickham’s section on Simplifying vs. Preserving Subsetting to learn more. ↩