Example: Data Frame Basics

Data frames are one of the most heavily used data structures in R.

Creation of a data frame

A data frame is created from scratch by supplying vectors to the data.frame function. Here are some examples:

x <- c(2.5, 3.5, 3.4)
y <- c(5, 10, 1)
my_df <- data.frame(x, y)
my_df

##     x  y
## 1 2.5  5
## 2 3.5 10
## 3 3.4  1

colnames(my_df) <- c("Floats", "Integers")

my_other_df <- data.frame(X = c(2, 3, 4), Y = c("A", "B", "C"))
my_other_df

##   X Y
## 1 2 A
## 2 3 B
## 3 4 C

The colnames function allows you to supply column names to an existing data frame. You can also set column names directly in data.frame(), as shown by the named arguments X and Y above.

Dimensions of a data frame

To get the dimensions of a data frame, use the ncol (number of columns), nrow (number of rows) or str (structure) function:

ncol(my_other_df)

## [1] 2

nrow(my_other_df)

## [1] 3

str(my_other_df)

## 'data.frame':	3 obs. of  2 variables:
##  $ X: num  2 3 4
##  $ Y: chr  "A" "B" "C"

Displaying and accessing the content of a data frame

Access values in a data frame using indices in square brackets (e.g., df[3,4]) or a column name after a $ sign (e.g., df$columnName). Here is an example:

my_other_df[1,]  # Shows first row

##   X Y
## 1 2 A

my_other_df[,2]  # Shows second column

## [1] "A" "B" "C"

my_other_df$Y  # Shows second column

## [1] "A" "B" "C"

A data frame has rows and columns. With two indices, use df[rows, columns]: the first index selects rows, and the second selects columns. Leaving an index empty selects all entries in that dimension. With a single index, df[index] selects columns.

In the examples below, df represents a data frame, x represents a row number, and y represents a column number.

Here are some possible combinations:

  • Single row, all columns: df[x,]
  • Single column, all rows: df[,y]
  • Single row and column: df[x,y]
  • All except one row, all columns: df[-x,]
  • Selected rows, all columns: df[c(x1, x2, x3),]
  • Consecutive rows, all columns: df[c(x1:x2),]

Use positive indices to select rows or columns and negative indices to exclude them. To select or exclude several rows or columns, supply a vector of indices using c().

my_other_df[c(1,3),]  # Shows rows 1 and 3

##   X Y
## 1 2 A
## 3 4 C

my_other_df[c(1,2),]  # Shows rows 1 to 2

##   X Y
## 1 2 A
## 2 3 B

To display the first or last rows, use head or tail. By default, these functions display six rows. You can change this number with the second argument. Let us look at the first two rows:

head(my_other_df, 2)

##   X Y
## 1 2 A
## 2 3 B

And now let’s look at the last two rows:

tail(my_other_df, 2)

##   X Y
## 2 3 B
## 3 4 C

Changing, adding or deleting an element of a data frame

In order to change an element of a data frame (individual value or entire vectors like rows or columns), you have to access it following the logic above. To add or delete a column, you have to supply/remove a vector to the specified position.

Other more specific changes will be covered later.

# overwrite an element
my_other_df$X[3] <- 400  # same as my_other_df[3,1] <- 400
my_other_df

##     X Y
## 1   2 A
## 2   3 B
## 3 400 C

# change an entire column
my_other_df[,1] <- c("200", "300", "401")  # same as my_other_df$X <- c("200", "300", "401")
my_other_df

##     X Y
## 1 200 A
## 2 300 B
## 3 401 C

The values are in quotation marks, so X becomes a character column.

# add a new column
my_other_df$z <- c(255, 300, 100)
my_other_df

##     X Y   z
## 1 200 A 255
## 2 300 B 300
## 3 401 C 100

# delete a column
my_other_df$z <- NULL
my_other_df

##     X Y
## 1 200 A
## 2 300 B
## 3 401 C

Assigning NULL to a data-frame column deletes that column.

For more information, see the accompanying Base R course on data types and object types. Package documentation is another useful resource.

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