5.2: Creating or Modifying Variables Using Mutate()
- Page ID
- 8727
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Often we will want to either create a new variable based on an existing variable, or modify the value of an existing variable. Within the tidyverse, we do this using a function called mutate(). Let’s start with a toy example by creating a data frame containing a single variable.
toy_df <- data.frame(x = c(1,2,3,4))
glimpse(toy_df)## Observations: 4
## Variables: 1
## $ x <dbl> 1, 2, 3, 4Let’s say that we wanted to create a new variable called y that would contain the value of x multiplied by 10. We could do this using mutate() and then assign the result back to the same data frame:
toy_df <- toy_df %>%
# create a new variable called y that contains x*10
mutate(y = x*10)
glimpse(toy_df)## Observations: 4
## Variables: 2
## $ x <dbl> 1, 2, 3, 4
## $ y <dbl> 10, 20, 30, 40We could also overwrite a variable with a new value:
toy_df2 <- toy_df %>%
# create a new variable called y that contains x*10
mutate(y = y + 1)
glimpse(toy_df2)## Observations: 4
## Variables: 2
## $ x <dbl> 1, 2, 3, 4
## $ y <dbl> 11, 21, 31, 41We will use mutate() often so it’s an important function to understand.
Here we can use it with our example data frame to create a new variable that is the sum of several other variables.
myDataFrame <-
myDataFrame %>%
mutate(total = x + y + z)
kable(myDataFrame)| n | x | y | z | total |
|---|---|---|---|---|
| russ | 1 | 4 | 7 | 12 |
| lucy | 2 | 5 | 8 | 15 |
| jaclyn | 3 | 6 | 9 | 18 |
| tyler | 4 | 7 | 10 | 21 |
mutate() is a function that creates a new variable in a data frame using the existing variables. In this case, it creates a variable called total that is the sum of the existing variables x, y, and z.
5.2.1 Remove a column using the select() function
Adding a minus sign to the name of a variable within the select() command will remove that variable, leaving all of the others.
myDataFrame <-
myDataFrame %>%
dplyr::select(-total)
kable(myDataFrame)| n | x | y | z |
|---|---|---|---|
| russ | 1 | 4 | 7 |
| lucy | 2 | 5 | 8 |
| jaclyn | 3 | 6 | 9 |
| tyler | 4 | 7 | 10 |