mutate

This lesson is called mutate, part of the Fundamentals of R course. This lesson is called mutate, part of the Fundamentals of R course.

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# mutate

![](slides/images/mutate.png)

We use `mutate` we make new variables or change existing ones. 

Create a **new variable with a specific value**

Create a new variable called `completed_survey` and make all responses to it "Yes".

```{r}
nhanes %>%
  mutate(completed_survey = "Yes")
```

Copy your code from above and then add a line where you select only the `completed_survey` variable. Don't forget the pipe (%>%)!

```{r}
nhanes %>%
  mutate(completed_survey = "Yes") %>% 
  select(completed_survey)
```

Create a **new variable based on other variables**

Create a new variable called `pct_days_phys_health_bad` and calculate it as the percentage of self-reported days of bad physical health in the last 30 days. Remember that the `days_phys_hlth_bad` variable is a measure of the **number** of self-reported days of bad physical health in the last 30 days. Then, use `select` to show the `days_phys_hlth_bad` and `pct_days_phys_health_bad` variables.

```{r}
nhanes %>% 
  mutate(pct_days_phys_health_bad = days_phys_hlth_bad / 30) %>% 
  select(days_phys_hlth_bad, pct_days_phys_health_bad)
```

Change an **existing variable**

Round the `height` variable to a whole number. Then, use `select` to show only the `height` variable.

```{r}
nhanes %>%
  mutate(height = round(height, digits = 0)) %>% 
  select(height)
```

Complete the mutate sections of the data-wrangling-and-analysis-exercises.Rmd file.

Learn More

General Data Wrangling and Analysis Resources

Because most material that discusses data wrangling and analysis with the dplyr packges does so in a way that covers all of the verbs discussed in this course, I have chosen not to separate them by lesson. Instead, here are some helpful resources for learning more about all of the tidyverse verbs discussed in this course:

Chapter 5 of R for Data Science

RStudio Cloud primer on working with data

Tidyverse for Beginners by Danielle Navarro

Learning Statistics with R by Danielle Navarro

Introduction to the Tidyverse by Alison Hill

A gRadual intRoduction to data wRangling by Chester Ismay and Ted Laderas

Working in the Tidyverse by Desi Quintans and Jeff Powell

Christine Monnier video tutorials on dplyr

Have any questions? Put them below and we will help you out!

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Bohdanna Kinasevych • March 27, 2021

how do I add multiple columns together to create a new variable without having to write out each existing variable in the mutate function. For example if I have 10 questions on a survey and I want to create a composite score based on a sum of these 10 questions, is there a way to do this without having to write out: var1 + var2 + var3 .... + var10?

Eduardo Rodriguez • March 30, 2021

Hi David, how do I change a value format? For example, in the course assignment we divide by 30 and get a long decimal value. How would I format it so that it only shows two decimal places or even, out of curiosity, as a percentage?

Harold Stanislaw • March 31, 2021

Question regarding overwriting a variable with itself. I'm always nervous about doing this, in case I mess up. On the other hand, I can see where creating new variables and keeping the old ones can easily get out of hand and/or require using the drop pipe a lot. If I want to overwrite a variable onto itself is there an easy way to undo the mutate if I make a mistake?

Lina Khan • September 27, 2021

When the values are changed, like rounding values, how come it doesn't show in the dataset tab? Or, have I forgotten something? For example, the output still shows the rounded values when I select the variable. But other than clean_names, the changes don't appear in the dataset. Thanks!

David Keyes Founder • September 28, 2021

I typically keep both a numeric and a character variable. Here's a quick explanation of why.

Lindsay Quarles • October 21, 2021

Can you help me better understand the round function? I don't understand the digits part? Does it only apply to decimals? What if you want to round to the nearest whole number what would you put? Or to round to the nearest 10?

Yenn Lee • July 19, 2022

Hi, I have a column that I would like to split into three. The data in the existing column looks like the following, and I would like it to feed into three new columns: id, bio, and username.

{"id":"1234567","bio":"John Smith, first-year undergraduate in Physics","username":"JSAstronaut"}

All rows follow this exact format (although the bio part may be longer), so I feel there must be a simple way to do, and I was just wondering if anyone has suggestions. Or is this going to be covered at a later point in the courses? So far I have tried the separate function in tidyr, but it hasn't quite turned out to be the way I hoped.

I'd appreciate any pointers. Thank you!

Tatiana Bustos • July 27, 2022

Hi What if I wanted to create a new variable for a subset of groups with one type and then another set of groups with another For example, if I wanted to only create a new variable for "Females" with country = United States, but then select only "Males" with country = Canada. I tried to do the following as a best guess nhanes %>% mutate(completed_survey = "Yes") select(contains("female"))