How to use R mutate_all or mutate_if to derive variables for rows selected with a condition?

To use mutate_all or mutate_if to derive variables for rows selected with a condition, you can use the ifelse() function within the mutate statement.

Here's an example:

library(dplyr)

# Create sample data frame
df <- data.frame(
  a = c(1, 2, 3, 4),
  b = c(5, 6, 7, 8),
  c = c(9, 10, 11, 12)
)

# Use mutate_all to multiply all columns by 2 for rows where a is greater than 2
df_new <- df %>% 
  mutate_all(
    ~ifelse(a > 2, . * 2, .)
  )

# Use mutate_if to multiply numeric columns by 2 for rows where a is greater than 2
df_new2 <- df %>% 
  mutate_if(
    is.numeric,
    ~ifelse(a > 2, . * 2, .)
  )

In this example, we first create a sample data frame df with three columns (a, b, c) and four rows.

Next, we use mutate_all to multiply all columns by 2 for rows where the value in column "a" is greater than 2. We achieve this by using the ~ifelse(a > 2, . * 2, .) syntax within the mutate_all statement. This means that for each column, if the value in column "a" is greater than 2, then the column value is multiplied by 2. Otherwise, the column value remains the same.

We can also achieve the same result using mutate_if which allows us to apply the function only to selected columns (in this case numeric columns). The is.numeric function is used to select only the numeric columns.

To use mutate_all to derive new values for the whole data frame only for values selected with a condition, you can use the ifelse() function within the mutate_all statement, and specify the condition in the first argument of ifelse().

Here's an example:

library(dplyr)

# Create sample data frame
df <- data.frame(
  a = c(1, 2, 3, 4),
  b = c(5, 6, 7, 8),
  c = c(9, 10, 11, 12)
)

# Use mutate_all to multiply all values by 2 for values greater than 5
df_new <- df %>%
  mutate_all(
    ~ifelse(. > 5, . * 2, .)
  )

In this example, we first create a sample data frame df with three columns (a, b, c) and four rows.

Next, we use mutate_all to multiply all values by 2 for values greater than 5. We achieve this by using the ~ifelse(. > 5, . * 2, .) syntax within the mutate_all statement. This means that for each value in the data frame, if the value is greater than 5, then the value is multiplied by 2. Otherwise, the value remains the same.

Note that the . in the ifelse() function refers to the current value being evaluated. The mutate_all function applies this function to every element in the data frame.