How can one apply ifelse to alter a column in data.table while grouping it?

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To apply ifelse to alter a column in data.table while grouping it, you can use the := operator to create a new column or replace an existing column based on a condition. Here's an example:

library(data.table)

# create sample data.table
dt <- data.table(x = c("A", "B", "C", "D"), y = c(1, 2, 3, 4))

# apply ifelse to alter 'y' column based on condition of 'x' column
dt[, y_new := ifelse(x == "A", y * 2, y)]

# output updated data.table
dt

In this example, we're altering the y column based on the condition that x == "A". If the condition is met, we're multiplying y by 2 and otherwise keeping it the same. The resulting updated data.table includes a new column y_new with the altered values.