Creating a new column to a data frame using a formula from another variable

Question

I want to create a new column to a data frame using a formula from another variable.
Example:
I have a data set \"aa\" is;

x    y 
2    3 
4             


        

Answer 1

you can use mutate from the package dplyr

library(dplyr)
aa <- aa %>% mutate(z = x+y-2)

Hope it helps.

Answer 2

You should probably read some basic tutorials on R other than An Introduction to R as despite what is written there the $ notation is more sensible and easier to understand than attach/detach. Try this in the meantime.

aa <- data.frame(x = c(2, 4, 6), y = c(3, 5, 7))

Which gives:

> aa
  x y
1 2 3
2 4 5
3 6 7

Then enter:

aa$z <- (aa$x + aa$y) - 2

Which gives:

> aa
  x y  z
1 2 3  3
2 4 5  7
3 6 7 11

Answer 3

If you want to evaluate an expression in the context, of a data frame, you can use with and within.

aa$z <- with(aa, x + y - 2)

or

aa <- within(aa, z <- x + y - 2)

Or, if your expression is in the form of a text string (you should see if there are other ways to write your code; evaluating arbitrary text strings can lead to lots of problems):

aa$z <- eval(parse(text="x + y - 2"), aa)