How to obtain all the variables in an R code expression?

Question

How can an expression in R be decoded to get all the variables involved?

For example if you have:

z<-x+y;

get_all_variables(z);

[1] \'x\' \'y\'
         


        

Answer 1

You can use all.vars, but you need to quote your expression:

all.vars(quote(x + y))
# [1] "x" "y"

You can't just use z as you describe it contains an evaluated expression (i.e. the result of the expression), not the expression itself. You can write a function that removes one step:

get_all_variables <- function(expr) all.vars(substitute(expr))
get_all_variables(x + y)
# [1] "x" "y"

But you will not be able to recover the expression from z, unless you create z by z <- quote(x + y) or some such.

If you have the expression in a string, then you can use @sunny's technique combined with all.vars:

all.vars(parse(text="z <- x + y"))
# [1] "z" "x" "y"

Though obviously you get z as well. As always, don't evaluate arbitrary text with parse in case someone Bobby Tables you.

Answer 2

So far as I know, you would need to store the expression in a string. Then you can parse it with getParseData:

txt <- "z<-x+y"

sf <- srcfile("txt")   
df = getParseData(sf)    
df$text[df$token=="SYMBOL"][2:3]

For further edification, here is what the result of getParseData looks like in its entirety:

   line1 col1 line2 col2 id parent       token terminal text
11     1    1     1    6 11      0        expr    FALSE     
1      1    1     1    1  1      3      SYMBOL     TRUE    z
3      1    1     1    1  3     11        expr    FALSE     
2      1    2     1    3  2     11 LEFT_ASSIGN     TRUE   <-
10     1    4     1    6 10     11        expr    FALSE     
4      1    4     1    4  4      6      SYMBOL     TRUE    x
6      1    4     1    4  6     10        expr    FALSE     
5      1    5     1    5  5     10         '+'     TRUE    +
7      1    6     1    6  7      9      SYMBOL     TRUE    y
9      1    6     1    6  9     10        expr    FALSE