Kalinkin Alexey - 1 year ago 82

R Question

I have a math expression, for example:

`((2-x+3)^2+(x-5+7)^10)^0.5`

I need to replace the

`^`

`pow`

`(\([^()]*)*(\s*\([^()]*\)\s*)+([^()]*\))*`

I don't know how to improve this. Can you advice me something to solve that problem?

The expected output:

`pow(pow(2-x+3,2)+pow(x-5+7,10),0.5)`

Answer Source

Here is a solution that follows the parse tree recursively and replaces `^`

:

```
#parse the expression
#alternatively you could create it with
#expression(((2-x+3)^2+(x-5+7)^10)^0.5)
e <- parse(text = "((2-x+3)^2+(x-5+7)^10)^0.5")
#a recursive function
fun <- function(e) {
#check if you are at the end of the tree's branch
if (is.name(e) || is.atomic(e)) {
#replace ^
if (e == quote(`^`)) return(quote(pow))
return(e)
}
#follow the tree with recursion
for (i in seq_along(e)) e[[i]] <- fun(e[[i]])
return(e)
}
#deparse to get a character string
deparse(fun(e)[[1]])
#[1] "pow((pow((2 - x + 3), 2) + pow((x - 5 + 7), 10)), 0.5)"
```

This would be much easier if `rapply`

worked with expressions/calls.

**Edit:**

OP has asked regarding performance. It is very unlikely that performance is an issue for this task, but the regex solution is not faster.

```
library(microbenchmark)
microbenchmark(regex = {
v <- "((2-x+3)^2+(x-5+7)^10)^0.5"
x <- grepl("(\\(((?:[^()]++|(?1))*)\\))\\^(\\d*\\.?\\d+)", v, perl=TRUE)
while(x) {
v <- sub("(\\(((?:[^()]++|(?1))*)\\))\\^(\\d*\\.?\\d+)", "pow(\\2, \\3)", v, perl=TRUE);
x <- grepl("(\\(((?:[^()]++|(?1))*)\\))\\^(\\d*\\.?\\d+)", v, perl=TRUE)
}
},
BrodieG = {
deparse(f(parse(text = "((2-x+3)^2+(x-5+7)^10)^0.5")[[1]]))
},
Roland = {
deparse(fun(parse(text = "((2-x+3)^2+(x-5+7)^10)^0.5"))[[1]])
})
#Unit: microseconds
# expr min lq mean median uq max neval cld
# regex 321.629 323.934 335.6261 335.329 337.634 384.623 100 c
# BrodieG 238.405 246.087 255.5927 252.105 257.227 355.943 100 b
# Roland 211.518 225.089 231.7061 228.802 235.204 385.904 100 a
```

I haven't included the solution provided by @digEmAll, because it seems obvious that a solution with that many data.frame operations will be relatively slow.

**Edit2:**

Here is a version that also handles `sqrt`

.

```
fun <- function(e) {
#check if you are at the end of the tree's branch
if (is.name(e) || is.atomic(e)) {
#replace ^
if (e == quote(`^`)) return(quote(pow))
return(e)
}
if (e[[1]] == quote(sqrt)) {
#replace sqrt
e[[1]] <- quote(pow)
#add the second argument
e[[3]] <- quote(0.5)
}
#follow the tree with recursion
for (i in seq_along(e)) e[[i]] <- fun(e[[i]])
return(e)
}
e <- parse(text = "sqrt((2-x+3)^2+(x-5+7)^10)")
deparse(fun(e)[[1]])
#[1] "pow(pow((2 - x + 3), 2) + pow((x - 5 + 7), 10), 0.5)"
```