How to write an equation as a function argument? - r

Question

The function I created runs correctly on my computer. However, I'm trying to check for errors by devtools :: check_built () and I'm having problems with the myeq argument. The letters d and h represent my columns simple1 $ Diameter (cm) and simple1 $ Height (m), respectively.
indvol(x = simple1, mens="plot", myeq = 0.000065661*d^(2.475293)*h^(0.300022))
> head(simple1)
Plot Individual Specie Height (m) Diameter (cm)
1 1 1 Cariocar braziliense 7.5 22.60
2 1 1 Cariocar braziliense 7.5 25.78
3 1 1 Cariocar braziliense 7.5 46.15
4 1 1 Cariocar braziliense 7.5 9.55
5 1 2 Qualya parvifora 2.0 5.73
6 1 3 Magonia pubescens 4.0 5.73
The following error appears:
Error in indvol(x = simple1, mens = "plot", myeq = 6.5661e-05 * d^(2.475293) * :
object 'd' not found
Execution halted
The part dealing with the myeq argument within the function is:
if(mens=="plot"){
colnames(x)[5]<-"d"
colnames(x)[4]<-"h"
d<-x[,5]
h<-x[,4]
}
x$`Volume (m3)` <- eval(substitute(myeq), envir=x); x
Would anyone know how to fix this problem?

Answer 1

You have to capture the expression without evaluating it. You can do this with match.call():
indvol <- function(x, mens, myeq)
{
mc <- as.list(match.call()[-1])
if (mens == "plot")
{
colnames(x)[5] <- "d"
colnames(x)[4] <- "h"
d <- x[, 5]
h <- x[, 4]
}
x$`Volume (m3)` <- eval(mc$myeq, envir = x)
return(x)
}
So the function now works as expected:
indvol(x = simple1, mens="plot", myeq = 0.000065661*d^(2.475293)*h^(0.300022))
#> Plot Individual Specie h d Volume (m3)
#> 1 1 1 Cariocar braziliense 7.5 22.60 0.270184396
#> 2 1 1 Cariocar braziliense 7.5 25.78 0.374268984
#> 3 1 1 Carioca braziliense 7.5 46.15 1.581822308
#> 4 1 1 Carioca braziliense 7.5 9.55 0.032036171
#> 5 1 2 Qualya parvifora 2.0 5.73 0.006085243
#> 6 1 3 Magoni pubescens 4.0 5.73 0.007491927
As a design point though, it might be better to allow users to just use the column names of the passed data frame into the function.
Data
simple1 <- structure(list(Plot = c(1L, 1L, 1L, 1L, 1L, 1L), Individual = c(1L,
1L, 1L, 1L, 2L, 3L), Specie = c("Cariocar braziliense", "Cariocar braziliense",
"Carioca braziliense", "Carioca braziliense", "Qualya parvifora",
"Magoni pubescens"), `Height (m)` = c(7.5, 7.5, 7.5, 7.5, 2, 4),
`Diameter (cm)` = c(22.6, 25.78, 46.15, 9.55, 5.73, 5.73)),
class = "data.frame", row.names = c("1", "2", "3", "4", "5", "6"))