calculating derivative in R using D function in mosaic package

Question

I installed mosaic package available in R to calculate derivative by typing install.packages('mosaic'). I created a function using makeFun and then i tried to calculate derivative in the following way

y1 <- makeFun(a +b *x  ~x, a=2, b=2)
dy1.dx <- D(a + b* x ~ x, a=2, b=2)
dy1.dx

but the console shows

" dy1.dx <- D(a+b*x~x, a=2, b=2)
Error in D(a + b * x ~ x, a = 2, b = 2) : unused arguments (a = 2, b = 2)"

How can I correct it?

Answer 1

using the base R's stats package. No need to install additional packages like mosaic .

D will give the derivative of expression, so using expression() function, we create an expression and pass it to the D function.

Then eval will evaluate the expression and substitute will substitute values of a and b in the expression.

get the derivative for an expression with respect to x:

stats::D(expression(a + b * x), "x")
# b

evaluate the expression after substituting with values in the derivative. b is substituted with the value 2.

eval(substitute( stats::D(expression(a + b * x), "x"), list(a=2, b = 2) ))
# [1] 2

Another example:

stats::D(expression(a + a*b * x), "x")
# a * b
eval(substitute( D(expression(a + a*b * x), "x"), list(a=3, b = 2) ))
# 3 * 2
eval(eval(substitute( D(expression(a + a*b * x), "x"), list(a=3, b = 2) )))
# 6

Answer 2

D used in your example is from library(mosaicCalc) and not from base stat. Install and call the library. Your function works normally.

require(mosaicCalc)
#> Loading required package: mosaicCalc
#> Warning: package 'mosaicCalc' was built under R version 3.6.3
#> Loading required package: mosaicCore
#> Warning: package 'mosaicCore' was built under R version 3.6.3
#> Registered S3 method overwritten by 'mosaic':
#>   method                           from   
#>   fortify.SpatialPolygonsDataFrame ggplot2
#> 
#> Attaching package: 'mosaicCalc'
#> The following object is masked from 'package:stats':
#> 
#>     D
dy1.dx <- D(a + b * x ~ x, a = 2, b = 2)
dy1.dx
#> function (x, a = 2, b = 2) 
#> b