[R] how to get higher derivatives with "deriv"

[Wu Gong]

## specify the function string
f.str <- "x^alpha"

## higher derivatives 
DD <- function(f.str, x = 2, alpha=3,order = 1){
expr.s <- parse(text=f.str)
while(order>=1) {
expr.s <- D(expr.s,"x")
order <- order-1}
eval(expr.s)
}

compute
DD(f.str,x=1,alpha=0.5,order=1)

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A R learner.
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