Alex Alex - 4 months ago 28

R Question

I need to find derivative of function defined like

`f<-function(x){x^2}`

After that I want to have a derivative function

`g`

`g(5) # entered`

10 # answer

without Yacas or similar heavy software. I find it surprising that R can find the derivative of

`x^2`

`f<-function(x){x^2}`

I have tried to do something like this:

`f<-function(x){x^2}`

g<-Deriv(f,"x")

`Error in list2env(list(), NULL, <environment>) :`

names(x) must be a character vector of the same length as x

I have also tried this:

`> f1<-'x^2'`

> g<-Deriv(f1,"x")

> g

#Result:

"2*x"

Can I transform my function f to a one like f1 and reverse the process?

Is there another way?

Answer

You could define your function as an expression:

```
f <- expression(x^2)
```

Such an expression can be treated like a function by using `eval()`

:

```
#> eval(f,list(x=2))
#[1] 4
```

In this case, the advantage of an expression instead of a function is that the former allows for the use of symbolic calculus to obtain the first derivative:

```
g <- D(f,"x")
#> g
#2 * x
```

To evaluate this expression of the first derivative (more precisely its class is a `call`

), you can use the same syntax as was done before for `f`

:

```
#> eval(g,list(x=5))
#[1] 10
```