# User defined function with operators in R

I want to use a user specified function and apply the function to a list of values. I am assuming that the user will provide a "formula" as a character string containing the names of variables and operators, for example. `"a * b %% c - d / e ^ f + g %/% h"`

...

The following toy example works

```
prmlist <- list(a=1:10, b=21:30, c=31:40, d=4, e=5, f=6, g=7, h=8)
with(prmlist, a * b %% c - d / e ^ f + g %/% h)
```

The problem starts when I want to use this approach inside a function. To do this, I have to get the "formula" specified by the user inside the function. The character string seems obvious. The question is how to evaluate it inside a function. do.call () doesn't seem to fit as operators are really a function. I was hoping for something simple like

```
my.formula <- "a * b %% c - d / e ^ f + g %/% h"
with(prmlist, eval(my.formula))
```

will work, but it doesn't.

source to share

Instead of operators, `substitute()`

you can declare your own operators:

```
my.formula <- substitute(a * b %% c - d / e ^ f + g %/% h)
with(prmlist, eval(my.formula))
[1] 20.99974 43.99974 68.99974 95.99974 124.99974 155.99974 188.99974
[8] 223.99974 260.99974 299.99974
```

Update: if the command is a string, you can use `parse`

:

```
myCmd <- "a * b %% c - d / e ^ f + g %/% h"
with(prmlist, eval( parse(text=myCmd) ))
[1] 20.99974 43.99974 68.99974 95.99974 124.99974 155.99974 188.99974
[8] 223.99974 260.99974 299.99974
```

source to share