Vineetha -4 years ago 116

R Question

I'm trying to make a small R package with my limited knowledge in R programming. I am trying to use the following argument:

`formula=~a+b*X`

where X is vector, 'a' and 'b' are constants in a function call.

What I'm wondering is once I input the formula, I want to extract

`(a,b)`

I would really appreciate any guidance.

I'm looking for something similar to model.matrix() output. The above mentioned formula can be more generalized to accommodate 'n' number of variables, say,

~2+3*X +4*Y+...+2*Z

In the output, I need the coefficients (2 3 4 ...2) as a vector and [1 X Y ... Z] as a covariate matrix.

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Answer Source

The question is not completely clear so we will assume that the question is, given a formula using standard formula syntax, how do we parse out the variables names (or in the second answer the variable names and constants) giving as output a character vector containing them.

**1) all.vars** Try this:

```
fo <- a + b * X # input
all.vars(fo)
```

giving:

```
[1] "a" "b" "X"
```

**2) strapplyc** Also we could do it with string manipulation. In this case it also parses out the constants.

```
library(gsubfn)
fo <- ~ 25 + 35 * X # input
strapplyc(gsub(" ", "", format(fo)), "-?[0-9.]+|[a-zA-Z0-9._]+", simplify = unlist)
```

giving:

```
[1] "25" "35" "X"
```

**Note:** If all you are trying to do is to *evaluate* the RHS of the formula as an R expression then it is just:

```
X <- 1:3
fo <- ~ 1 + 2 * X
eval(fo[[2]])
```

giving:

```
[1] 3 5 7
```

**Update:** Fixed and added second solution and Note.

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