# How to make lm display in your output by a formula passed to it as a variable

Please consider the following example code (from the doc `lm`

):

```
ctl <- c(4.17,5.58,5.18,6.11,4.50,4.61,5.17,4.53,5.33,5.14)
trt <- c(4.81,4.17,4.41,3.59,5.87,3.83,6.03,4.89,4.32,4.69)
group <- gl(2,10,20, labels=c("Ctl","Trt"))
weight <- c(ctl, trt)
lm(weight ~ group)
form1 <- weight ~ group
lm(form1)
do.call("lm",list(formula=form1))
```

With the following (abbreviated) output:

```
> lm(weight ~ group)
Call:
lm(formula = weight ~ group)
> lm(form1)
Call:
lm(formula = form1)
> do.call("lm",list(formula=form1))
Call:
lm(formula = weight ~ group)
```

As you can see, the second call `lm`

does not display the formula, but the variable containing the formula. With some experimentation, I came up with a third solution, but I find it a little tedious.

**The question, therefore, is** : is there any other way to make the `lm`

formula display instead of a variable (i.e. without using the method `do.call`

I used above).

(The reason I want is because I'm working on a Sweave document, so I don't see the calls, only the outputs, and then lm tells you which formula is being used very conveniently.)

**EVEN BEST SOLUTION** - Thanks to @Aaron

```
lm <- function(...) {
mf <- match.call()
mf[[1]] <- quote(stats::lm)
env <- parent.frame()
mf$formula <- eval(mf$formula, env)
eval(mf, env)
}
```

**SOLUTION:**
Based on the answer by G. Grothendieck, I came up with the following function:

```
lm <- function(...) {
mf <- match.call()
mf[[1]] <- quote(stats::lm)
env <- parent.frame()
fm <- eval(mf, env)
fm$call$formula <- formula(fm)
fm
}
```

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Place this at the top of your Sweave file:

```
<<preliminaries,echo=FALSE,results=hide>>=
lm <- function(fo, ...) { fm <- stats::lm(fo, ...); fm$call <- fo; fm }
@
```

and then call `lm`

normally on the rest of the file.

Using the example in the question and assume we have defined `lm`

as above:

```
> lm(form1)
Call:
weight ~ group
Coefficients:
(Intercept) groupTrt
5.032 -0.371
```

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You can change `print.lm`

:

```
body(print.lm)[[2]] <-
as.call(expression({
ca<-x$call
ca$formula <- formula(x)
cat("\nCall:\n", paste(deparse(ca), sep = "\n", collapse = "\n"), "\n\n", sep = "")
})[[1]])
```

then

```
> lm(form1)
Call:
lm(formula = weight ~ group)
Coefficients:
(Intercept) groupTrt
5.032 -0.371
```

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