R help - Jun 2010 - Specifying formula inside a function

Hello,

How does one specify a formula to lm inside a function (with variable
names not known in advance) and have the formula appear explicitly in
the output?

For example,

f <- function(d) {
  in.model <- sample(c(0,1), ncol(d)-1, replace=T)
  current.model <- lm(paste(names(d)[1], "~",
paste(names(d[2:ncol(d)])[which(in.model == 1)], collapse= "+")),
data=d)  #***
  return(current.model)
}
x1 <- rnorm(50,0,1)
x2 <- rnorm(50,0,1)
x3 <- rnorm(50,0,1)
x4 <- rnorm(50,0,1)
y <- rnorm(50,0,1)
d <- data.frame(y, x1, x2, x3, x4)
f(d)

Call:
lm(formula = paste(names(d)[1], "~",
paste(names(d[2:ncol(d)])[which(in.model ==     1)], collapse = "+")),
data = d)

Coefficients:
(Intercept)           x3           x4
    -0.1087       0.2830       0.1024

How can I specify the formula in the line marked *** so that the
output will show "formula = y ~ x3 + x4" instead of "formula
paste..."?

Thanks for any help you can give.

Regards,
Mark
-- 
Mark Seeto
Statistician

National Acoustic Laboratories
A Division of Australian Hearing

Here is one way.

f <- function(d) {
  y <- names(d)[1]
  xs <- names(d)[-1]
  nx <- length(xs)
  xs <- sort(sample(xs, sample(1:nx, 1)))
  form <- as.formula(paste(y, "~", paste(xs,
collapse="+")))
  Call <- substitute(lm(FORM, data = d), list(FORM = form))
  eval(Call)
}
d <-  within(data.frame(y = rnorm(50)), {
  x1 <- rnorm(50)
  x2 <- rnorm(50)
  x3 <- rnorm(50)
  x4 <- rnorm(50)
})

f(d)


-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Mark Seeto
Sent: Thursday, 10 June 2010 2:18 PM
To: r-help at r-project.org
Subject: [R] Specifying formula inside a function

Hello,

How does one specify a formula to lm inside a function (with variable
names not known in advance) and have the formula appear explicitly in
the output?

For example,

f <- function(d) {
  in.model <- sample(c(0,1), ncol(d)-1, replace=T)
  current.model <- lm(paste(names(d)[1], "~",
paste(names(d[2:ncol(d)])[which(in.model == 1)], collapse= "+")),
data=d)  #***
  return(current.model)
}
x1 <- rnorm(50,0,1)
x2 <- rnorm(50,0,1)
x3 <- rnorm(50,0,1)
x4 <- rnorm(50,0,1)
y <- rnorm(50,0,1)
d <- data.frame(y, x1, x2, x3, x4)
f(d)

Call:
lm(formula = paste(names(d)[1], "~",
paste(names(d[2:ncol(d)])[which(in.model ==     1)], collapse = "+")),
data = d)

Coefficients:
(Intercept)           x3           x4
    -0.1087       0.2830       0.1024

How can I specify the formula in the line marked *** so that the
output will show "formula = y ~ x3 + x4" instead of "formula
paste..."?

Thanks for any help you can give.

Regards,
Mark
-- 
Mark Seeto
Statistician

National Acoustic Laboratories
A Division of Australian Hearing

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and provide commented, minimal, self-contained, reproducible code.

Hello,
> How does one specify a formula to lm inside a function (with variable
> names not known in advance) and have the formula appear explicitly in
> the output?
> 
> For example,
> 
> f <- function(d) {
>   in.model <- sample(c(0,1), ncol(d)-1, replace=T)
>   current.model <- lm(paste(names(d)[1], "~",
> paste(names(d[2:ncol(d)])[which(in.model == 1)], collapse= "+")),
> data=d)  #***
>   return(current.model)
> }
> x1 <- rnorm(50,0,1)
> x2 <- rnorm(50,0,1)
> x3 <- rnorm(50,0,1)
> x4 <- rnorm(50,0,1)
> y <- rnorm(50,0,1)
> d <- data.frame(y, x1, x2, x3, x4)
> f(d)
> 
> Call:
> lm(formula = paste(names(d)[1], "~",
> paste(names(d[2:ncol(d)])[which(in.model ==     1)], collapse =
"+")),
> data = d)
> 
> Coefficients:
> (Intercept)           x3           x4
>     -0.1087       0.2830       0.1024
> 
> How can I specify the formula in the line marked *** so that the
> output will show "formula = y ~ x3 + x4" instead of "formula
> paste..."?
> 
Well, there could very well be some tricks you can pull with ?substitute, or 
others, but I can't seem to figure it out.  Instead of tricks, it might be 
easier/clearer to assign a class to your object, say 'test', and write a
print
method for that based on print.lm.  There very well may be drawbacks to this 
that I am not realizing :).

f <- function(d) {
   in.model <- sample(c(0,1), ncol(d)-1, replace=TRUE)
   current.model <- lm(paste(names(d)[1], "~",
                             paste(names(d[2:ncol(d)])[which(in.model == 1)], 
collapse= "+")),
                       data=d)
   class(current.model) <- c("test", "lm")
   current.model
}

# slight modification to print.lm
print.test <- function (x, digits = max(3, getOption("digits") -
3), ...)
{
     cat("\nCall:\n", deparse(x$terms), "\n\n", sep =
"")
     if (length(coef(x))) {
         cat("Coefficients:\n")
         print.default(format(coef(x), digits = digits), print.gap = 2,
             quote = FALSE)
     }
     else cat("No coefficients\n")
     cat("\n")
     invisible(x)
}

# should give you what you want...
f(d)