gtools (version 2.0.9)

defmacro: Define a macro

Description

Define a macro.

Usage

defmacro(..., expr)

Arguments

...

macro argument list

expr

macro body

Value

• A macro function.

Details

When evaluated, a macros created with defmacro substitutes the provided arguments into the parsed source code included in expr, and then evaluates the results. This is particularly useful for programmatically creating new functions.

"Note that because [the defmacro code] works on the parsed expression, not on a text string, defmacro avoids some of the problems with C preprocessor macros. In mul <- defmacro(a, b, expr={a*b}) a C programmer might expect mul(i, j + k) to expand (incorrectly) to i*j + k. In fact it expands correctly, to the equivalent of i*(j + k)." (From the reference below.) For a more complete description of the differences between functions and macros, please see the reference below.

References

The code for this function is directly taken from:

Lumley T. "Programmer's Niche: Macros in {R}", R News, 2001, Vol 1, No. 3, pp 11--13, http://CRAN.R-project.org/doc/Rnews/

See Also

function substitute, eval, parse, source, parse,

Examples

####
# macro for replacing a specified missing value indicator with NA
# within a dataframe
###
setNA <- defmacro(df, var, values,
                  expr={
                         df$var[df$var %in% values] <- NA
                       })

###
# create example data, 999 is missing value indicator
###
d <- data.frame(
   Grp=c("Trt", "Ctl", "Ctl", "Trt", "Ctl", "Ctl", "Trt", "Ctl", "Trt", "Ctl"),
   V1=c(1, 2, 3, 4, 5, 6, 999, 8,   9,  10),
   V2=c(1, 1, 1, 1, 1, 2, 999, 2, 999, 999)
               )
d

###
# Try it out
###
setNA(d, V1, 999)
setNA(d, V2, 999)
d


###
# Macro for creating a plotting function
###
plotit <- defmacro( df, var, col="red", title="", expr=
  {
    plot( df$var ~ df$Grp, type="b", col=col, main=title )
  } )

# Notes:
# 1 - The macro expansion of 'col' will correctly replaces the
# object name 'col' (right hand side of col=col) and not the argument
# name (left hand side of col=col) in the function call to plot
# 2 - The plot's y axis label 

plotit( d, V1)
plotit( d, V1, "blue" )

# works with variables too
color <- "cyan"
plotit( d, V1, color, title="Crazy Plot")

  # An equivalent function is somewhat messier, since it must either explicitly
  # construct the y axis label, duplicating some of the work of the plot
  # function:
  plotit <- function( df, var, col="red", title="" )
  {
      dname <- deparse(substitute(df))
      vname <- deparse(substitute(var))
      plot( df[[vname]] ~ df$Grp, type="b", col=col, title=title,
            ylab=paste( dname, "$", vname, sep='' ) )
  }
  # or we explicitly construct the call and then call eval.  The code for
  # the latter approach is # omiited since this is quite messy and
  # requires a lot of work.