Recently I was writing a code allowing to plot multiple ggplot2 plots on one page. I wanted to replicate standard behavior of plot function that plots graphs in sequence according to mfrow/ mfcol option in par. The solution lead me to think of emulating C-like local static variables in R.

There are several solutions to this problem but I think that a nice one is by adding attributes to a function. Here is a simple example:

f **<-** **function****(**x**)** **{**

y **<-** attr**(**f, “sum”**)**

**if** **(**is.null**(**y**))** **{**

y **<-** 0

**}**

y **<-** x **+** y

attr**(**f, “sum”**)** **<<-** y

return**(**y**)**

**}**

It can be applied as follows:

> **for** **(**i **in** 1**:**5**)** cat**(**i, “: “, f**(**i**)**, “\n”, sep**=**“”**)**

1: 1

2: 3

3: 6

4: 10

5: 15

As it can be seen attribute “sum” is static but it can be thought of as local because it is not stored directly as a variable in global environment.

And here is the application of the concept to the problem of plotting several qplots in a sequence:

library**(**ggplot2**)**

library**(**grid**)**

# setup the ploting grid and plotting sequence

mplot.setup **<-** **function****(**nrow, ncol, by.row **=** **TRUE)** **{**

attributes**(**mplot.seq**)** **<<-** list**(**nrow **=** nrow, ncol **=** ncol,

pos **=** 0, by.row **=** by.row**)**

grid.newpage**()**

pushViewport**(**viewport**(**layout **=** grid.layout**(**nrow, ncol**)))**

**}**

# plot at given grid location

mplot **<-** **function****(**graph, row, col**)** **{**

print**(**graph, vp **=** viewport**(**layout.pos.row **=** row,

layout.pos.col **=** col**))**

**}**

# plot the at the next position in the sequence

mplot.seq **<-** **function****(**graph**)** **{**

pos **<-** attr**(**mplot.seq, “pos”**)**

nrow **<-** attr**(**mplot.seq, “nrow”**)**

ncol **<-** attr**(**mplot.seq, “ncol”**)**

**if** **(**attr**(**mplot.seq, “by.row”**))** **{**

col **<-** 1 **+** **(**pos %% ncol**)**

row **<-** 1 **+** **((**pos %/% ncol**)** %% nrow**)**

**}** **else** **{**

row **<-** 1 **+** **(**pos %% nrow**)**

col **<-** 1 **+** **((**pos %/% nrow**)** %% ncol**)**

**}**

attr**(**mplot.seq, “pos”**)** **<<-** pos **+** 1

mplot**(**graph, row, col**)**

**}**

# application example

mplot.setup**(**2,4, **FALSE****)**

**for** **(**i **in** 1**:**4**)** **{**

mplot.seq**(**qplot**(**iris**[**,i**]**, xlab **=** names**(**iris**)[**i**]))**

mplot.seq**(**qplot**(**iris**[**,5**]**, iris**[**,i**]**, geom **=** “boxplot”,

xlab **=** “Species”, ylab **=** names**(**iris**)[**i**])** **+** coord_flip**())**

**}**

The following plot is produced by the above code:

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