express: Programmatically Generate Expression Objects
In mwrowe/microRutils: Mike Rowe's Miscellaneous R Utility Functions

Description Usage Arguments Details Value Author(s) Examples

express() converts a character vector into a set of expressions.

1	express(char.expressions)

char.expressions

A character vector where each element is an expression as a character string that you wish to evaluate.

This function makes it possible to use plotmath symbols, but stored as characters, which means they can be created and manipulated programmatically.

The text-drawing functions (text, mtext, axis, legend, etc.) can accept an expression for their text argument, which is then interpreted and drawn as a mathematical expression.

However, expressions are usually entered literally, which means you can't manipulate them programmatically. If the text argument is a character object, it gets printed literally. This function gets around that.

Ridiculously simple once you figure out how to do it, but it took me a while; hence this function. as.expression() doesn't do this, by the way...

Unfortunately, you still have to build valid plotmath expressions; plotmath doesn't handle spaces well, for example.

Returns a length one compound expression object.

M.W.Rowe, mwr.stats@gmail.com

par(mar=c(6, 6, 1, 1))
plot(0, 0, xlim=sym(), ylim=sym(), xaxt="n", yaxt="n", mgp=c(4,0.2,0),
   xlab="axis(1, (-9:9)/10, tick.labels, las=2, cex.axis=0.8)",
   ylab="axis(2, (-9:9)/10, express(tick.labels), las=1, cex.axis=0.8)")
tick.labels <- paste("x >=", (-9:9)/10)
# this is what you get if you just use tick.labels the regular way:
axis(1, (-9:9)/10, tick.labels, las=2, cex.axis=0.8)
# but if you express() them... voila!
axis(2, (-9:9)/10, express(tick.labels), las=1, cex.axis=0.8)