# Value of an R object in an expression

December 15, 2010
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(This article was first published on binfalse » R, and kindly contributed to R-bloggers)

Just wanted to create an expression, existing of some mathematical annotation and a value of an R object. Wasn’t that intuitive!

Each single goal is easy to reach, for example to combine a value of an R object with text just use paste:

 123 > n = 5 > paste ("n=", n, "!") [1] "n= 5 !"

To have a plot title with an $\alpha_1$ you can use expression:

 1 > plot(1:5, runif(5, 1, 4), main=expression("this is a " * alpha[1] * " example"))

But to let the the title of a plot contain objects and Greek letters isn’t that easy. Those of you who think it’s just about combining paste and expression might try it on their own and come back head-ached after few minutes of unsuccessful testings.

The problem is, that expression interprets chars as expression and not as object identifier, of course, how should it know whether you mean the var alpha or the Greek letter!? The solution is called substitute! With substitute you can replace objects inline, here is a small example:

 123 > var=10 > substitute(paste("here is the content: ", v), list(v=var)) paste("here is the content: ", 10)

You see, substitute got a list what to substitute and replaces the v in paste with the content of var. Run eval to evaluate to result:

 123 > var=10 > eval(substitute(paste("here is the content: ", v), list(v=var))) [1] "here is the content:  10"

Now it’s easy to create a more complex plot title:

 12 > var=10 > plot(1:5, runif(5, 1, 4), main=substitute(paste("here ", lambda[1], "=", v, " and ", epsilon^2, "=", b, "!"), list(v=var, b=var^2)))

Go out and produce imposing graphs! (-;

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