# [R] Problem with mathematical expression and loop

Duncan Murdoch murdoch at stats.uwo.ca
Tue Mar 31 17:23:49 CEST 2009

```On 3/31/2009 9:15 AM, Denis Aydin wrote:
> Hi to all
>
> I use a loop to plot 9 different histograms of 9 different
> transformations of one dataset (x).
>
> I want to label the histograms with the mathematical expression of each
> transformation (e.g. x^3). For that I've prepared a vector with the
> labeling names for "expression".
>
>  > trans.expr <- c("x^3", "x^2", "x", "frac(1,x)", "frac(1,x^2)",
>  > "frac(1,x^3)", "sqrt(x)", "log(x)", "frac(1,sqrt(x))")
>
>  > for(i in 1:9){
>  >
>  > hist(x[i], main=expression(trans.expr[i]))
>  >
>  > }
>
> But if I want to pass the names to "expression", it always prints
> "trans.expr[i]" instead of the mathematical expressions.

Don't use a character vector, use expressions from the beginning:

trans.expr <- expression(x^3, x^2, x, frac(1,x), frac(1,x^2),
frac(1,x^3), sqrt(x), log(x), frac(1,sqrt(x)))
for (i in 1:9) hist(x[i], main=trans.expr[i])

If you really need to use strings, then you need parse() to convert them
to expressions.

Duncan Murdoch
>
> I also tried to use "noquote" to remove the quotes but it didn't work.
>
> Does anyone knows a solution to that problem?
>
> Any help is appreciated.
>
> Denis
>
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