Boxplot - accessing column names with variables

Question

How can I access a column by using a variable that contains the name of the column?

Let's assume we have a data frame DF with 3 columns: Var1 Var2 Var3, where Var3 contains numerical data and Var1 as well as Var2 contain a few factors.

We would like to produce 2 boxplots using a temporary variable that contains the name of the column:

temp<-"Var3"
boxplot(DF[temp]) #(<--that works).

If I use the same method to obtain a boxplot for each factor in Var2, it doesn't:

boxplot(DF[temp]~DF$Var2) #(<-- does not work).

How can I get this working?

Annotation: If I use the name "Var3" directly, it does work and shows several boxplots:

boxplot(DF$Var3~DF$Var2).

Greg Snow · Accepted Answer · 2011-10-27 17:27:50Z

7

Try using double brackets instead of single brackets:

tmp1 <- 'Sepal.Width'
tmp2 <- 'Species'
boxplot( iris[[tmp1]] ~ iris[[tmp2]] )

answered Oct 27, 2011 at 17:27

Greg Snow

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John Over a year ago

Thanks for this suggestion. Before I tried [,[tmp1]] which did not work. Your solution is even easier to carry out and it works nicely.

Ramnath · Accepted Answer · 2011-10-27 16:13:08Z

1

You could simply do this. The with statement instructs boxplot to look for variables inside DF, the get statement accesses the object with name tmp.

with(DF, boxplot(get(tmp) ~ Var2))

Here is an illustrative example

tmp <- 'wt'
with(mtcars, boxplot(get(tmp) ~ cyl))

enter image description here

answered Oct 27, 2011 at 16:13

Ramnath

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John Over a year ago

Dear Ramnath, thanks a lot for your answer! I especially liked that you give a detailed description on what the different elements in your code are doing.

James · Accepted Answer · 2011-10-27 16:56:24Z

1

You can use paste to construct the formula, and then convert to a formula for the boxplot call:

boxplot(as.formula(paste(temp,"Var2",sep="~")),DF)

edited Oct 27, 2011 at 16:56

answered Oct 27, 2011 at 16:32

James

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1 Comment

John Over a year ago

It seems there are many different options to perform the task. I tried your solution and it works fine. Very interesting method.

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