using apply() to draw multiple lines with abline()

[Bonnie Dixon]

Dear D-RUG group mind;

Does anyone know why using an apply() statement to draw multiple lines on a plot with abline() returns "NULL" in addition to drawing the lines?  I am creating a plot in an R Markdown PDF document, and my document would look nicer if it didn't have "NULL" printed along with the plot.  

Here is an example of the problem:

> plot(1)

> x <- cbind(c(0.8, 1.0, 1.2), c(0, 0, 0))

> apply(x, 1, abline)

NULL

Bonnie 

[Vince S. Buffalo]

Hi Bonnie,

This is because the function abline isn't returning anything each iteration in apply(). apply() needs to return something so it returns NULL. This isn't necessarily a problem; the output can be ignored. If it bothers you, you can use a for loop or capture the output to a variable in local scope:

# maybe something like:

> local(a <-apply(x, 1, abline))

> a # doesn't pollute your environment

Error: object 'a' not found

HTH,

Vince

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Vince Buffalo

Ross-Ibarra Lab (www.rilab.org)

Plant Sciences, UC Davis

[Bonnie Dixon]

Thanks for the tip, Vince.  I just tried using local(), and that prevented the word NULL from being printed in the console, but it was still printed in my R Markdown document.  So, next I tried doing it as a for loop, and that worked beautifully.  No more "NULL' in my document.  Thanks again!

Bonnie

[Vince S. Buffalo]

For Rmarkdown/knitr you should be able to set results="hide" in a chunk to not print the results of a calculation too. 

cheers,

Vince

[Noam Ross]

Alternative the a_ply, l_ply, and d_ply functions in the plyr package are designed for this use. They should return no value.

[Bonnie Dixon]

I had been assuming that hiding the results would hide the plot too, but I just tried it and, indeed, results="hide" prints the plot, but not the "NULL" result in the document.  That is great to know.

And, thanks, Noam for letting me know about the a_ply, etc. functions in plyr.  That makes it even simpler.

Bonnie