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  3. Introduction to Writing Functions in R

Exercise

Returning invisibly

When the main purpose of a function is to generate output, like drawing a plot or printing something in the console, you may not want a return value to be printed as well. In that case, the value should be invisibly returned.

The base R plot function returns NULL, since its main purpose is to draw a plot. This isn't helpful if you want to use it in piped code: instead it should invisibly return the plot data to be piped on to the next step.

Recall that plot() has a formula interface: instead of giving it vectors for x and y, you can specify a formula describing which columns of a data frame go on the x and y axes, and a data argument for the data frame. Note that just like lm(), the arguments are the wrong way round because the detail argument, formula, comes before the data argument.

plot(y ~ x, data = data)

Instructions 1/2

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  • Use the cars dataset and the formula interface to plot(), draw a scatter plot of dist versus speed.