Lesson 2: First R Steps

The R command prompt is >. Again, it will be shown here, but you don’t type it. It is just there in your R window to let you know R is inviting you to submit a command. (If you are using RStudio, you’ll see it in the Console pane.)

So, just type 1+1 then hit Enter. Sure enough, it prints out 2 (you were expecting maybe 12108?):

> 1 + 1
[1] 2

But what is that [1] here? It’s just a row label. We’ll go into that later, not needed quite yet.

Example: Nile River data

R includes a number of built-in datasets, mainly for illustration purposes. One of them is Nile, 100 years of annual flow data on the Nile River.

Let’s find the mean flow:

> mean(Nile)
[1] 919.35

Here mean is an example of an R function, and in this case Nile is an argument — fancy way of saying “input” — to that function. That output, 919.35, is called the return value or simply value. The act of running the function is termed calling the function.

Another point to note is that we didn’t need to call R’s print function. We could have typed,

> print(mean(Nile))

Function calls in R (and other languages) work “from the inside out.” Here we are asking R to find the mean of the Nile data, then print the result.

But whenever we are at the R > prompt, any expression we type will be printed out anyway, so there is no need to call print.

Since there are only 100 data points here, it’s not unwieldy to print them out. Again, all we have to do is type Nile, no need to call print:

> Nile
Time Series:
Start = 1871 
End = 1970 
Frequency = 1 
  [1] 1120 1160  963 1210 1160 1160  813 1230 1370 1140  995  935 1110  994 1020
 [16]  960 1180  799  958 1140 1100 1210 1150 1250 1260 1220 1030 1100  774  840
 [31]  874  694  940  833  701  916  692 1020 1050  969  831  726  456  824  702
 [46] 1120 1100  832  764  821  768  845  864  862  698  845  744  796 1040  759
 [61]  781  865  845  944  984  897  822 1010  771  676  649  846  812  742  801
 [76] 1040  860  874  848  890  744  749  838 1050  918  986  797  923  975  815
 [91] 1020  906  901 1170  912  746  919  718  714  740

Now you can see how the row labels work. There are 15 numbers per row here, so the second row starts with the 16th, indicated by [16]. The third row starts with the 31st output number, hence the [31] and so on.

Note that a set of numbers such as Nile is called a vector.

A first graph

R has great graphics, not only in base R but also in wonderful user-contributed packages, such as ggplot2 and lattice. But we’ll stick with base-R graphics for now, and save the more powerful yet more complex ggplot2 for a later lesson.

We’ll start with a very simple, non-dazzling one, a no-frills histogram:

> hist(Nile)

No return value for the hist function (there is one, but it is seldom used, and we won’t go into it here), but it does create the graph.

R has lots of online help, which you can access via ?. E.g. typing

> ?hist

will tell you to full story on all the options available for the hist function. Again, there are far too many for you to digest for now (most users don’t ever find a need for the more esoteric ones), but it’s a vital resource to know.