Exercises for 1.1: Getting started

Exercise 1.1.1: Addition

The first command is simple addition, so the exercise is to add some numbers together. Open up R, and use it to do a few basic sums. In particular, it’s worth trying the following:

  • Try adding more than two numbers.
  • Try playing around with the number of spaces between numbers and the plus sign, and verify that it doesn’t actually matter.
  • Try hitting enter when you’re half way through the command (e.g., 10+). R will wait for you to finish before trying to execute it.

Solution 1.1.1:

For this exercise, the solution set displays the R prompt (i.e., the >) and displays the results more or less exactly the way that the would appear in R. For the later exercises, I’ll tidy the output a bit more to make it a little more human-friendly. The basic convention you’ll see throughout the output is that the grey boxes contain R commands, and the white boxes contain R output. (This is a pretty standard format for “R Markdown” files, which I talk about briefly at the end of the workshop).

First, here’s the basic addition from the slides:

> 10+3
## [1] 13

(Note that when you see the output on screen you don’t see the ## part).

Here’s an example of adding more than two numbers:

> 10+3+4
## [1] 17

Here’ an example showing that spacing doesn’t really matter all that much if you do it sensibly:

> 10         +3
## [1] 13

Finally, here’s an example where I’ve hit enter before finishing the command. Sometimes (but not always), R is “smart” enough to figure out that the command isn’t complete, so it waits for the user to finish before continuing:

> 10+
+     3
## [1] 13

Exercise 1.1.2: Other arithmetic operations

  • Multiply 3 by 14 and subtract 5 from the result
  • Raise 5 to the power of 4
  • Divide 267563 by 1235

Solution 1.1.2:

Here’s the answer to the first part:

3*14 - 5
## [1] 37

Notice that - as I mentioned above - the solution set now drops the > from the output. I’ll do it this way from now on.

Next, here’s 5 raised to the power of 4

5^4
## [1] 625

And finally, here’s the silly division problem:

267563 / 1235
## [1] 216.6502

Exercise 1.1.3: Logical operations

  • Use logical operations to get R to agree that “two plus two equals five” is FALSE
  • Use logical operations to test whether 8 raised to the power 13 is less than 15 ^ 9

Solution 1.1.3:

2+2 == 5
## [1] FALSE
8^13 < 15^9
## [1] FALSE

Exercise 1.1.4: Using functions

  • use the sqrt() function to calculate the square root of 789
  • round 2.456 to the nearest whole number using round()
  • round 2.456 to two decimal places using round()
  • R has a factorial() function that calculates the factorial function, n! (e.g., 5! = 5*4*3*2*1). Use it to calculate 25!, and take note of the way that R formats the answer.
  • Use factorial() to (try to) calculate 2000!, and note the answer that R gives.

Solution 1.1.4:

Here’s the square root calculation:

sqrt(789)
## [1] 28.08914

Here are the solutions to the problems involving rounding:

round(2.456)
## [1] 2
round(x = 2.456, digits = 2)
## [1] 2.46

Here’s what happens with the factorial calculations:

factorial(25)
## [1] 1.551121e+25
factorial(2000)
## Warning in factorial(2000): value out of range in 'gammafn'
## [1] Inf

The warning message there is worth commenting on. Notice that it says that the warning comes from a function called gammafn, not from factorial? What that means is that the factorial function is using gammafn to do some of the calculations, and it is that function that has issued the complaint. That’s very typical of R warnings and error messages. The message that gets printed to the screen comes from the function that issued the complaint, which might not be the one that you called.