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Introduction

The following are a set of self-test questions designed to complement the beginnerConcepts tutorial. These exercises are only meant to be attempted after you have finished the beginnerConcepts tutorial.

Loading in Data

We will be using several pre-built in datasets that come with R for this exercise. You can load them into R using the data( ) function.

# Data from the 1974 Motor Trend US magazine. 
> data(mtcars)

# Measurements of the girth, height and volume of felled black cherry trees.
> data(trees)
  
# The average amount of precipitation (rainfall) in inches for U.S. cities.
> data(precip)

Note that you would normally expect to define the name of new objects using the <- operator, but the data( ) function creates a new object for you automatically.

# Take a quick look at the structure of each dataset.
# You can use the head( ) function to only display the first few rows of the dataset.
> head(mtcars)
                   mpg cyl disp  hp drat    wt  qsec vs am gear carb
Mazda RX4         21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
Datsun 710        22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1

# you can also use the tail( ) function to only display the last few rows of the dataset
> tail(trees)
   Girth Height Volume
26  17.3     81   55.4
27  17.5     82   55.7
28  17.9     80   58.3
29  18.0     80   51.5
30  18.0     80   51.0
31  20.6     87   77.0

> head(precip)
 Mobile      Juneau     Phoenix Little Rock Los Angeles  Sacramento 
   67.0        54.7         7.0        48.5        14.0        17.2

Problem Set 1

  1. What class of object is mtcars? What function did you use to find out?

  2. Is precip defined as a 1-dimensional array or a vector? How did you find out?

  3. How would you convert the data.frame trees into a matrix?

  4. What is the name of the 14th city in the precip dataset?

  5. What function would you use if you wanted to combine all three data sets into a single object?

  6. Does precip consist of numeric data? How did you find out?

  7. Code four different ways to subscript the 2nd row and 7th column of mtcars using bracket notation - i.e., 17.02.

  8. How would you change the precipitation values of "Juneau", "Phoenix", and "Sacramento" to 23, 46, and 12 in the precip dataset. (Hint: You will need to use subscripts and the <- operator).

  9. Are there any trees in the trees dataset with more girth than volume? How did you find out?

  10. Take the sum of all elements in column height of the trees dataset, call this value A. Take the sum of all elements in row Valiant of the mtcars dataset, call this value B. Take the sum of the first 8 elements of the precip dataset, call this value C. Divide C by B and add A. What is your final answer?