---
title: "Introduction - Tasks"
author: "Florian Oswald, Gustave Kenedi and Pierre Villedieu"
date: "`r Sys.Date()`"
output: html_document
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

## Task 1

1\. Create a new R script (File $\rightarrow$ New File $\rightarrow$ R Script). Save it somewhere as `lecture_intro.R`.

2\. Type the following code in your script and run it. To run the code press `Ctrl` or `Cmd` + `Enter` (you can either highlight the code or just put your cursor at the end of the line)
```{r}
4 * 8
```

3\. Type the following code in your script and run it. What happens if you only run the first line of the code?
```{r}
x = 5 # equivalently x <- 5
x
```

```{r}
x = 5
```

**If I only run the first line of code, the object `x` is created in my environment but no output appears in the console. This is because I am not asking `R` to output anything; the only thing I am asking it is to create an object `x` equal to $5$.**

Congratulations, you have created your first `R` "object"! Everything is an object in R! Objects are assigned using `=` or `<-`.

4\. Create a new object named `x_3` to which you assign the cube of `x`. Note that to assign you need to use `=` or `<-`. Use code to compute the cube, not a calculator.

```{r}
x_3 = x^3
x_3
```


## Task 2

1\. Find out (using `help()` or google) how to import a .csv file. Do NOT use the "Import Dataset" button, nor install a package.

**There are many ways to import a .csv file. The simplest way is to use the `read.csv()` function.**

2\. Import [gun_murders.csv](https://www.dropbox.com/s/zuk0qcfm3kyzs4e/gun_murders.csv?dl=1)<sup>1</sup> in a new object `murders`. This file contains data on gun murders by US state in 2010. (Hint: objects are created using `=` or `<-`).

```{r}
link <- "https://www.dropbox.com/s/zuk0qcfm3kyzs4e/gun_murders.csv?dl=1"

murders <- read.csv(link)
```

3\. Ensure that `murders` is a data.frame by running:

```{r, error = TRUE}
class(murder)
```

**Hmm I get an error: "object 'murder' not found." Why is that? It's simple, there's a typo: it should be `murders` not `murder`. Everytime you see an error that says objet `x` not found it simply means that there is no object named `x` in your environment. So either you haven't created it yet or there's a small typo in your code. Always read what the error message says!**

```{r}
class(murders)
```

4\. Find out what variables are contained in `murders` by running

```{r}
names(murders)
```

5\. View the contents of `murders` by clicking on `murders` in your workspace. What does the `total` variable correspond to?

```{r}
View(murders)
```

**The `total` variable corresponds to the total number of gun murders by state (in 2010).**


## Task 3

1\. How many observations are there in `murders`?

```{r}
nrow(murders)
```

2\. How many variables? What are the data types of each variable?

```{r}
str(murders)
```

3\. Remember that the colon operator `1:10` is just short for *construct a sequence from `1` to `10`* (i.e. 1, 2, 3, etc). Create a new object `murders_2` containing the rows 10 to 25 of `murders`.

```{r}
murders_2 <- murders[10:25,]
nrow(murders_2)
```

4\. Create a new object `murders_3` which only contains the columns `state` and `total`. (Recall that `c` creates vectors.)

```{r}
murders_3 <- murders[, c("state", "total")]
```

5\. Create a `total_percap` variable equal to the number of murders per 10,000 inhabitants by running the following code.
```{r}
murders$total_percap = (murders$total / murders$population) * 10000
names(murders)
```

Congratulations, you've created your first variable! Click on the `murders` object to see the new variable.