---
title: "Import and Tidy Data"
author: "Dr. Hua Zhou"
date: "Jan 30, 2018"
subtitle: Biostat M280
output:
  ioslides_presentation:
    smaller: true
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(fig.width = 5, fig.height = 3.5, fig.align = 'center',
  cache = TRUE)
```

## Workflow

A typical data science project:

<p align="center">
<img src="./data-science-wrangle.png" height="275">
</p>

# Tibble | r4ds chapter 10

## Tibbles

- Tibbles extend data frames in R and form the core of tidyverse.
    ```{r}
    library("tidyverse")
    ```

## Create tibbles

- Convert a regular data frame to tibble:
    ```{r}
    # a regular data frame
    head(iris)
    ```
    
----

- 
    ```{r}
    as_tibble(iris)
    ```

- Convert a tibble to data frame:
    ```{r, eval = FALSE}
    as.data.frame(tb)
    ```

----

- Create tibble from individual vectors:
    ```{r}
    tibble(
      x = 1:5, 
      y = 1, 
      z = x ^ 2 + y
    )
    ```

----

- Transposed tibbles:
    ```{r}
    tribble(
      ~x, ~y, ~z,
      #--|--|----
      "a", 2, 3.6,
      "b", 1, 8.5
    )
    ```
  
## Printing of a tibble

- By default, tibble prints the first 10 rows and all columns that fit on screen.
    ```{r}
    tibble(
      a = lubridate::now() + runif(1e3) * 86400,
      b = lubridate::today() + runif(1e3) * 30,
      c = 1:1e3,
      d = runif(1e3),
      e = sample(letters, 1e3, replace = TRUE)
    )
    ```

----

- To change number of rows and columns to display:
    ```{r}
    nycflights13::flights %>% 
      print(n = 10, width = Inf)
    ```
    
----

- To change the default print setting:
    - `options(tibble.print_max = n, tibble.print_min = m)`: if more than `m` rows, print only `n` rows.
    - `options(dplyr.print_min = Inf)`: print all row.
    - `options(tibble.width = Inf)`: print all columns.

## Subsetting

- 
    ```{r}
    df <- tibble(
      x = runif(5),
      y = rnorm(5)
    )
    ```

- Extract by name:
    ```{r}
    df$x
    df[["x"]]
    ```

----

- Extract by position：
    ```{r}
    df[[1]]
    ```

- Pipe:
    ```{r}
    df %>% .$x
    df %>% .[["x"]]
    ```

# Data import | r4ds chapter 11

## 

- readr package implements functions that turn flat files into tibbles.

    - `read_csv()`, `read_csv2()` (semicolon seperated files), `read_tsv()`,     `read_delim()`.

    - `read_fwf()` (fixed width files), `read_table()`.

    - `read_log()` (Apache style log files). 

- An example file [heights.csv](http://hua-zhou.github.io/teaching/biostatm280-2018winter/slides/08-tidy/heights.csv):
    ```{bash}
    head heights.csv
    ```

----

- Read from a local file [heights.csv](http://hua-zhou.github.io/teaching/biostatm280-2018winter/slides/08-tidy/heights.csv):
    ```{r}
    (heights <- read_csv("heights.csv"))
    ```
    
----

- Read from inline csv file:
    ```{r}
    read_csv("a,b,c
    1,2,3
    4,5,6")
    ```

- Skip first `n` lines:
    ```{r}
    read_csv("The first line of metadata
      The second line of metadata
      x,y,z
      1,2,3", skip = 2)
    ```
    
----

- Skip comment lines:
    ```{r}
    read_csv("# A comment I want to skip
      x,y,z
      1,2,3", comment = "#")
    ```
    
- No header line:
    ```{r}
    read_csv("1,2,3\n4,5,6", col_names = FALSE)
    ```

----

- No header line and specify colnames:
    ```{r}
    read_csv("1,2,3\n4,5,6", col_names = c("x", "y", "z"))
    ```
    
- Specify the symbol representing missing values:
    ```{r}
    read_csv("a,b,c\n1,2,.", na = ".")
    ```
    
## Writing to a file

- Write to csv:
    ```{r, eval = FALSE}
    write_csv(challenge, "challenge.csv")
    ```
    
- Write (and read) RDS files:
    ```{r, eval = FALSE}
    write_rds(challenge, "challenge.rds")
    read_rds("challenge.rds")
    ```

## Other types of data

- **haven** reads SPSS, Stata, and SAS files.

- **readxl** reads excel files (both .xls and .xlsx).

- **DBI**, along with a database specific backend (e.g. **RMySQL**, **RSQLite**, **RPostgreSQL** etc) allows you to run SQL queries against a database and return a data frame.

- **jsonlite** reads json files.

- **xml2** reads XML files.

# Tidy data | r4ds chapter 12

## Tidy data

There are three interrelated rules which make a dataset tidy:

- Each variable must have its own column.

- Each observation must have its own row.

- Each value must have its own cell.

<p align="center">
<img src="./tidy-1.png" height="200">
</p>

----

- Example table1
    ```{r}
    table1
    ```
is tidy.

----

- Example table2
    ```{r}
    table2
    ```
is not tidy.    

----

- Example table3
    ```{r}
    table3
    ```
is not tidy.
    
----

- Example table4a
    ```{r}
    table4a
    ```
is not tidy.    

- Example table4b
    ```{r}
    table4b
    ```
is not tidy.


## Gathering

<p align="center">
<img src="./tidy-9.png" height="200">
</p>

- `gather` columns into a new pair of variables.
    ```{r}
    table4a %>%
      gather(`1999`, `2000`, key = "year", value = "cases")
    ```

----

- We can gather table4b too and then join them
    ```{r}
    tidy4a <- table4a %>% 
      gather(`1999`, `2000`, key = "year", value = "cases")
    tidy4b <- table4b %>% 
      gather(`1999`, `2000`, key = "year", value = "population")
    left_join(tidy4a, tidy4b)
    ```

## Spreading

<p align="center">
<img src="./tidy-8.png" height="200">
</p>

- Spreading is the opposite of gathering.
    ```{r}
    spread(table2, key = type, value = count)
    ```

## Separating

<p align="center">
<img src="./tidy-17.png" height="200">
</p>

- 
    ```{r}
    table3 %>% 
      separate(rate, into = c("cases", "population"))
    ```

----

- Seperate into numeric values:
    ```{r}
    table3 %>% 
      separate(rate, into = c("cases", "population"), convert = TRUE)
    ```

----

- Separate at a fixed position:
    ```{r}
    table3 %>% 
      separate(year, into = c("century", "year"), sep = 2)
    ```

## Unite

<p align="center">
<img src="./tidy-18.png" height="200">
</p>

- 
    ```{r}
    table5
    ```

----

- `unite()` is the inverse of `separate()`. 
    ```{r}
    table5 %>% 
      unite(new, century, year, sep = "")
    ```