tidyverse_dplyr_apply.Rmd

# dplyr进阶 {#tidyverse-dplyr-apply}


本章主要关注dplyr的一些应用。



## 导入数据

今天讲一个关于企鹅的数据故事。

```{r message = FALSE, warning = FALSE}
library(tidyverse)
library(palmerpenguins)

penguins <- penguins %>% drop_na()
```


## 变量含义

|variable          |class     |description |
|:-----------------|:---------|:-----------|
|species           |character | 企鹅种类 (Adelie, Gentoo, Chinstrap) |
|island            |character | 所在岛屿 (Biscoe, Dream, Torgersen) |
|bill_length_mm    |double    | 嘴峰长度 (单位毫米) |
|bill_depth_mm     |double    | 嘴峰深度 (单位毫米)|
|flipper_length_mm |integer   | 鰭肢长度 (单位毫米) |
|body_mass_g       |integer   | 体重 (单位克) |
|sex               |character | 性别 |
|year              |integer   | 记录年份 |



```{r out.width = '86%', echo = FALSE}
knitr::include_graphics("images/culmen_depth.png")
```


## 简单回顾 
### 选择"bill_"开始的列

```{r, eval=FALSE}
penguins %>% select(bill_length_mm, bill_depth_mm)
```


```{r}
penguins %>% select(starts_with("bill_"))
```



### 选择"_mm"结尾的列

```{r, eval=FALSE}
penguins %>% select(bill_length_mm, bill_depth_mm, flipper_length_mm)
```


```{r}
penguins %>% select(ends_with("_mm"))
```



### 选择含有"length"的列
```{r, eval=FALSE}
penguins %>% select(bill_length_mm, flipper_length_mm)
```


```{r}
penguins %>% select(contains("length"))
```



### 选择数值型的列

```{r}
penguins %>% select(where(is.numeric))
```


### 选择字符串类型的列

```{r}
penguins %>% select(where(is.character))
```


### 选择字符串类型以外的列

```{r}
penguins %>% select(!where(is.character))
```

### 可以用多种组合来选择

```{r}
penguins %>% select(species, starts_with("bill_"))
```


### 返回向量还是数据框

对应数据框`my_tibble`， 注意返回向量还是数据框的区别

- 返回向量
```{r, eval = FALSE}
my_tibble[["x"]] 

my_tibble$x 

my_tibble %>% 
  pull(x)
```


- 返回数据框
```{r, eval = FALSE}
my_tibble["x"]

my_tibble %>% 
  select(x)
```


### 选择全部为0的列
```{r}
tb <- tibble(
  x = 1:5,
  y = 0,
  z = 5:1,
  w = 0
)
tb



myfun <- function(x) sum(x) == 0
tb %>%
  select(where(myfun))



# or
tb %>%
  select(where(~sum(.x) == 0))
```

**课堂练习**：剔除全部为NA的列
```{r}
df <- tibble(
  x = c(NA, NA, NA),
  y = c(2, 3, NA),
  z = c(NA, 5, NA)
)


df %>%
  select(where(~ !all(is.na(.x))))

# or
df %>%
  filter(
    if_any(everything(), ~ !is.na(.x))
  )
```



### 寻找男企鹅

函数 `filter()` 中的逻辑运算符

Operator  | Meaning
----------|--------
`==`      | Equal to
`>`       | Greater than
`<`       | Less than
`>=`      | Greater than or equal to
`<=`      | Less than or equal to
`!=`      | Not equal to
`%in%`    | in 
`is.na`   | is a missing value (NA)
`!is.na`  | is not a missing value
`&`       | and
`|`       | or


```{r}
penguins %>% filter(sex == "male")
```


```{r}
penguins %>% filter(species %in% c("Adelie", "Gentoo"))
```


```{r}
penguins %>%
  filter(species == "Adelie" &  bill_length_mm > 40)

penguins %>%
  filter(species == "Adelie", bill_length_mm > 40)
```


**课堂练习**，说出以下代码的含义
```{r, eval=FALSE}
penguins %>%
  filter(species == "Adelie", bill_length_mm == max(bill_length_mm) )
```



## 更多应用

希望介绍一个技术，对应一个应用场景


### 弱水三千，只取一瓢
```{r}
penguins %>% 
  head()

penguins %>% 
  tail()
```


```{r}
penguins %>% 
  slice(1)
```


```{r}
penguins %>% 
  group_by(species) %>% 
  slice(1)
```


### 嘴峰长度最大那一行

三种方法

```{r}
penguins %>%
  filter(bill_length_mm == max(bill_length_mm) )
```


```{r}
penguins %>% 
  arrange(desc(bill_length_mm)) %>% 
  slice(1)
```


```{r}
penguins %>% 
  slice_max(bill_length_mm)
```


### separate
```{r}
tb <- tibble::tribble(
  ~day, ~price,
  1,   "30-45",
  2,   "40-95",
  3,   "89-65",
  4,   "45-63",
  5,   "52-42"
)

```

```{r}
tb1 <- tb %>% 
  separate(price, into = c("low", "high"), sep = "-")
tb1
```

### unite
```{r}
tb1 %>% 
  unite(col = "price", c(low, high), sep = ":", remove = FALSE)
```


### distinct

`distinct()`处理的对象是data.frame；功能是**筛选不重复的row**；返回data.frame

```{r}
df <- tibble::tribble(
  ~x, ~y, ~z,
  1, 1, 1,
  1, 1, 2,
  1, 1, 1,
  2, 1, 2,
  2, 2, 3,
  3, 3, 1
)
df
```


```{r}
df %>%
  distinct()
```


```{r}
df %>%
  distinct(x)


df %>%
  distinct(x, y)
```


```{r}
df %>%
  distinct(x, y, .keep_all = TRUE) # 只保留最先出现的row
```


```{r, eval=FALSE}
df %>%
  distinct(
    across(c(x, y)),
    .keep_all = TRUE
  )
```


```{r}
df %>%
  group_by(x) %>%
  distinct(y, .keep_all = TRUE)
```


`n_distinct()`处理的对象是vector；功能是**统计不同的元素有多少个**；返回一个数值

```{r}
c(1, 1, 1, 2, 2, 1, 3, 3) %>% n_distinct()
```


```{r}
df$z %>% n_distinct()
```


```{r}
df %>%
  group_by(x) %>%
  summarise(
    n = n_distinct(z)
  )
```


### 有关NA的计算

`NA`很讨厌，凡是它参与的四则运算，结果都是`NA`，

```{r}
sum(c(1, 2, NA, 4))
```

所以需要事先把它删除，增加参数说明 `na.rm = TRUE`
```{r}
sum(c(1, 2, NA, 4), na.rm = TRUE)
```

```{r}
mean(c(1, 2, NA, 4), na.rm = TRUE)
```


### 寻找企鹅中的胖子

```{r}
penguins %>% 
  mutate(
    body = if_else(body_mass_g > 4200, "you are fat", "you are fine")
)
```

**随堂练习**：用考试成绩的均值代替缺失值

```{r}
df <- tibble::tribble(
    ~name,     ~type, ~score,
  "Alice", "english",    80,
  "Alice",    "math",    NA,
    "Bob", "english",    70,
    "Bob",    "math",    69,
  "Carol", "english",    NA,
  "Carol",    "math",    90
  )
df
```


```{r}
df %>% 
  group_by(type) %>% 
  mutate(mean_score = mean(score, na.rm = TRUE)) %>% 
  mutate(newscore = if_else(is.na(score), mean_score, score))
```


### 给企鹅身材分类

```{r}
penguins %>% 
  mutate(
   body = case_when(
     body_mass_g < 3500                       ~ "best",
     body_mass_g >= 3500 & body_mass_g < 4500 ~ "good",
     body_mass_g >= 4500 & body_mass_g < 5500 ~ "general",
     TRUE                                     ~ "other"
  )
)
```


**随堂练习**：按嘴峰长度分成A, B, C, D 4个等级

```{r}
penguins %>% 
  mutate(
    degree = case_when(
      bill_length_mm < 35                        ~ "A",
      bill_length_mm >= 35 & bill_length_mm < 45 ~ "B",
      bill_length_mm >= 45 & bill_length_mm < 55 ~ "C",
      TRUE                                       ~ "D"
  )
)
```



### 每种类型企鹅有多少只？

知识点：`n()`函数，统计当前分组数据框的行数

```{r}
penguins %>%
  summarise(
    n = n()
  ) 
```


```{r}
penguins %>%
  group_by(species) %>%
  summarise(
    n = n()
  )
```


统计某个变量中**各组**出现的次数，可以使用`count()`函数

```{r}
penguins %>% count(species)
```


不同性别的企鹅各有多少
```{r}
penguins %>% count(sex, sort = TRUE)
```

可以统计不同组合出现的次数
```{r}
penguins %>% count(island, species)
```


可以在`count()`里构建新变量，并利用这个新变量完成统计。
比如，统计嘴巴长度大于40的企鹅个数

- 常规做法
```{r}
penguins %>%
  filter(bill_length_mm > 40) %>% 
  summarise(
    n = n()
  )
```

- `count()`做法
```{r}
penguins %>% count(longer_bill = bill_length_mm > 40)
```

解析思路：`bill_length_mm > 40` 比较算符 返回逻辑型向量，向量里面只有TRUR和FALSE两种值，因此上面的代码相当于统计TRUE有多少个，FALSE有多少个？



### 强制转换

矢量中的元素必须是相同的类型，但如果不一样呢，会发生什么？
这个时候R会**强制转换**成相同的类型。这就涉及数据类型的转换层级

- character > numeric > logical
- double > integer

比如这里会强制转换成字符串类型
```{r}
c("foo", 1, TRUE)
```

这里会强制转换成数值型
```{r}
c(1, TRUE, FALSE)
```

```{r}
c(TRUE, TRUE, FALSE) %>% sum()
```

**随堂练习**：补全下面代码，求嘴峰长度大于40mm的占比？

```{r}
penguins %>% 
  mutate(is_bigger40 = bill_length_mm > 40)
```


```{r, eval=FALSE, echo=FALSE}
penguins %>% 
  mutate(is_bigger40 = bill_length_mm > 40) %>% 
  summarise(
    peop = sum(is_bigger40) / n()
  )
```

## across()之美

我们想知道，嘴巴长度和厚度的均值

```{r}
penguins %>%
  summarize(
    length = mean(bill_length_mm)
  )
```

接着添加下个变量
```{r}
penguins %>%
  summarize(
    length = mean(bill_length_mm),
    depth  = mean(bill_length_mm)
  )
```

长度和厚度惊人的相等。我是不是发现新大陆了？


### across()函数
更安全、更简练的写法，王老师的最爱

```{r}
penguins %>%
  summarize(
     across(c(bill_depth_mm, bill_length_mm), mean)
 )
```

翅膀的长度加进去看看

```{r}
penguins %>%
  summarize(
     across(c(bill_depth_mm, bill_length_mm, flipper_length_mm), mean)
 )
```


还可以更简练喔
```{r}
penguins %>%
  summarize(
    across(ends_with("_mm"), mean)
 )
```


::: {.rmdnote}

`across()`函数用法

```{r, eval = FALSE}
across(.cols = everything(), .fns = NULL, ..., .names = NULL)
```

- 用在 `mutate()` 和`summarise()` 函数里面
- `across()` 对**多列**执行**相同**的函数操作，返回**数据框**

::: 



### 数据中心化

```{r}
penguins %>%
  mutate(
    bill_length_mm = bill_length_mm - mean(bill_length_mm),
    bill_depth_mm  = bill_depth_mm  - mean(bill_depth_mm)
    )
```


更清晰的办法

```{r}
centralized <- function(x) {
  x - mean(x)
}


penguins %>%
  mutate(
    across(c(bill_length_mm, bill_depth_mm), centralized)
  )
```


### 数据标准化

```{r}
std <- function(x) {
  (x - mean(x, na.rm = TRUE)) / sd(x, na.rm = TRUE)
}


penguins %>%
  mutate(
    across(c(bill_length_mm, bill_depth_mm), std)
  )
```


或者使用更简洁的方法
```{r}
# using across() and purrr style
penguins %>%
  summarise(
    across(starts_with("bill_"), ~ (.x - mean(.x)) / sd(.x))
  )
```



### 多列多个统计函数

```{r}
penguins %>%
  group_by(species) %>%
  summarise(
    across(ends_with("_mm"), list(mean = mean, sd = sd), na.rm = TRUE)
  )
```


**随堂练习**：以sex分组，对"bill_"开头的列，求出每列的最大值和最小值


```{r}
penguins %>%
  group_by(sex) %>%
  summarise(
    across(starts_with("bill_"), list(max = max, min = min), na.rm = TRUE)
  )
```



在第 \@ref(tidyverse-beauty-of-across1) 章到第 \@ref(tidyverse-beauty-of-across4) 章会继续讲王老师的最爱`across()`函数。