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title: "R Operators & Pipes"
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```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

# Introduction

Load packages:
```{r, message=FALSE}
#install.packages('tidyverse')
library(tidyverse)
```


Load data:  
```{r}
#data we used in the last problem set
load(url('https://github.com/anyone-can-cook/rclass1/raw/master/data/recruiting/recruit_event_somevars.RData'))  

#data we are using in this lecture 
load(url("https://github.com/ozanj/rclass/raw/master/data/prospect_list/wwlist_merged.RData"))
```

Resources used to create this document:

- https://www.datamentor.io/r-programming/precedence-associativity/  
- https://stat.ethz.ch/R-manual/R-devel/library/base/html/Syntax.html  
- https://www.w3adda.com/r-tutorial/r-operator-precedence


# Operator Precedence and Associativity  

It is important to understand operator precedence and associativity when working with data in general, but particularly when we use logical operators like `&` or `|` to filter data.  

<br>  

**Operator Precedence**  

Operator Precedence refers to the order in which operators are executed. Operators with higher precedence are evaluated first and operators with the lowest precedence are evaluated last. If you recall [PEMDAS](https://study.com/academy/lesson/what-is-pemdas-definition-rule-examples.html#:~:text=PEMDAS%20is%20an%20acronym%20for,until%20the%20calculation%20is%20complete.), multiplication `*` takes precedence over addition `+`.   

```{r}
1 + 2 * 6
```

If we add a parenthesis to the equation, our answer changes.  
```{r}
(1 + 2) * 6
```

<br>   

**Operator Associativity**  

When you are working with operators with the same precedence, the execution of the operators is determined through associativity.   

> Operators with same precedence follows operator associativity defined for its operator group. In R, operators can either follow left-associative, right-associative or have no associativity. Operators with left-associative are evaluted from left to right, operators with right-associative are evaluated from right to left and operators with no associativity, does not follow any predefined order.  

*Credit: [R Operator Precedence](https://www.w3adda.com/r-tutorial/r-operator-precedence)*

```{r}

8 / 4 / 2
```

```{r}
8 / (4 / 2)
```

*Credit: [R Operator Precedence and Associativity](https://www.w3adda.com/r-tutorial/r-operator-precedence), [Operator Syntax and Precedence](https://stat.ethz.ch/R-manual/R-devel/library/base/html/Syntax.html)*
  
<br>   
  
**R logical operator precedence**  

According to this [link](https://stat.ethz.ch/R-manual/R-devel/library/base/html/Syntax.html), the `&` takes precedence over `|`. And both logical operators are left-associative, meaning they are evaluated from left to right.  

*Credit [R Operator Precedence](https://www.datamentor.io/r-programming/precedence-associativity/)*  

<br>  

**Now let's see an example using the data frame from problem set 3**   


Why do we get different results?
```{r}
nrow(subset(df_event, event_state %in% c("CA", "FL", "MA") & event_type == "public hs" | total_students_pri > 1000))

nrow(subset(df_event, event_state %in% c("CA", "FL", "MA") & (event_type == "public hs" | total_students_pri > 1000)))
```


Let's try to put our code into words.   

The first code reads. Return observations where the recruiting visit took place at a public high school in the state of California, Florida, or Massachusetts **OR** a private high school where the total number of students is greater than 1000. 

```{r}
tail(subset(df_event, event_state %in% c("CA", "FL", "MA") & event_type == "public hs" | total_students_pri > 1000, select = c(event_state, event_type, total_students_pri)))
```

The second code reads. Return observations where the recruiting visit took place at a public high school **OR** private high school where the total number of students is greater than 1000 in the state of California, Florida, or Massachusetts.

```{r}
tail(subset(df_event, event_state %in% c("CA", "FL", "MA") & (event_type == "public hs" | total_students_pri > 1000), select = c(event_state, event_type, total_students_pri)))
```


# Practice with pipes    

**wwlist** data frame  

- De-identified list of prospective students purchased by Western Washington University from College Board
- We collected these data using public records requests request

```{r}
dim(wwlist)
names(wwlist)
#glimpse(wwlist)
#str(wwlist)
```

Let's use `select()`, `filter()`, and `arrange()` to do the following using the Base R approach:  

- Sort `wwlist` descending by `total_students`  
- Select the following variables: `hs_state`,  `hs_city`, `hs_name`, `school_type`, `total_students`  
- Filter for private schools `(school_type == "private")`  
- Print the first 10 observations
```{r}
head(select(arrange(filter(wwlist, school_type == "private"), desc(total_students)), hs_state, hs_city, hs_name, school_type, total_students), n = 10)
```

```{r}
df_temp <- filter(wwlist,school_type == "private")
df_temp2 <- arrange(df_temp,desc(total_students))
head(select(df_temp2, hs_state, hs_city, hs_name, school_type, total_students),n=10)
rm(df_temp,df_temp2)
```

Now let's use pipes `%` 
```{r}
wwlist %>%
  filter(school_type == "private") %>%
  arrange(desc(total_students)) %>%
  select(state, hs_name, school_type, total_students) %>%
  head(n = 10)
```

<br>  

**Your turn**  

Use`select()`, `filter()`, and `arrange()` to do the following using both the Base R & tidyverse approach:

- Sort `wwlist` descending by `med_inc_zip`  
- Select the following variables: `hs_state`,  `hs_city`, `hs_name`, `school_type`, `med_inc_zip`, `ethn_code`, `med_inc_state`  
- Filter for public schools `(school_type == "public")` in the state of New York `hs_state == "NY"`
- Print the first 10 observations

**Base R**
```{r}

```

**Tidyverse using pipes**  
```{r}

```


<br>  

Now let's use`select()`, `filter()`, and `arrange()` to do the following using both the Base R & tidyverse approach:  

- Sort `wwlist` descending by `med_inc_zip`  
- Select the following variables: `hs_state`,  `hs_city`, `hs_name`, `school_type`, `med_inc_zip`, `ethn_code`, `med_inc_state`  
- Filter for public schools `(school_type == "public")` where the `med_inc_zip` is less than the `med_inc_state`
- Print the first 10 observations

**Base R**
```{r}

```

**Tidyverse using pipes**
```{r}

```

<br>  

**Bonus** question   

- Write down a question you have about the data.  
- Using any or all of the following functions `select()`, `filter()`, `arrange()`, how would you go about subsetting and sorting the data to answer your question?  

Write down your question below:  


Now try to work through your question. 
```{r}

```