build an end to end Shiny Real Estate app using R and the zillow API. Explain each step then at the end Print out the entire code and indicate the online sandbox which allows this to be tested. Include unit tests and include error handling
Here's a step-by-step guide to building an end-to-end Shiny Real Estate app using R and the Zillow API:

**Step 1: Get a Zillow API key**

To use the Zillow API, you need to sign up for a Zillow account and obtain an API key. You can do this by going to the Zillow API website and following the instructions.

**Step 2: Install required R packages**

You'll need to install the following R packages:

* `httr` for making API requests
* `jsonlite` for parsing JSON data
* `shiny` for building the Shiny app
* `DT` for displaying data tables
* `leaflet` for displaying maps

You can install these packages using the following code:
```R
install.packages(c("httr", "jsonlite", "shiny", "DT", "leaflet"))
```
**Step 3: Create a function to retrieve Zillow data**

Create a function that takes in a ZIP code and returns a data frame with information about the median home value, average listing price, and other relevant metrics. You can use the Zillow API's `GetDeepSearchResults` endpoint to retrieve this data.

Here's an example function:
```R
get_zillow_data <- function(zip_code, api_key) {
  url <- "http://www.zillow.com/webservice/GetDeepSearchResults.htm"
  query <- list(
    "zws-id
build an end to end Shiny Real Estate app using R and the zillow API. Explain each step then at the end Print out the entire code and indicate the online sandbox which allows this to be tested. Include unit tests and include error handling.

To build an end-to-end Shiny Real Estate app using R and the Zillow API, follow these steps:

1. Install and load required packages:

```R
install.packages(c("shiny", "httr", "jsonlite", "dplyr", "stringr", "testthat"))
library(shiny)
library(httr)
library(jsonlite)
library(dplyr)
library(stringr)
```

2. Create a function to fetch data from the Zillow API:

```R
get_zillow_data <- function(address, api_key) {
  url <- paste0("https://www.zillow.com/webservice/GetSearchResults.htm?zws-id=", api_key, "&address=", address)
  response <- GET(url)
  content <- content(response, "text")
  data <- fromJSON(content)

  if (!is.null(data$response$results$result)) {
    return(data$response$results$result[[1]])
  } else {
    return(NULL)
  }
}
```

3. Create a unit test for the function:

```R
test_get