# Conditional Lists and Keys

You will often want to display multiple similar components from a collection of data. 

In this chapter, you will learn:

- How to use iterators in `rsx!`
- How to filter and transform data into a list of Elements
- How to create efficient lists with keys

## Rendering data from lists

Thinking back to our analysis of the `r/reddit` page, we notice a list of data that needs to be rendered: the list of posts. This list of posts is always changing, so we cannot just hardcode the lists into our app like:

```rust
rsx!(
    div {
        Post {/* some properties */}
        Post {/* some properties */}
        Post {/* some properties */}
    }
)
```

Instead, we need to transform the list of data into a list of Elements. 

For convenience, `rsx!` supports any type in curly braces that implements the `IntoVnodeList` trait. Conveniently, every iterator that returns something that can be rendered as an Element also implements `IntoVnodeList`. 

As a simple example, let's render a list of names. First, start with our input data:

```rust
let names = ["jim", "bob", "jane", "doe"];
```

Then, we create a new iterator by calling `iter` and then `map`. In our `map` function, we'll place render our template.

```rust
let name_list = names.iter().map(|name| rsx!(
    li { "{name}" }
));
```

Finally, we can include this list in the final structure:

```rust
rsx!(
    ul {
        {name_list}
    }
)
```
The HTML-rendered version of this list would follow what you would expect:
```html
<ul>
    <li> jim </li>
    <li> bob </li>
    <li> jane </li>
    <li> doe </li>
</ul>
```

### Rendering our posts with a PostList component

Let's start by modeling this problem with a component and some properties. 

For this example, we're going to use the borrowed component syntax since we probably have a large list of posts that we don't want to clone every time we render the Post List.

```rust
#[derive(Props, PartialEq)]
struct PostListProps<'a> {
    posts: &'a [PostData]
}
```
Next, we're going to define our component:

```rust
fn App(cx: Scope<PostList>) -> Element {
    // First, we create a new iterator by mapping the post array
    let posts = cx.props.posts.iter().map(|post| rsx!{
        Post {
            title: post.title,
            age: post.age,
            original_poster: post.original_poster
        }
    });

    // Finally, we render the post list inside of a container
    cx.render(rsx!{
        ul { class: "post-list"
            {posts}
        }
    })
}
```


## Filtering Iterators

Rust's iterators are extremely powerful, especially when used for filtering tasks. When building user interfaces, you might want to display a list of items filtered by some arbitrary check.

As a very simple example, let's set up a filter where we only list names that begin with the letter "J". 

Let's make our list of names:

```rust
let names = ["jim", "bob", "jane", "doe"];
```

Then, we create a new iterator by calling `iter`, then `filter`, then `map`. In our `filter` function, we'll only allow "j" names, and in our `map` function, we'll  render our template.

```rust
let name_list = names
    .iter()
    .filter(|name| name.starts_with('j'))
    .map(|name| rsx!( li { "{name}" }));
```

Rust's iterators provide us tons of functionality and are significantly easier to work with than JavaScript's map/filter/reduce.

For keen Rustaceans: notice how we don't actually call `collect` on the name list. If we `collected` our filtered list into new Vec, then we would need to make an allocation to store these new elements. Instead, we create an entirely new _lazy_ iterator which will then be consumed by Dioxus in the `render` call. 

The `render` method is extraordinarily efficient, so it's best practice to let it do most of the allocations for us.

## Keeping list items in order with key


The examples above demonstrate the power of iterators in `rsx!` but all share the same issue: a lack of "keys". Whenever you render a list of elements, each item in the list must be **uniquely identifiable**. To make each item unique, you need to give it a "key".

In Dioxus, keys are strings that uniquely identifies it among other items in that array:

```rust
rsx!( li { key: "a" } )
```

Keys tell Dioxus which array item each component corresponds to, so that it can match them up later. This becomes important if your array items can move (e.g. due to sorting), get inserted, or get deleted. A well-chosen key helps Dioxus infer what exactly has happened, and make the correct updates to the screen


NB: the language from this section is strongly borrowed from [React's guide on keys](https://reactjs.org/docs/lists-and-keys.html).
### Where to get your key

Different sources of data provide different sources of keys:

- _Data from a database_: If your data is coming from a database, you can use the database keys/IDs, which are unique by nature.
- _Locally generated data_: If your data is generated and persisted locally (e.g. notes in a note-taking app), use an incrementing counter or a package like `uuid` when creating items.

### Rules of keys

- Keys must be unique among siblings. However, it’s okay to use the same keys for Elements in different arrays.
- Keys must not change or that defeats their purpose! Don’t generate them while rendering.

### Why does Dioxus need keys?

Imagine that files on your desktop didn’t have names. Instead, you’d refer to them by their order — the first file, the second file, and so on. You could get used to it, but once you delete a file, it would get confusing. The second file would become the first file, the third file would be the second file, and so on.

File names in a folder and Element keys in an array serve a similar purpose. They let us uniquely identify an item between its siblings. A well-chosen key provides more information than the position within the array. Even if the position changes due to reordering, the key lets Dioxus identify the item throughout its lifetime.

### Gotcha
You might be tempted to use an item’s index in the array as its key. In fact, that’s what Dioxus will use if you don’t specify a key at all. But the order in which you render items will change over time if an item is inserted, deleted, or if the array gets reordered. Index as a key often leads to subtle and confusing bugs.

Similarly, do not generate keys on the fly, `gen_random`. This will cause keys to never match up between renders, leading to all your components and DOM being recreated every time. Not only is this slow, but it will also lose any user input inside the list items. Instead, use a stable ID based on the data.

Note that your components won’t receive key as a prop. It’s only used as a hint by Dioxus itself. If your component needs an ID, you have to pass it as a separate prop:
```rust
Post { key: "{key}", id: "{id}" }
```

## Moving on

In this section, we learned:
- How to render lists of data
- How to use iterator tools to filter and transform data
- How to use keys to render lists efficiently

Moving forward, we'll finally cover user input and interactivity.
