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# core::ops::index::Index

A trait for indexing operations (`container[index]`) where the input type is mutated.
This trait should be implemented when you want to implement indexing operations on a type that's
mutated by a read access. This is useful for any type depending on a [`Felt252Dict`](./core-dict-Felt252Dict), where
dictionary accesses are modifying the data structure itself.
`container[index]` is syntactic sugar for `container.index(index)`.

## Signature

```rust theme={null}
pub trait Index
```

## Examples

The following example implements `Index` on a `Stack` type. This `Stack` is implemented based on
a [`Felt252Dict`](./core-dict-Felt252Dict), where dictionary accesses are modifying the dictionary itself. As such, we
must implement the `Index` trait instead of the `IndexView` trait.

```rust theme={null}
use core::ops::Index;

#[derive(Destruct, Default)]
struct Stack {
    items: Felt252Dict,
    len: usize
}

#[generate_trait]
impl StackImpl of StackTrait {
    fn push(ref self: Stack, item: u128) {
        self.items.insert(self.len.into(), item);
        self.len += 1;
    }
}

impl StackIndex of Index {
     type Target = u128;

     fn index(ref self: Stack, index: usize) -> Self::Target {
         if index >= self.len {
             panic!("Index out of bounds");
         }
         self.items.get(index.into())
     }
 }

let mut stack: Stack = Default::default();
stack.push(1);
assert!(stack[0] == 1);
```

## Trait functions

### index

Performs the indexing (`container[index]`) operation.

#### Panics

May panic if the index is out of bounds.

#### Signature

```rust theme={null}
fn index(ref self: C, index: I) -> IndexTarget
```

## Trait types

### Target

The returned type after indexing.

#### Signature

```rust theme={null}
type Target;
```