# Collections

## Shielded Arrays

Arrays of shielded types work like standard Solidity arrays. Keys (indices) must be non-shielded — using a shielded type as an array index is a compiler error.

They come in two forms:

* **Dynamic** (`suint256[]`, `sbool[]`, `saddress[]`, etc.) — the length is stored as shielded.
* **Fixed-size** (`suint256[5]`, `sbool[4]`, `saddress[3]`, etc.) — the length is a compile-time constant and publicly visible.

```solidity
suint256[] private balances;     // dynamic — shielded length
sbool[4] private flags;          // fixed — length 4 is public

function example(uint256 i) public {
    balances[i] = suint256(100);   // valid — uint256 index
    flags[0] = sbool(true);        // valid — literal index
}
```

{% hint style="warning" %}
Even with dynamic shielded arrays, an upper bound on the length may be visible to observers monitoring gas costs, since gas usage scales with array operations.
{% endhint %}

## Shielded Mappings

Mappings can have shielded values but **keys cannot be shielded types**. Using a shielded type as a mapping key is a compiler error. The standard `mapping` syntax applies.

```solidity
mapping(address => suint256) private balances;    // valid
mapping(uint256 => sbool) private flags;          // valid
mapping(address => saddress) private recipients;  // valid

mapping(saddress => uint256) private lookup;      // INVALID — shielded key
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.seismic.systems/seismic-solidity/collections.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.