# Legacy - 08 - Stack Data Queries

Stack data queries are opcodes that return information directly relating to the stack or to the data item at the top of the stack.

### Stack Depth Check

The opcode OP\_DEPTH will leave a value on the stack which indicates the depth of the stack. For example, if the stack had 3 items on it, calling OP\_DEPTH would leave the integer '3' as a new item at the top of the stack.

<table><thead><tr><th width="132">Word</th><th width="95">Input</th><th width="132">Output</th><th>Description</th></tr></thead><tbody><tr><td>OP_DEPTH</td><td>items</td><td>items, qty items</td><td>Counts the number of stack items onto the stack and places the value on the top</td></tr></tbody></table>

<figure><img src="/files/YFiFIh6CpPelzDpd3JUN" alt=""><figcaption></figcaption></figure>

#### Example:

`OP_DEPTH OP_1 OP_EQUAL OP_IF`

`<pubkey>`

`OP_ELSE`

`OP_DUP OP_HASH160 <pubkeyhash> OP_EQUALVERIFY`

`OP_ENDIF`

`OP_CHECKSIG`

In this example, the script checks the depth of the stack. If there is just 1 item on the stack, it assumes it is a signature and checks it against a public key stored in the script. If there is more than one item on the stack, it assumes this is a public key and signature, which it checks against a stored public key hash.

### Size Query <a href="#size-query" id="size-query"></a>

OP\_SIZE evaluates the first item on the stack and adds its length to the top of the stack.

<table><thead><tr><th width="118">Word</th><th width="91">Input</th><th width="103">Output</th><th>Description</th></tr></thead><tbody><tr><td>OP_SIZE</td><td>item</td><td>item, item size</td><td>Pushes the string length of the top element of the stack (without popping it).</td></tr></tbody></table>

<figure><img src="/files/AI2nQ10n2Rt1Kt8hln4y" alt=""><figcaption></figcaption></figure>

#### Example:

`OP_SIZE OP_10 OP_LESSTHANOREQUAL OP_VERIFY`

In this example, the script checks that an item input to the stack is less than or equal to 10 bytes long. If the string is longer than 10 bytes long, the script fails. This can be useful when testing input conditions from user entered forms that are directly submitted to script.


---

# 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://hub.bsvblockchain.org/higher-learning/bsv-academy/chapter-3-the-opcodes/08-stack-data-queries.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.