# Mutation

**EVM**

```solidity
contract Counter {
  mapping(address user => uint256 count) public counter;
  
  function increment(address to, uint256 amount) external {
    counter[to] += amount;
  }
}
```

**Solana**

**First time**

The flag `init` or `init_if_needed` is required to initialize the `Counter` account.

Notes on some terminologies:

* seeds - It is used to derive the address (Program Derived Address). It can be static or dynamic and it can contain multiple elements
* seeds::program - It is used to derive the address for other programs
* bump - Bump seed is a provided value that ensures the account's address is off the Ed25519 curve. An address that's off the curve does not have a corresponding private key
* space - Each account occupies some space and it tells Solana how much space is needed. There is always an extra 8 bytes for Anchor Discriminator. This [post](https://solana.stackexchange.com/questions/4948/what-is-anchor-8-bytes-discriminator#4949) explains it well.

```rust
#[account]
pub struct Counter {
  pub user: Pubkey,
  pub count: u64,
}

#[derive(Accounts)]
pub struct Increment {
  #[account(init, seeds = [b"user", user.key()], bump, space = 8 + 32 + 8)
  pub counter: Account<'info, Counter>,
  
  pub user: Signer<'info>,
}

pub fn increment(ctx: Context<Increment>, amount: u64) -> Result<()> {
  ctx.accounts.counter = ctx.accounts.counter.checked_add(amount).unwrap();
  Ok(())
}
```

**Not the first time**

The flag `mut` is required to let Solana know that the account is mutable. The program will [fail silently](https://github.com/coral-xyz/anchor/issues/326) if the flag is not present!

```rust
#[account]
pub struct Counter {
  pub user: Pubkey,
  pub count: u64,
}

#[derive(Accounts)]
pub struct Increment {
  #[account(mut, seeds = [b"user", user.key()])
  pub counter: Account<'info, Counter>,
  
  pub user: Signer<'info>,
}

pub fn increment(ctx: Context<Increment>, amount: u64) -> Result<()> {
  ctx.accounts.counter = ctx.accounts.counter.checked_add(amount).unwrap();
  Ok(())
}
```


---

# 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://0xkowloon.gitbook.io/anchor-for-evm-developers/mutation.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.