# Make card attributes random · Decentralized marketplace with FT4 · Learn Chromia

> True randomness is hard to achieve on a blockchain. Every node must be able to replay each operation and reach the same outcome, so every transaction and…

Source: https://chromia.com/learn/marketplace-course/module-nft/randomness/

Decentralized marketplace with FT4Lesson 9 of 15

1. [01Decentralized marketplace with FT4](https://chromia.com/learn/marketplace-course/introduction/)
2. [02Set up your project](https://chromia.com/learn/marketplace-course/setup/)

Module 1: Registering accounts and assets

1. [03Overview](https://chromia.com/learn/marketplace-course/module-ft4/)
2. [04Register payment token](https://chromia.com/learn/marketplace-course/module-ft4/register-token/)
3. [05Create user accounts](https://chromia.com/learn/marketplace-course/module-ft4/register-account/)

Module 2: Model NFTs in Rell

1. [06Overview](https://chromia.com/learn/marketplace-course/module-nft/)
2. [07Define the NFT model](https://chromia.com/learn/marketplace-course/module-nft/nft/)
3. [08Mint NFTs](https://chromia.com/learn/marketplace-course/module-nft/mint-nfts/)
4. [09Make card attributes random](https://chromia.com/learn/marketplace-course/module-nft/randomness/)

Module 3: Create the marketplace

1. [10Overview](https://chromia.com/learn/marketplace-course/module-assets/)
2. [11Charge a fee for mystery card purchases](https://chromia.com/learn/marketplace-course/module-assets/buy-mystery-card/)
3. [12Put a card up for sale on the marketplace](https://chromia.com/learn/marketplace-course/module-assets/list-card/)
4. [13Buy a card on the marketplace](https://chromia.com/learn/marketplace-course/module-assets/buy-listed-card/)
5. [14Test our marketplace](https://chromia.com/learn/marketplace-course/module-assets/test-marketplace/)
6. [15Test using Chromia CLI](https://chromia.com/learn/marketplace-course/module-assets/test-cli/)

[Learn](https://chromia.com/learn/)[Decentralized marketplace with FT4](https://chromia.com/learn/marketplace-course/)

09 / 15Module 2: Model NFTs in RellAdvanced

# Make card attributes random

True randomness is hard to achieve on a blockchain. Every node must be able to replay each operation and reach the same outcome, so every transaction and function call has to return identical results everywhere. That determinism is the opposite of genuine unpredictability.

You can still approximate randomness by deriving values from transaction data; the code below uses the transaction RID. The result is not truly random, but it works for simple cases. This lesson is optional. To stay focused on the marketplace, skip it and give your cards fixed attributes.

### Add random functions

To give the `buy_mystery_card` operation random `strength` and `health` values, start with a function that produces a pseudo-random number:

src/rell\_marketplace/nft.rell

```
// Generate a random integer within a specified range
function random(high: integer, seed: integer): integer {
    // Ensure high is not zero to avoid division by zero
    if (high == 0) return 0;

    // Calculate the random value using the provided seed
    return (op_context.last_block_time - op_context.block_height - op_context.op_index + seed) % high;
}
```

The transaction RID serves as the seed source. You take a substring of it for each random number you need, and those substrings become the seeds for `health` and `strength`.

src/rell\_marketplace/nft.rell

```
function extract_seeds() {
    // Extract seeds for randomness from the transaction RID
    val seeds = op_context.transaction.tx_rid.to_hex();
    val health_seed: integer = integer.from_hex(seeds.sub(0, 4));
    val strength_seed: integer = integer.from_hex(seeds.sub(4, 8));
    return (health_seed, strength_seed);
}
```

Next, add one function per attribute. `generate_health` and `generate_strength` each take their extracted seed and return a random value for `health` or `strength`.

src/rell\_marketplace/nft.rell

```
// Generate random health within a specified range
function generate_health(seed: integer): integer {
    val rand = random(30, seed) + 1; // Generate a random value between 1 and 30
    val baseline = 30; // Consistent baseline for all cards
    return baseline + rand;
}

// Generate random strength within a specified range
function generate_strength(seed: integer): integer {
    val rand = random(30, seed) + 1; // Generate a random value between 1 and 30
    val baseline = 50; // Consistent baseline for all cards
    return baseline + rand * 2; // Adjust range to 50-110
}
```

Finally, `mint_card` uses these generated values to create a new NFT card with random `strength` and `health`.

src/rell\_marketplace/nft.rell

```
// Mint a new card with random attributes
function mint_card(account) {
    // Create a new NFT for the specified account
    val nft = create nft(account);

    // Extract seeds for randomness
    val (health_seed, strength_seed) = extract_seeds();

    // Generate random health and strength based on the provided seeds
    val health = generate_health(health_seed);
    val strength = generate_strength(strength_seed);

    // Create the NFT card with the generated attributes
    create nft_card(
        .nft = nft,
        .strength = strength,
        .health = health
    );
}
```

Written for this site from Chromia’s public course. Code samples are the course’s own.

[← Previous**Mint NFTs**](https://chromia.com/learn/marketplace-course/module-nft/mint-nfts/)[Next →**Module 3: Create the marketplace**](https://chromia.com/learn/marketplace-course/module-assets/)