Packing variables for gas savings in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a packed struct with two uint128 variables.

Blockchain / Solidity

struct Data {
    uint128 a;
    uint128 [1];
}

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Ae

Bc

Cd

Db

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that is not declared or inconsistent.

Using a type other than uint128 which affects packing.

The struct has two variables: a and b, both uint128 to pack efficiently.

2fill in blank

medium

Complete the code to pack three variables efficiently in a struct.

Blockchain / Solidity

struct Packed {
    uint64 x;
    uint64 y;
    [1] z;
}

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Auint64

Buint32

Cuint128

Duint256

Attempts:

3 left

💡 Hint

Common Mistakes

Using a larger type like uint128 or uint256 which wastes space.

Using a smaller type like uint32 which may not be intended.

Using uint64 for z keeps all three variables at 64 bits each, allowing efficient packing into 192 bits total.

3fill in blank

hard

Fix the error in the struct to pack variables efficiently without wasting space.

Blockchain / Solidity

struct Example {
    uint256 a;
    uint128 [1];
    uint128 c;
}

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Ab

Bd

Ce

Df

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that breaks naming conventions.

Misordering variables causing inefficient packing.

The variable name should be b to keep consistent naming and packing order.

4fill in blank

hard

Fill both blanks to declare a struct with packed variables for gas savings.

Blockchain / Solidity

struct Storage {
    [1] flag;
    [2] count;
}

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Abool

Buint256

Cuint8

Daddress

Attempts:

3 left

💡 Hint

Common Mistakes

Using large types like uint256 unnecessarily.

Using types that do not pack well together.

Using bool and uint8 packs variables tightly, saving gas.

5fill in blank

hard

Fill all three blanks to create a packed struct with an address and two small integers.

Blockchain / Solidity

struct PackedData {
    [1] owner;
    [2] level;
    [3] score;
}

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Auint256

Buint8

Cuint16

Daddress

Attempts:

3 left

💡 Hint

Common Mistakes

Using large integer types for small values wastes gas.

Mixing types that do not pack well together.

The address type is 20 bytes, uint8 and uint16 are small integers that pack efficiently.