Variable packing in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to pack two uint128 variables into one uint256 variable.

Blockchain / Solidity

uint256 packed = (uint256(a) << [1]) | uint256(b);

Drag options to blanks, or click blank then click option'

A128

B64

C256

D32

Attempts:

3 left

💡 Hint

Common Mistakes

Using 64 instead of 128 bits for shifting.

Not shifting at all.

Shifting by 256 bits which is too large.

We shift a by 128 bits to the left to pack it into the higher half of the 256-bit variable.

2fill in blank

medium

Complete the code to unpack the higher 128 bits from a packed uint256 variable.

Blockchain / Solidity

uint128 a = uint128(packed >> [1]);

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A256

B32

C128

D64

Attempts:

3 left

💡 Hint

Common Mistakes

Shifting by 64 bits instead of 128.

Not shifting before casting.

Shifting by 256 bits which clears all bits.

We shift right by 128 bits to get the higher 128 bits into the lower part, then cast to uint128.

3fill in blank

hard

Fix the error in unpacking the lower 128 bits from the packed uint256 variable.

Blockchain / Solidity

uint128 b = uint128(packed [1] 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF);

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A&

B^

C|

D>>

Attempts:

3 left

💡 Hint

Common Mistakes

Using bitwise OR which sets bits incorrectly.

Using XOR which toggles bits.

Using right shift which moves bits instead of masking.

We use bitwise AND & with a mask to extract the lower 128 bits.

4fill in blank

hard

Fill both blanks to pack three uint85 variables into one uint256 variable.

Blockchain / Solidity

uint256 packed = (uint256(x) << [1]) | (uint256(y) << [2]) | uint256(z);

Drag options to blanks, or click blank then click option'

A170

B85

C128

D64

Attempts:

3 left

💡 Hint

Common Mistakes

Using 128 bits shift which is too large for 85-bit variables.

Using 64 bits shift which is too small.

Mixing up the order of shifts.

Shift x by 170 bits (85*2) and y by 85 bits to pack three 85-bit values.

5fill in blank

hard

Fill all three blanks to unpack three uint85 variables from a packed uint256 variable.

Blockchain / Solidity

uint85 x = uint85(packed >> [1]);
uint85 y = uint85((packed >> [2]) & [3]);

Drag options to blanks, or click blank then click option'

A170

B85

C0x1FFFFFFFFFFFFFFFFFF

D128

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect mask size.

Using wrong shift amounts.

Not masking after shifting.

Shift x by 170 bits, y by 85 bits, and mask with 0x1FFFFFFFFFFFFFFFFFF to extract 85 bits.

Practice

1.

What is the main benefit of variable packing in blockchain smart contracts?

easy

A. It reduces storage space and lowers gas fees.

B. It increases the number of variables allowed.

C. It makes the code run faster on all blockchains.

D. It automatically encrypts stored data.

Solution

1. Step 1: Understand variable packing purpose

   Variable packing groups smaller variables to use less storage space.

2. Step 2: Connect storage saving to gas fees

   Less storage means lower gas fees because blockchain charges for storage.

3. Final Answer:

   It reduces storage space and lowers gas fees. -> Option A

4. Quick Check:

   Variable packing = saves space and gas [OK]

Hint: Variable packing saves storage and gas fees [OK]

Common Mistakes:

• Thinking it increases variable count
• Assuming it speeds up all code execution
• Believing it encrypts data automatically

2.

Which of the following Solidity variable declarations best uses variable packing?

uint8 a;
uint16 b;
uint32 c;

easy

A. Declare all as uint256 for best packing.

B. Declare as uint32 c; uint16 b; uint8 a; in this order.

C. Declare as uint8 a; uint16 b; uint32 c; in this order.

D. Declare as uint16 b; uint8 a; uint32 c; in this order.

Solution

1. Step 1: Order variables from smallest to largest

   Variable packing works best when smaller variables come first to fill storage slots efficiently.

2. Step 2: Check given orders

   Declare as uint8 a; uint16 b; uint32 c; in this order. orders variables from uint8 (smallest) to uint32 (largest), maximizing packing.

3. Final Answer:

   Declare as uint8 a; uint16 b; uint32 c; in this order. -> Option C

4. Quick Check:

   Smallest to largest order = best packing [OK]

Hint: Order variables smallest to largest for packing [OK]

Common Mistakes:

• Placing largest variables first wastes space
• Using all uint256 wastes storage
• Mixing order without size consideration

3.

Consider this Solidity struct packed into one storage slot:

struct Data {
  uint8 x;
  uint16 y;
  uint8 z;
}
Data d = Data(1, 300, 2);

What is the total storage size used by d?

medium

A. 128 bits (16 bytes)

B. 256 bits (32 bytes)

C. 64 bits (8 bytes)

D. 32 bits (4 bytes)

Solution

1. Step 1: Calculate bits for each variable

   uint8 = 8 bits, uint16 = 16 bits, uint8 = 8 bits; total = 8+16+8 = 32 bits.

2. Step 2: Understand Solidity storage slot size

   Solidity packs variables into 256-bit slots, but here total variables use only 32 bits, so storage used is 32 bits.

3. Final Answer:

   32 bits (4 bytes) -> Option D

4. Quick Check:

   8+16+8 = 32 bits used [OK]

Hint: Sum bits of variables to find packed size [OK]

Common Mistakes:

• Assuming full 256 bits always used
• Adding bytes instead of bits incorrectly
• Confusing variable sizes

4.

Identify the error in this Solidity contract snippet related to variable packing:

contract Example {
  uint256 a;
  uint8 b;
  uint16 c;
}

Why is this not optimized for variable packing?

medium

A. Variables are not ordered from smallest to largest size.

B. uint256 cannot be packed with smaller types.

C. uint8 and uint16 must be declared as uint256.

D. All variables must be declared as uint8 for packing.

Solution

1. Step 1: Check variable order for packing

   Variable packing requires ordering from smallest to largest to fill storage slots efficiently.

2. Step 2: Analyze given order

   Here, uint256 a is first (largest), then smaller uint8 and uint16, which wastes space.

3. Final Answer:

   Variables are not ordered from smallest to largest size. -> Option A

4. Quick Check:

   Order smallest to largest for packing [OK]

Hint: Order variables smallest to largest to fix packing [OK]

Common Mistakes:

• Thinking uint256 can't be packed
• Believing all must be uint8
• Ignoring variable order importance

5.

You want to store these variables in a Solidity contract efficiently:

bool isActive;
uint8 count;
uint256 total;
uint16 code;

Which variable order best uses variable packing to minimize storage slots?

hard

A. uint256 total; uint16 code; uint8 count; bool isActive;

B. bool isActive; uint8 count; uint16 code; uint256 total;

C. uint16 code; uint256 total; bool isActive; uint8 count;

D. uint8 count; bool isActive; uint256 total; uint16 code;

Solution

1. Step 1: Order variables from smallest to largest size

   bool (1 bit), uint8 (8 bits), uint16 (16 bits), uint256 (256 bits) is the correct size order.

2. Step 2: Check options for this order

   bool isActive; uint8 count; uint16 code; uint256 total; lists variables in this order, maximizing packing into fewer storage slots.

3. Final Answer:

   bool isActive; uint8 count; uint16 code; uint256 total; -> Option B

4. Quick Check:

   Smallest to largest order = best packing [OK]

Hint: Place smallest variables first, largest last for packing [OK]

Common Mistakes:

• Placing uint256 first wastes slots
• Ignoring bool size as smallest
• Mixing order without size logic