Reference types behavior in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a reference type variable in Solidity.

Blockchain / Solidity

string memory [1] = "hello";

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

Agreeting

Baddress

Cbool

Duint

Attempts:

3 left

💡 Hint

Common Mistakes

Using a type name instead of a variable name.

Using a value instead of a variable name.

In Solidity, string memory declares a reference type variable named greeting.

2fill in blank

medium

Complete the code to assign a reference type variable to another in Solidity.

Blockchain / Solidity

string memory name1 = "Alice";
string memory [1] = name1;

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

Aname2

Bbool

Caddress

Duint

Attempts:

3 left

💡 Hint

Common Mistakes

Using a type name instead of a variable name.

Using the same variable name twice.

Assigning name1 to name2 copies the reference type string in memory.

3fill in blank

hard

Fix the error in the function parameter declaration for a reference type in Solidity.

Blockchain / Solidity

function setName(string [1] newName) public {
    name = newName;
}

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

Aaddress

Buint

Cstorage

Dmemory

Attempts:

3 left

💡 Hint

Common Mistakes

Using storage instead of memory for function parameters.

Omitting the data location keyword.

Function parameters of reference types must be declared with memory to indicate temporary data.

4fill in blank

hard

Fill both blanks to correctly declare and assign a dynamic array reference type in Solidity.

Blockchain / Solidity

uint[] [1];
[2] = new uint[](5);

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

Anumbers

Caddress

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for declaration and assignment.

Using invalid types for the array variable.

The variable numbers is declared as a dynamic array and then initialized with length 5.

5fill in blank

hard

Fill all three blanks to create a mapping from address to a dynamic array of uint in Solidity.

Blockchain / Solidity

mapping([1] => [2]) public [3];

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

Aaddress

Buint[]

Cbalances

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect types for mapping keys or values.

Using an invalid variable name for the mapping.

This mapping links each address to a dynamic array of uint called balances.

Practice

1. In blockchain programming, what happens when you assign one reference type variable to another?

easy

A. The variables become independent and unrelated.

B. A new copy of the data is created.

C. The original data is deleted.

D. Both variables point to the same data location.

Solution

1. Step 1: Understand reference assignment

   Assigning one reference type variable to another makes both point to the same data location in memory.

2. Step 2: Effect of changes through references

   Changes made through either variable affect the same original data because they share the reference.

3. Final Answer:

   Both variables point to the same data location. -> Option D

4. Quick Check:

   Reference assignment shares data location = A [OK]

Hint: Reference types share data location, not copy data [OK]

Common Mistakes:

• Thinking assignment copies data
• Assuming variables become independent
• Believing original data is deleted

2. Which of the following is the correct syntax to copy a reference type variable in Solidity (blockchain language)?

easy

A. uint[] memory newArray = oldArray;

B. uint[] newArray = oldArray.copy();

C. uint[] storage newArray = oldArray;

D. uint[] newArray = copy(oldArray);

Solution

1. Step 1: Understand storage vs memory in Solidity

   Storage variables hold data permanently on blockchain, memory variables are temporary copies.

2. Step 2: Correct way to copy reference

   Assigning a storage reference variable to another storage variable copies the reference, not the data.

3. Final Answer:

   uint[] storage newArray = oldArray; -> Option C

4. Quick Check:

   Storage assignment copies reference = B [OK]

Hint: Use 'storage' keyword to copy reference, not 'memory' [OK]

Common Mistakes:

• Using 'memory' copies data, not reference
• Assuming .copy() method exists
• Trying to use undefined copy functions

3. Consider this Solidity code snippet:

uint[] storage arr1 = someArray;
uint[] storage arr2 = arr1;
arr2.push(10);
uint length = arr1.length;

What is the value of length after execution?

medium

A. Length is unchanged from original someArray length.

B. Length increases by 1 due to push on arr2.

C. Length decreases by 1 due to push on arr2.

D. Code causes a compilation error.

Solution

1. Step 1: Understand reference assignment

   arr2 is assigned as a storage reference to arr1, so both point to the same array.

2. Step 2: Effect of push on arr2

   Pushing 10 to arr2 modifies the shared array, increasing its length by 1.

3. Final Answer:

   Length increases by 1 due to push on arr2. -> Option B

4. Quick Check:

   Shared reference means push affects both = C [OK]

Hint: Changes via one reference affect all references [OK]

Common Mistakes:

• Thinking arr1 length stays same
• Assuming arr2 is a copy
• Ignoring reference sharing behavior

4. Identify the error in this Solidity code snippet:

function updateArray(uint[] memory arr) public {
  uint[] storage ref = arr;
  ref.push(5);
}

medium

A. Cannot assign memory array to storage reference directly.

B. Cannot push to a storage array.

C. Function must be declared view.

D. No error; code runs fine.

Solution

1. Step 1: Check assignment between memory and storage

   In Solidity, you cannot assign a memory array directly to a storage reference variable.

2. Step 2: Understand push operation

   Push is allowed on storage arrays, but assignment error occurs before push.

3. Final Answer:

   Cannot assign memory array to storage reference directly. -> Option A

4. Quick Check:

   Memory to storage assignment invalid = A [OK]

Hint: Memory and storage types cannot be assigned directly [OK]

Common Mistakes:

• Assuming memory can be assigned to storage
• Thinking push is disallowed on storage
• Ignoring data location mismatch

5. You want to create a function in Solidity that returns a modified copy of a storage array without changing the original. Which approach correctly uses reference types to achieve this?

hard

A. Create a new memory array, copy elements from storage, modify it, then return.

B. Assign storage array to a storage reference, modify it, then return.

C. Return the storage array directly after modifying it.

D. Use a global variable to store the modified array.

Solution

1. Step 1: Understand data locations for copying

   To avoid changing original storage array, create a new memory array and copy elements.

2. Step 2: Modify and return the memory array

   Modify the memory array safely and return it without affecting storage data.

3. Final Answer:

   Create a new memory array, copy elements from storage, modify it, then return. -> Option A

4. Quick Check:

   Use memory copy to avoid changing storage = D [OK]

Hint: Copy to memory, modify, then return to keep original safe [OK]

Common Mistakes:

• Modifying storage reference directly
• Returning storage array after modification
• Using global variables unnecessarily