Reference types behavior in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity code snippet?

Consider the following Solidity contract snippet. What will be the value of arr[0] after calling modifyArray()?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    uint[] public arr = [1, 2, 3];

    function modifyArray() public {
        uint[] memory temp = arr;
        temp[0] = 99;
    }
}

A 1

B Compilation error

C 0

D 99

💡 Hint

Think about whether temp is a reference or a copy.

In Solidity, memory creates a copy of the array. Modifying temp does not affect the original arr. So, arr[0] remains 1.

❓ Predict Output

intermediate

What happens when modifying a storage reference in Solidity?

Given this Solidity code, what will be the value of arr[0] after modifyArray() is called?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    uint[] public arr = [1, 2, 3];

    function modifyArray() public {
        uint[] storage temp = arr;
        temp[0] = 99;
    }
}

A Compilation error

B 0

C 1

D 99

💡 Hint

Consider what storage means for temp.

temp is a storage reference to arr. Modifying temp changes arr directly. So, arr[0] becomes 99.

🔧 Debug

advanced

Why does this Solidity function revert when modifying a struct?

Examine the code below. Why does calling updatePerson() cause a revert?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract People {
    struct Person {
        string name;
        uint age;
    }

    Person public person = Person("Alice", 30);

    function updatePerson() public {
        Person memory p = person;
        p.age = 31;
    }
}

A Because <code>person</code> is not updated, so revert occurs

B Because the function tries to modify a copy, not the original, no revert occurs

C Because <code>memory</code> structs cannot be modified

D Because <code>string</code> in <code>memory</code> cannot be assigned

💡 Hint

Think about whether modifying p affects person.

The function modifies a memory copy of person. This does not affect the stored person. No revert happens; the change is simply lost.

📝 Syntax

advanced

Which option correctly declares a storage reference to a mapping in Solidity?

Given the mapping mapping(address => uint) balances;, which declaration correctly creates a storage reference to it inside a function?

A mapping(address => uint) memory ref = balances;

B mapping(address => uint) ref = balances;

C mapping(address => uint) storage ref = balances;

D mapping(address => uint) calldata ref = balances;

💡 Hint

Remember that mappings can only be stored in storage or calldata, not memory.

Mappings cannot be stored in memory. Only storage or calldata are allowed. To create a reference, storage is required.

🚀 Application

expert

How many items are in the array after this Solidity function runs?

Consider this Solidity contract. After calling addItem(), how many elements does items contain?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Inventory {
    string[] public items;

    function addItem() public {
        string[] storage ref = items;
        ref.push("apple");
        string[] memory copy = ref;
        // copy.push("banana"); // This line causes a compilation error
    }
}

A Compilation error

B 2

C 0

D 1

💡 Hint

Think about which array is modified and which is a copy.

ref is a storage reference to items, so pushing "apple" adds to items. copy is a memory copy; pushing "banana" to it does not affect items. However, pushing to a memory array is not allowed and causes a compilation error.

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