Transfer and approve flow in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the transfer function signature.

Blockchain / Solidity

function transfer(address [1], uint256 amount) public returns (bool);

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Afrom

Bto

Cspender

Downer

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'from' instead of 'to' for the recipient address.

The transfer function sends tokens to the to address.

2fill in blank

medium

Complete the code to approve a spender to spend tokens.

Blockchain / Solidity

function approve(address [1], uint256 amount) public returns (bool);

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Aspender

Bowner

Crecipient

Dto

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'owner' instead of 'spender'.

The approve function allows the spender to spend tokens on behalf of the owner.

3fill in blank

hard

Fix the error in the transferFrom function parameter name.

Blockchain / Solidity

function transferFrom(address [1], address to, uint256 amount) public returns (bool);

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Ato

Bspender

Cowner

Dfrom

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'to' or 'spender' instead of 'from'.

The transferFrom function moves tokens from the from address to to.

4fill in blank

hard

Fill both blanks to complete the allowance mapping declaration.

Blockchain / Solidity

mapping([1] => mapping([2] => uint256)) private _allowances;

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Aaddress

Buint256

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using uint256 or bool as keys instead of address.

The _allowances mapping tracks how many tokens an owner allows a spender to use. Both keys are addresses.

5fill in blank

hard

Fill all three blanks to complete the transferFrom function body snippet.

Blockchain / Solidity

require(_allowances[[1]][[2]] >= amount, "Allowance too low");
_allowances[[3]][[2]] -= amount;

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Afrom

Bspender

Cto

Downer

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up 'to' and 'spender' keys.

The allowance is checked and reduced for the from owner and spender. The from address's allowance is decreased.

Practice

1. What is the main purpose of the approve function in a token contract?

easy

A. To allow another address to spend tokens on your behalf

B. To send tokens directly to another address

C. To check the balance of an address

D. To mint new tokens to an address

Solution

1. Step 1: Understand the role of approve

   The approve function lets a token owner give permission to another address to spend tokens from their balance.

2. Step 2: Differentiate from other functions

   transfer sends tokens directly, approve only sets allowance, so the spender can use transferFrom later.

3. Final Answer:

   To allow another address to spend tokens on your behalf -> Option A

4. Quick Check:

   approve = permission grant [OK]

Hint: Approve sets spending permission, transfer sends tokens [OK]

Common Mistakes:

• Confusing approve with transfer
• Thinking approve sends tokens immediately
• Mixing approve with balance checking

2. Which of the following is the correct syntax to approve 100 tokens for spender address 0xABC... in Solidity?

easy

A. approve(100, 0xABC...);

B. approve(address spender, uint amount);

C. approve(0xABC..., 100);

D. approve(100);

Solution

1. Step 1: Recall approve function parameters

   The approve function takes two parameters: the spender's address and the amount to approve.

2. Step 2: Match parameters order

   The correct order is approve(address spender, uint amount), so calling approve(0xABC..., 100); is correct.

3. Final Answer:

   approve(0xABC..., 100); -> Option C

4. Quick Check:

   approve(spender, amount) syntax [OK]

Hint: approve(spender, amount) order matters [OK]

Common Mistakes:

• Swapping parameters order
• Using variable names instead of values
• Calling approve with only one argument

3. Given the following Solidity code snippet, what will be the balance of recipient after execution?

address owner = msg.sender;
address spender = 0x123...;
uint amount = 50;

// Owner approves spender
approve(spender, amount);

// Spender transfers tokens from owner to recipient
transferFrom(owner, recipient, 30);

medium

A. 80 tokens

B. 50 tokens

C. 0 tokens

D. 30 tokens

Solution

1. Step 1: Understand approve and transferFrom interaction

   The owner approves spender to spend 50 tokens. Then spender transfers 30 tokens to recipient.

2. Step 2: Calculate recipient balance after transferFrom

   Since spender transfers 30 tokens, recipient's balance increases by 30 tokens.

3. Final Answer:

   30 tokens -> Option D

4. Quick Check:

   transferFrom moves approved amount [OK]

Hint: transferFrom moves approved tokens only [OK]

Common Mistakes:

• Assuming full approved amount is transferred
• Confusing approve amount with transfer amount
• Ignoring transferFrom effect on recipient balance

4. Identify the error in this Solidity code snippet for transferring tokens using approve and transferFrom:

approve(spender, 100);
transferFrom(msg.sender, recipient, 50);

medium

A. approve should be called after transferFrom

B. transferFrom should be called by spender, not msg.sender

C. recipient address is missing

D. approve amount is less than transfer amount

Solution

1. Step 1: Check who calls transferFrom

   transferFrom must be called by the spender who was approved, not the token owner (msg.sender here).

2. Step 2: Confirm approve and transferFrom roles

   Owner calls approve, spender calls transferFrom to move tokens.

3. Final Answer:

   transferFrom should be called by spender, not msg.sender -> Option B

4. Quick Check:

   transferFrom caller = spender [OK]

Hint: transferFrom caller must be approved spender [OK]

Common Mistakes:

• Calling transferFrom as token owner
• Mixing approve and transferFrom order
• Ignoring caller permissions

5. You want to allow a decentralized exchange (DEX) contract to spend exactly 200 tokens from your wallet, but only once. Which sequence of calls correctly implements this using the transfer and approve flow?

hard

A. Call approve(DEX, 200) once, then DEX calls transferFrom for 200 tokens

B. Call transfer(DEX, 200) directly, no approve needed

C. Call approve(DEX, 200) multiple times before each trade

D. Call transferFrom(DEX, yourAddress, 200) yourself

Solution

1. Step 1: Understand one-time spending permission

   To let DEX spend tokens once, you approve exactly 200 tokens once.

2. Step 2: Let DEX transfer tokens

   DEX uses transferFrom to move 200 tokens from your wallet to itself or others.

3. Final Answer:

   Call approve(DEX, 200) once, then DEX calls transferFrom for 200 tokens -> Option A

4. Quick Check:

   approve once, transferFrom once [OK]

Hint: Approve once, then let spender transferFrom once [OK]

Common Mistakes:

• Transferring tokens directly without approval
• Approving multiple times unnecessarily
• Calling transferFrom as token owner