Transfer and approve flow in Blockchain / Solidity - Mini Project: Build & Apply

Transfer and approve flow

📖 Scenario: You are building a simple blockchain token system where users can transfer tokens to each other and approve others to spend tokens on their behalf.

🎯 Goal: Create a basic token transfer and approval flow using a dictionary to track balances and allowances.

📋 What You'll Learn

Create a dictionary called balances with exact user balances

Create a dictionary called allowances to track approved spending

Write a function transfer to move tokens between users

Write a function approve to set spending allowance

Write a function transfer_from to spend tokens using allowance

Print the final balances after transfers and approvals

💡 Why This Matters

🌍 Real World

This project models how cryptocurrencies and tokens manage transfers and approvals on blockchains.

💼 Career

Understanding transfer and approve flows is essential for blockchain developers working on smart contracts and token standards.

1

Set up initial balances

Create a dictionary called balances with these exact entries: 'Alice': 100, 'Bob': 50, 'Charlie': 75.

Blockchain / Solidity

# Create the balances dictionary with exact values
# Your code here

Hint

Use curly braces to create a dictionary with keys and values exactly as shown.

2

Set up allowances dictionary

Create an empty dictionary called allowances to track approved spending.

Blockchain / Solidity

balances = {'Alice': 100, 'Bob': 50, 'Charlie': 75}
# Create an empty allowances dictionary
# Your code here

Hint

Use empty curly braces to create an empty dictionary.

3

Write transfer, approve, and transfer_from functions

Write three functions: transfer(sender, receiver, amount) to move tokens if sender has enough balance; approve(owner, spender, amount) to set allowance; and transfer_from(spender, owner, receiver, amount) to transfer tokens using allowance if allowed and owner has enough balance.

Blockchain / Solidity

balances = {'Alice': 100, 'Bob': 50, 'Charlie': 75}
allowances = {}
# Write transfer, approve, and transfer_from functions below
# Your code here

Hint

Use dictionary methods like get to safely access balances and allowances.

4

Perform transfers, approvals, and print final balances

Call transfer('Alice', 'Bob', 20), then approve('Bob', 'Charlie', 15), then transfer_from('Charlie', 'Bob', 'Alice', 10). Finally, print the balances dictionary.

Blockchain / Solidity

balances = {'Alice': 100, 'Bob': 50, 'Charlie': 75}
allowances = {}

def transfer(sender, receiver, amount):
    if balances.get(sender, 0) >= amount:
        balances[sender] -= amount
        balances[receiver] = balances.get(receiver, 0) + amount

def approve(owner, spender, amount):
    if owner not in allowances:
        allowances[owner] = {}
    allowances[owner][spender] = amount

def transfer_from(spender, owner, receiver, amount):
    allowed = allowances.get(owner, {}).get(spender, 0)
    if allowed >= amount and balances.get(owner, 0) >= amount:
        balances[owner] -= amount
        balances[receiver] = balances.get(receiver, 0) + amount
        allowances[owner][spender] -= amount

# Call transfer, approve, transfer_from and print balances
# Your code here

Hint

Make sure to call the functions in the correct order and print the balances dictionary.

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