Structs in Blockchain / Solidity - Mini Project: Build & Apply

Working with Structs in Blockchain

📖 Scenario: You are building a simple blockchain application that stores information about digital assets. Each asset has a name, an owner, and a value. You will use structs to organize this data clearly.

🎯 Goal: Create a struct to represent a digital asset, store multiple assets, and display their details.

📋 What You'll Learn

Create a struct called Asset with fields name (string), owner (string), and value (uint).

Create a list called assets to hold multiple Asset structs.

Add a configuration variable minValue to filter assets by their value.

Use a loop to select assets with value greater than or equal to minValue.

Print the details of the selected assets.

💡 Why This Matters

🌍 Real World

Structs help organize complex data in blockchain apps, like digital assets, transactions, or user profiles.

💼 Career

Understanding structs is key for blockchain developers to build clear, maintainable smart contracts and decentralized apps.

1

Define the Asset struct and create assets list

Define a struct called Asset with fields name (string), owner (string), and value (uint). Then create a list called assets with these exact entries: {"name": "ArtPiece", "owner": "Alice", "value": 100}, {"name": "CryptoKitty", "owner": "Bob", "value": 50}, and {"name": "RareCoin", "owner": "Charlie", "value": 200}.

Blockchain / Solidity

// Define struct Asset and create list assets with 3 entries
// Your code here

Hint

Use struct keyword to define Asset. Create an array of Asset with the given entries.

2

Add a minimum value filter

Create a variable called minValue of type uint and set it to 100.

Blockchain / Solidity

struct Asset {
    string name;
    string owner;
    uint value;
}

Asset[] assets = [
    Asset("ArtPiece", "Alice", 100),
    Asset("CryptoKitty", "Bob", 50),
    Asset("RareCoin", "Charlie", 200)
];

// Create uint minValue and set to 100
// Your code here

Hint

Use uint minValue = 100; to set the minimum value filter.

3

Select assets with value >= minValue

Create a new list called selectedAssets of type Asset[]. Use a for loop with variable i to go through assets. Inside the loop, use an if statement to check if assets[i].value is greater than or equal to minValue. If yes, add assets[i] to selectedAssets.

Blockchain / Solidity

struct Asset {
    string name;
    string owner;
    uint value;
}

Asset[] assets = [
    Asset("ArtPiece", "Alice", 100),
    Asset("CryptoKitty", "Bob", 50),
    Asset("RareCoin", "Charlie", 200)
];

uint minValue = 100;

// Create Asset[] selectedAssets and add assets with value >= minValue
// Your code here

Hint

Use a for loop and if condition to filter assets. Use push to add to selectedAssets.

4

Print selected assets details

Use a for loop with variable j to go through selectedAssets. Inside the loop, print the asset's name, owner, and value in this exact format: Asset: [name], Owner: [owner], Value: [value].

Blockchain / Solidity

struct Asset {
    string name;
    string owner;
    uint value;
}

Asset[] assets = [
    Asset("ArtPiece", "Alice", 100),
    Asset("CryptoKitty", "Bob", 50),
    Asset("RareCoin", "Charlie", 200)
];

uint minValue = 100;

Asset[] selectedAssets;

for (uint i = 0; i = minValue) {
        selectedAssets.push(assets[i]);
    }
}

// Print details of selectedAssets
// Your code here

Hint

Use a for loop and string concatenation to prepare each selected asset's details for output. Solidity does not support console.log; instead, return the strings or emit events.

Practice

1.

What is the main purpose of using structs in blockchain programming?

easy

A. To perform mathematical calculations faster

B. To group related data together under one name

C. To create loops for repeating tasks

D. To store only single values like integers

Solution

1. Step 1: Understand what structs do

   Structs are used to group different pieces of related data into one unit.

2. Step 2: Compare options

   Only To group related data together under one name correctly describes grouping related data. Other options describe unrelated tasks.

3. Final Answer:

   To group related data together under one name -> Option B

4. Quick Check:

   Structs = Group related data [OK]

Hint: Think of structs as folders holding related info [OK]

Common Mistakes:

• Confusing structs with loops or calculations
• Thinking structs store only single values
• Assuming structs speed up math operations

2.

Which of the following is the correct syntax to define a struct named Person with fields name (string) and age (uint)?

?

easy

A. struct Person(string name, uint age);

B. struct Person = (string name, uint age);

C. Person struct { string name; uint age; }

D. struct Person { string name; uint age; }

Solution

1. Step 1: Recall struct syntax in blockchain languages

   Structs are defined with the keyword struct, followed by the name and curly braces containing fields with types and names.

2. Step 2: Check each option

   struct Person { string name; uint age; } matches the correct syntax. Options B, C, and D use incorrect punctuation or order.

3. Final Answer:

   struct Person { string name; uint age; } -> Option D

4. Quick Check:

   Correct struct syntax = struct Person { string name; uint age; } [OK]

Hint: Remember: struct Name { type field; } [OK]

Common Mistakes:

• Using parentheses instead of braces
• Assigning struct like a variable
• Omitting semicolons after fields

3.

Given the struct and code below, what will be the output?

struct Car {
  string brand;
  uint year;
}

Car myCar = Car("Tesla", 2020);

print(myCar.brand);

medium

A. Tesla

B. 2020

C. Car

D. Error: Cannot print struct field

Solution

1. Step 1: Understand struct initialization

   The struct Car is created with brand "Tesla" and year 2020.

2. Step 2: Identify printed field

   The code prints myCar.brand, which is "Tesla".

3. Final Answer:

   Tesla -> Option A

4. Quick Check:

   Print myCar.brand = Tesla [OK]

Hint: Print the field name, not the whole struct [OK]

Common Mistakes:

• Printing the whole struct instead of a field
• Confusing brand with year
• Expecting an error when printing a field

4.

What is wrong with the following struct usage?

struct Book {
  string title;
  uint pages;
}

Book myBook;
myBook.title = "Learn Blockchain";
myBook.pages = "300";

medium

A. Struct name should be lowercase

B. Missing semicolon after struct definition

C. Cannot assign string to pages which is uint

D. Cannot assign values after declaration

Solution

1. Step 1: Check field types and assignments

   The field pages is of type uint, but assigned a string "300".

2. Step 2: Identify type mismatch error

   Assigning a string to a number type causes a type error.

3. Final Answer:

   Cannot assign string to pages which is uint -> Option C

4. Quick Check:

   Type mismatch in assignment = Cannot assign string to pages which is uint [OK]

Hint: Match field types exactly when assigning values [OK]

Common Mistakes:

• Assigning string to a number field
• Ignoring type errors
• Thinking struct names must be lowercase

5.

You want to store multiple Employee structs in a list and then find the average age. Which approach correctly uses structs and arrays?

struct Employee {
  string name;
  uint age;
}

Employee[] employees;
employees.push(Employee("Alice", 30));
employees.push(Employee("Bob", 40));

uint totalAge = 0;
for (uint i = 0; i < employees.length; i++) {
  totalAge += employees[i].age;
}
uint averageAge = totalAge / employees.length;
print(averageAge);

hard

A. This code correctly calculates and prints average age

B. Cannot push structs into arrays

C. Loop should use i <= employees.length

D. Cannot access struct fields inside arrays

Solution

1. Step 1: Understand struct array usage

   The code creates an array of Employee structs and adds two employees.

2. Step 2: Check loop and calculation

   The loop sums ages correctly using employees[i].age and divides by length to get average.

3. Final Answer:

   This code correctly calculates and prints average age -> Option A

4. Quick Check:

   Struct arrays and loops = correct [OK]

Hint: Use array indexing and dot to access struct fields [OK]

Common Mistakes:

• Using <= instead of < in loop causing out-of-bounds
• Thinking structs can't be stored in arrays
• Trying to access struct fields without indexing