Structs in Blockchain / Solidity - Time & Space Complexity

Time Complexity: Structs

O(1)

Understanding Time Complexity

When working with structs in blockchain code, it's important to know how the time to access or process them changes as they grow.

We want to understand how the program's work grows when handling structs with more data.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

struct Transaction {
  address sender;
  address receiver;
  uint amount;
}

Transaction[] public transactions;

function addTransaction(address _to, uint _amount) public {
  transactions.push(Transaction(msg.sender, _to, _amount));
}
    

This code defines a struct for transactions and adds new transactions to a list.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Adding a new struct to the end of an array.
• How many times: Each call adds one transaction; no loops or traversals happen during add.

How Execution Grows With Input

Adding one transaction takes about the same time no matter how many transactions are already stored.

Input Size (n)	Approx. Operations
10	1 operation per add
100	1 operation per add
1000	1 operation per add

Pattern observation: The time to add a transaction stays about the same as the list grows.

Final Time Complexity

Time Complexity: O(1)

This means adding a new struct is quick and does not slow down as more data is stored.

Common Mistake

[X] Wrong: "Adding a struct to an array takes longer as the array grows because it has to check all items."

[OK] Correct: Adding to the end of an array is direct and does not require checking existing items, so time stays constant.

Interview Connect

Understanding how structs and arrays behave helps you explain how data is stored and accessed efficiently in blockchain programs.

Self-Check

"What if we searched for a transaction by sender in the array? How would the time complexity change?"

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