Array Declaration and Initialization in DSA Rust - Build from Scratch

Array Declaration and Initialization

📖 Scenario: You are helping a small grocery store keep track of the number of apples sold each day for a week.

🎯 Goal: Create an array to store the number of apples sold each day, then print the array to see the sales for the week.

📋 What You'll Learn

Create an array called apples_sold with 7 elements representing daily sales

Initialize the array with these exact values: 5, 8, 6, 7, 10, 4, 9

Create a variable called days and set it to 7

Use a for loop with variable i from 0 to days - 1 to print each day's sales

Print the sales in the format: Day 1: 5 apples

💡 Why This Matters

🌍 Real World

Arrays are used to store fixed-size collections of data, like daily sales, temperatures, or scores.

💼 Career

Understanding arrays and loops is essential for software development, data analysis, and many programming tasks.

1

Create the array with daily apple sales

Create an array called apples_sold with 7 elements initialized to these exact values: 5, 8, 6, 7, 10, 4, 9

DSA Rust

# Create the array apples_sold with the given values
# Your code here

Hint

Use let apples_sold = [5, 8, 6, 7, 10, 4, 9]; to create the array.

2

Add a variable for the number of days

Add a variable called days and set it to 7 to represent the number of days in the week

DSA Rust

fn main() {
    let apples_sold = [5, 8, 6, 7, 10, 4, 9];
    
    # Create the variable days and set it to 7
    # Your code here
}

Hint

Use let days = 7; to create the variable.

3

Loop through the array to access daily sales

Use a for loop with variable i from 0 to days - 1 to access each element in apples_sold

DSA Rust

fn main() {
    let apples_sold = [5, 8, 6, 7, 10, 4, 9];
    let days = 7;

    # Write a for loop from 0 to days - 1 using i
    # Your code here
}

Hint

Use for i in 0..days { } to loop through the array indices.

4

Print the daily apple sales

Inside the for loop, print the sales for each day in this exact format: Day 1: 5 apples where the day number is i + 1 and the sales is apples_sold[i]

DSA Rust

fn main() {
    let apples_sold = [5, 8, 6, 7, 10, 4, 9];
    let days = 7;

    for i in 0..days {
        # Print the sales for day i + 1
        # Your code here
    }
}

Hint

Use println!("Day {}: {} apples", i + 1, apples_sold[i]); inside the loop.

Practice

1. Which of the following is the correct way to declare an array of 5 integers initialized to zero in Rust?

easy

A. let arr = [5; 0];

B. let arr: [i32; 5] = [0; 5];

C. let arr: [i32] = [0; 5];

D. let arr = [0, 5];

Solution

1. Step 1: Understand Rust array syntax

   Rust arrays require a fixed size and type, declared as [type; size].

2. Step 2: Check initialization correctness

   [0; 5] means 5 elements all set to 0, which matches the declaration.

3. Final Answer:

   let arr: [i32; 5] = [0; 5]; -> Option B

4. Quick Check:

   Correct Rust array declaration = let arr: [i32; 5] = [0; 5]; [OK]

Hint: Use [value; size] to initialize arrays quickly [OK]

Common Mistakes:

• Missing size in array type declaration
• Using wrong syntax like [0, 5] for repeated values
• Declaring array without fixed size

2. What is the correct syntax to declare a mutable array of 3 floating-point numbers initialized to 1.0 in Rust?

easy

A. let mut arr: [f64; 3] = [1.0; 3];

B. let arr mut: [f64; 3] = [1.0; 3];

C. mut let arr: [f64; 3] = [1.0; 3];

D. let arr: mut [f64; 3] = [1.0; 3];

Solution

1. Step 1: Identify mutable variable syntax in Rust

   Mutable variables use 'let mut' before the variable name.

2. Step 2: Confirm array declaration and initialization

   Array type is [f64; 3] and initialized with [1.0; 3], which is correct.

3. Final Answer:

   let mut arr: [f64; 3] = [1.0; 3]; -> Option A

4. Quick Check:

   Mutable array syntax = let mut arr: [f64; 3] = [1.0; 3]; [OK]

Hint: Place 'mut' right after 'let' for mutable arrays [OK]

Common Mistakes:

• Placing 'mut' after variable name
• Using 'mut let' instead of 'let mut'
• Incorrect array type syntax

3. What will be the output of the following Rust code?

let arr = [2; 4];
println!("{:?}", arr);

medium

A. [2; 4]

B. [4, 4, 4, 4]

C. [2, 2, 2, 2]

D. Compilation error

Solution

1. Step 1: Understand array initialization [2; 4]

   This creates an array with 4 elements, each set to 2.

2. Step 2: Print array with debug format

   Using {:?} prints the array elements as a list.

3. Final Answer:

   [2, 2, 2, 2] -> Option C

4. Quick Check:

   Array with repeated values prints as list [OK]

Hint: Remember [value; size] repeats value size times [OK]

Common Mistakes:

• Confusing size and value positions
• Expecting syntax [2;4] to print literally
• Assuming compilation error due to syntax

4. Identify the error in the following Rust code snippet:

let arr: [i32; 3] = [1, 2];

medium

A. Array size mismatch error

B. Type mismatch error

C. Mutable variable required

D. No error, code is correct

Solution

1. Step 1: Check declared array size

   Array is declared with size 3 but initialized with only 2 elements.

2. Step 2: Understand Rust array size rules

   Rust requires the number of elements to match the declared size exactly.

3. Final Answer:

   Array size mismatch error -> Option A

4. Quick Check:

   Declared size must match initializer length [OK]

Hint: Array size and initializer length must be equal [OK]

Common Mistakes:

• Ignoring size mismatch
• Assuming Rust allows shorter initializers
• Confusing type mismatch with size error

5. You want to create a 2D array (matrix) of size 3x3 filled with the value 7 in Rust. Which code correctly declares and initializes this array?

hard

A. let matrix: [i32; 3][3] = [[7; 3]; 3];

B. let matrix: [[i32; 3]; 3] = [7; 9];

C. let matrix: [i32; 9] = [[7; 3]; 3];

D. let matrix: [[i32; 3]; 3] = [[7; 3]; 3];

Solution

1. Step 1: Understand 2D array type declaration

   A 3x3 array of i32 is declared as [[i32; 3]; 3], meaning 3 arrays each of size 3.

2. Step 2: Check initialization syntax

   [[7; 3]; 3] creates 3 arrays each filled with 3 sevens, forming a 3x3 matrix.

3. Final Answer:

   let matrix: [[i32; 3]; 3] = [[7; 3]; 3]; -> Option D

4. Quick Check:

   Nested arrays use [[type; cols]; rows] [OK]

Hint: Use nested [value; size] for multi-dimensional arrays [OK]

Common Mistakes:

• Incorrect type syntax like [i32; 3][3]
• Initializing flat array instead of nested
• Mismatch between declared size and initializer