Array Traversal Patterns in DSA C# - Build from Scratch

Array Traversal Patterns

📖 Scenario: You are helping a small shop owner who keeps track of daily sales in an array. They want to analyze the sales data by going through each day's sales one by one.

🎯 Goal: Build a simple program that creates an array of daily sales, sets a starting point, traverses the array to collect sales data, and finally prints the collected sales.

📋 What You'll Learn

Create an integer array called dailySales with these exact values: 100, 200, 150, 300, 250

Create an integer variable called startIndex and set it to 0

Use a for loop with variable i starting from startIndex to traverse dailySales and collect the sales into a new list called collectedSales

Print the collectedSales list elements separated by spaces

💡 Why This Matters

🌍 Real World

Array traversal is common when processing lists of data like daily sales, temperatures, or scores. It helps in analyzing or transforming data step-by-step.

💼 Career

Understanding array traversal is essential for software developers, data analysts, and anyone working with data processing or algorithms.

1

Create the daily sales array

Create an integer array called dailySales with these exact values: 100, 200, 150, 300, 250

DSA C#

// Create the dailySales array with the exact values
// Your code here

Hint

Use int[] dailySales = {100, 200, 150, 300, 250}; to create the array.

2

Set the starting index

Create an integer variable called startIndex and set it to 0

DSA C#

int[] dailySales = {100, 200, 150, 300, 250};
// Create startIndex variable and set it to 0
// Your code here

Hint

Use int startIndex = 0; to create the variable.

3

Traverse the array and collect sales

Use a for loop with variable i starting from startIndex to traverse dailySales and collect the sales into a new list called collectedSales

DSA C#

int[] dailySales = {100, 200, 150, 300, 250};
int startIndex = 0;
// Create a list called collectedSales
// Use a for loop from startIndex to dailySales.Length to add elements to collectedSales
// Your code here

Hint

Use List<int> collectedSales = new List<int>(); to create the list.
Use for (int i = startIndex; i < dailySales.Length; i++) to loop.
Inside the loop, add dailySales[i] to collectedSales.

4

Print the collected sales

Print the collectedSales list elements separated by spaces using Console.WriteLine

DSA C#

using System.Collections.Generic;

int[] dailySales = {100, 200, 150, 300, 250};
int startIndex = 0;
List collectedSales = new List();
for (int i = startIndex; i < dailySales.Length; i++)
{
    collectedSales.Add(dailySales[i]);
}
// Print collectedSales elements separated by spaces
// Your code here

Hint

Use Console.WriteLine(string.Join(" ", collectedSales)); to print the list elements separated by spaces.

Practice

1. What does the following C# code do?
int[] arr = {1, 2, 3, 4};
for (int i = 0; i < arr.Length; i++) {
Console.Write(arr[i] + " ");
}

easy

A. Prints all elements of the array from start to end

B. Prints all elements of the array in reverse order

C. Prints only the first element of the array

D. Causes a runtime error due to wrong loop condition

Solution

1. Step 1: Understand the loop range

   The loop starts at 0 and runs while i is less than arr.Length (4), so i goes 0,1,2,3.

2. Step 2: Print elements in order

   Each iteration prints arr[i], so it prints 1 2 3 4 in order.

3. Final Answer:

   Prints all elements of the array from start to end -> Option A

4. Quick Check:

   Loop from 0 to length-1 prints all elements [OK]

Hint: Loop from 0 to length-1 prints all elements in order [OK]

Common Mistakes:

• Thinking loop prints in reverse
• Confusing loop condition with <= instead of <
• Assuming only first element prints

2. Which of the following is the correct syntax to traverse an array arr in C# using a foreach loop?

easy

A. foreach (int i in arr) { Console.Write(i); }

B. foreach (arr in int i) { Console.Write(i); }

C. foreach (int arr in i) { Console.Write(i); }

D. foreach (int i; arr) { Console.Write(i); }

Solution

1. Step 1: Recall foreach syntax

   The correct syntax is: foreach (type variable in collection) { ... }

2. Step 2: Match syntax to options

   foreach (int i in arr) { Console.Write(i); } matches correct syntax: foreach (int i in arr) { Console.Write(i); }

3. Final Answer:

   foreach (int i in arr) { Console.Write(i); } -> Option A

4. Quick Check:

   foreach loops use 'variable in collection' syntax [OK]

Hint: Remember foreach uses 'variable in collection' syntax [OK]

Common Mistakes:

• Swapping variable and collection order
• Using semicolon instead of 'in'
• Incorrect variable declaration inside foreach

3. What is the output of the following code?

int[] arr = {10, 20, 30, 40};
for (int i = arr.Length - 1; i >= 0; i--) {
  Console.Write(arr[i] + " ");
}

medium

A. 40 30 20

B. 10 20 30 40

C. 40 30 20 10

D. Runtime error due to loop condition

Solution

1. Step 1: Analyze loop start and end

   Loop starts at arr.Length - 1 = 3 and runs while i >= 0, decrementing i each time.

2. Step 2: Trace printed elements

   Prints arr[3]=40, arr[2]=30, arr[1]=20, arr[0]=10 in that order.

3. Final Answer:

   40 30 20 10 -> Option C

4. Quick Check:

   Loop from end to start prints reversed array [OK]

Hint: Loop from length-1 down to 0 prints array in reverse [OK]

Common Mistakes:

• Confusing loop direction and printing forward
• Off-by-one errors in loop condition
• Assuming runtime error due to negative index

4. Find the error in this array traversal code:

int[] arr = {5, 10, 15};
for (int i = 0; i <= arr.Length; i++) {
  Console.Write(arr[i] + " ");
}

medium

A. Array declaration is incorrect

B. Variable i should start from 1, not 0

C. Console.Write should be Console.WriteLine

D. Loop condition should be i < arr.Length, not <= arr.Length

Solution

1. Step 1: Check loop condition

   Loop runs while i <= arr.Length, but arr indices go from 0 to arr.Length - 1.

2. Step 2: Identify out-of-range access

   When i == arr.Length (3), arr[3] is accessed causing runtime error (IndexOutOfRangeException).

3. Final Answer:

   Loop condition should be i < arr.Length, not <= arr.Length -> Option D

4. Quick Check:

   Array indices go 0 to length-1, use < not <= [OK]

Hint: Use i < arr.Length to avoid out-of-range errors [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Starting loop at 1 instead of 0
• Confusing Write and WriteLine methods

5. Given an integer array arr, which code snippet correctly prints only the even-indexed elements (0, 2, 4, ...) of the array?

hard

A. foreach (int i in arr) { if (i % 2 == 0) Console.Write(i + " "); }

B. for (int i = 0; i < arr.Length; i += 2) { Console.Write(arr[i] + " "); }

C. for (int i = 1; i < arr.Length; i += 2) { Console.Write(arr[i] + " "); }

D. for (int i = 0; i < arr.Length; i++) { if (arr[i] % 2 == 0) Console.Write(arr[i] + " "); }

Solution

1. Step 1: Understand requirement

   We want to print elements at even indexes: 0, 2, 4, ...

2. Step 2: Analyze options

   for (int i = 0; i < arr.Length; i += 2) { Console.Write(arr[i] + " "); } increments i by 2 starting at 0, so it accesses even indexes correctly.
   for (int i = 1; i < arr.Length; i += 2) { Console.Write(arr[i] + " "); } accesses odd indexes.
   foreach (int i in arr) { if (i % 2 == 0) Console.Write(i + " "); } checks element values for evenness, not indexes.
   for (int i = 0; i < arr.Length; i++) { if (arr[i] % 2 == 0) Console.Write(arr[i] + " "); } prints elements with even values, not indexes.

3. Final Answer:

   for (int i = 0; i < arr.Length; i += 2) { Console.Write(arr[i] + " "); } -> Option B

4. Quick Check:

   Increment index by 2 from 0 to get even indexes [OK]

Hint: Increment loop index by 2 starting at 0 for even indexes [OK]

Common Mistakes:

• Confusing even index with even element value
• Starting loop at 1 for even indexes
• Using foreach without index tracking