Array Traversal Patterns in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is array traversal?

Array traversal means visiting each element in an array one by one, usually from the first element to the last.

beginner

How do you traverse an array forwards in C#?

Use a for loop starting from index 0 up to the last index, accessing each element by its index.

beginner

What is reverse traversal of an array?

Reverse traversal means visiting array elements starting from the last element down to the first element.

intermediate

Why is it important to know different array traversal patterns?

Different problems need different ways to visit array elements, like forwards, backwards, skipping elements, or jumping by steps.

beginner

Show a simple C# code snippet to traverse an array and print all elements.

for (int i = 0; i < arr.Length; i++) { Console.WriteLine(arr[i]); }

Which index does array traversal usually start with?

A -1

B 1

C 0

D Length of array

Array indexes in C# start at 0, so traversal usually begins at index 0.

What happens if you traverse an array backwards?

A You visit elements from last to first

B You skip every other element

C You visit elements randomly

D You only visit the first element

Backward traversal means visiting elements starting from the last index down to the first.

Which loop is commonly used for array traversal in C#?

A foreach loop

B while loop

C do-while loop

D for loop

The for loop is commonly used because it gives control over the index.

What is the last index of an array with length 5?

A 4

B 5

C 0

D 6

Since indexing starts at 0, the last index is length - 1, which is 4.

Which traversal pattern would you use to print elements in reverse order?

A Start from 0 and go to last index

B Start from last index and go to 0

C Skip every other element

D Visit elements randomly

To print in reverse, start from the last element and move backwards to the first.

Explain how to traverse an array forwards and backwards with examples.

Think about the starting and ending index for each direction.

Describe why different array traversal patterns are useful in programming.

Consider real-life examples like reading a book front to back or back to front.

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