Array iteration with for and foreach in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of for loop iterating over an array

What is the output of the following C# code?

C Sharp (C#)

int[] numbers = {1, 2, 3};
for (int i = 0; i < numbers.Length; i++)
{
    Console.Write(numbers[i] + " ");
}

A 0 1 2

B 1 2 3

C 123

D 1 2 3 4

💡 Hint

Remember that Console.Write prints without a newline and the loop prints each element followed by a space.

The for loop iterates over each element in the array and prints it followed by a space, resulting in '1 2 3 '.

❓ Predict Output

intermediate

Output of foreach loop with array

What will this C# code print?

C Sharp (C#)

string[] fruits = {"apple", "banana", "cherry"};
foreach (string fruit in fruits)
{
    Console.Write(fruit[0]);
}

A abc

B applebanana cherry

C a b c

D apple

💡 Hint

Each fruit's first character is printed without spaces.

The foreach loop prints the first character of each fruit, resulting in 'abc'.

❓ Predict Output

advanced

Value of sum after mixed iteration

What is the value of sum after running this code?

C Sharp (C#)

int[] values = {2, 4, 6};
int sum = 0;
for (int i = 0; i < values.Length; i++)
{
    sum += values[i];
}
foreach (int v in values)
{
    sum += v;
}

A 6

B 12

C 18

D 24

💡 Hint

The for loop adds all elements once, then the foreach adds them again.

The sum is 2+4+6 = 12 from the for loop, then 12 more from the foreach, total 24.

❓ Predict Output

advanced

Output of modifying array inside foreach

What happens when this code runs?

C Sharp (C#)

int[] nums = {1, 2, 3};
foreach (int n in nums)
{
    n = n * 2;
    Console.Write(n + " ");
}
Console.WriteLine();
foreach (int n in nums)
{
    Console.Write(n + " ");
}

A

2 4 6 
1 2 3 

B

2 4 6 
2 4 6 

C

1 2 3 
1 2 3 

D Compilation error

💡 Hint

Foreach variable is read-only copy, modifying it does not change the array.

The first loop prints doubled values but does not change the array. The second loop prints original values.

🧠 Conceptual

expert

Why does modifying array elements inside foreach cause error?

Consider this code snippet: int[] arr = {1, 2, 3}; foreach (int x in arr) { x = x + 1; } Why does this code not modify the array elements?

A Because the compiler automatically resets the array after the loop finishes.

B Because arrays in C# are immutable and cannot be changed after creation.

C Because the foreach variable 'x' is a copy of each element, not a reference, so changes don't affect the array.

D Because the foreach loop does not allow any statements inside its block.

💡 Hint

Think about how foreach handles the loop variable.

In C#, foreach assigns each element's value to the loop variable as a copy. Modifying the variable does not change the original array element.

Practice

1. Which loop is best when you want to access each element in an array without needing the index in C#?

easy

A. foreach loop

B. for loop

C. while loop

D. do-while loop

Solution

1. Step 1: Understand loop purpose

   foreach is designed to iterate over each element directly without needing the index.

2. Step 2: Compare with other loops

   for requires index management, while and do-while are less direct for arrays.

3. Final Answer:

   foreach loop -> Option A

4. Quick Check:

   Direct element access = foreach [OK]

Hint: Use foreach when index is not needed for array items [OK]

Common Mistakes:

• Choosing for loop when index is not required
• Confusing while loops with foreach
• Thinking foreach can modify array size

2. Which of the following is the correct syntax for iterating over an array named numbers using a for loop in C#?

easy

A. for (int i = 0; i < numbers.Count; i++) { Console.WriteLine(numbers[i]); }

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

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

D. for (int i = 1; i <= numbers.Length; i++) { Console.WriteLine(numbers[i]); }

Solution

1. Step 1: Check loop start and end conditions

   Index starts at 0 and runs while less than numbers.Length (array length).

2. Step 2: Validate index usage

   Accessing numbers[i] is correct; using Count is invalid for arrays.

3. Final Answer:

   for (int i = 0; i < numbers.Length; i++) { Console.WriteLine(numbers[i]); } -> Option B

4. Quick Check:

   Index 0 to Length-1 = correct for loop [OK]

Hint: Use i < array.Length, not <= or Count for arrays [OK]

Common Mistakes:

• Using i <= numbers.Length causing out-of-range error
• Starting index at 1 instead of 0
• Using Count property on arrays

3. What will be the output of the following C# code?

int[] nums = {1, 2, 3};
int sum = 0;
foreach (int n in nums) {
  sum += n;
}
Console.WriteLine(sum);

medium

A. Error

B. 123

C. 0

D. 6

Solution

1. Step 1: Understand foreach iteration

   The loop adds each element (1, 2, 3) to sum one by one.

2. Step 2: Calculate sum

   sum = 0 + 1 + 2 + 3 = 6

3. Final Answer:

   6 -> Option D

4. Quick Check:

   Sum of array elements = 6 [OK]

Hint: Sum elements by adding each in foreach loop [OK]

Common Mistakes:

• Concatenating numbers instead of adding
• Forgetting to initialize sum to 0
• Using for loop incorrectly causing index errors

4. Identify the error in this C# code snippet:

int[] arr = {10, 20, 30};
for (int i = 0; i <= arr.Length; i++) {
  Console.WriteLine(arr[i]);
}

medium

A. The loop condition should be i < arr.Length, not i <= arr.Length

B. The array declaration is incorrect

C. Console.WriteLine cannot print array elements

D. The loop variable i should start at 1

Solution

1. Step 1: Check loop boundary

   Using i <= arr.Length causes index out of range because last valid index is Length - 1.

2. Step 2: Correct loop condition

   Change condition to i < arr.Length to avoid runtime error.

3. Final Answer:

   The loop condition should be i < arr.Length, not i <= arr.Length -> Option A

4. Quick Check:

   Index must be less than Length [OK]

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

Common Mistakes:

• Using <= instead of < in loop condition
• Starting index at 1 instead of 0
• Misunderstanding array length vs last index

5. You have an array string[] fruits = {"apple", "banana", "cherry"};. You want to create a new array with all fruit names in uppercase using a for loop. Which code correctly does this?

hard

A. string[] upperFruits = new string[fruits.Length];
for (int i = 1; i <= fruits.Length; i++) {
upperFruits[i] = fruits[i].ToUpper();
}

B. string[] upperFruits = new string[fruits.Length];
foreach (string f in fruits) {
upperFruits[i] = f.ToUpper();
}

C. string[] upperFruits = new string[fruits.Length];
for (int i = 0; i < fruits.Length; i++) {
upperFruits[i] = fruits[i].ToUpper();
}

D. string[] upperFruits = new string[fruits.Length];
for (int i = 0; i < fruits.Length; i++) {
upperFruits[i] = fruits.ToUpper();
}

Solution

1. Step 1: Initialize new array with same length

   We create upperFruits with the same size as fruits.

2. Step 2: Use for loop with correct index and method

   Loop from 0 to Length-1, assign fruits[i].ToUpper() to upperFruits[i].

3. Final Answer:

   string[] upperFruits = new string[fruits.Length]; for (int i = 0; i < fruits.Length; i++) { upperFruits[i] = fruits[i].ToUpper(); } -> Option C

4. Quick Check:

   Use for loop with index and ToUpper() per element [OK]

Hint: Use for loop with index to assign transformed elements [OK]

Common Mistakes:

• Using foreach without index to assign array elements
• Starting loop at 1 or using <= causing errors
• Calling ToUpper() on array instead of string element