Array iteration with for and foreach in C Sharp (C#)

Introduction

We use array iteration to look at each item in a list one by one. This helps us do something with every item, like print or check it.

When you want to print all items in a list of names.

When you need to add up all numbers in an array.

When you want to find a specific item in a list.

When you want to change or update each item in a list.

When you want to count how many items meet a condition.

Syntax

C Sharp (C#)

class Program
{
    static void Main()
    {
        int[] numbers = {1, 2, 3, 4, 5};

        // Using for loop
        for (int index = 0; index < numbers.Length; index++)
        {
            int currentNumber = numbers[index];
            // Do something with currentNumber
        }

        // Using foreach loop
        foreach (int currentNumber in numbers)
        {
            // Do something with currentNumber
        }
    }
}

The for loop uses an index to access each item by position.

The foreach loop goes through each item directly without using an index.

Examples

Both loops do nothing because the array is empty.

C Sharp (C#)

int[] emptyArray = {};

// for loop on empty array
for (int i = 0; i < emptyArray.Length; i++)
{
    Console.WriteLine(emptyArray[i]);
}

// foreach loop on empty array
foreach (int item in emptyArray)
{
    Console.WriteLine(item);
}

Both loops print the single item once.

C Sharp (C#)

int[] singleItemArray = {42};

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

// foreach loop
foreach (int item in singleItemArray)
{
    Console.WriteLine(item);
}

Both loops print only the first and last items.

C Sharp (C#)

int[] numbers = {10, 20, 30};

// for loop accessing first and last
for (int i = 0; i < numbers.Length; i++)
{
    if (i == 0 || i == numbers.Length - 1)
    {
        Console.WriteLine(numbers[i]);
    }
}

// foreach loop with condition
foreach (int number in numbers)
{
    if (number == 10 || number == 30)
    {
        Console.WriteLine(number);
    }
}

Sample Program

This program shows how to go through an array of scores using both for and foreach loops. It prints each score with its position for the for loop and just the score for the foreach loop.

C Sharp (C#)

using System;

class Program
{
    static void Main()
    {
        int[] scores = {85, 92, 78, 90, 88};

        Console.WriteLine("Scores using for loop:");
        for (int index = 0; index < scores.Length; index++)
        {
            Console.WriteLine($"Score at index {index}: {scores[index]}");
        }

        Console.WriteLine("\nScores using foreach loop:");
        foreach (int score in scores)
        {
            Console.WriteLine($"Score: {score}");
        }
    }
}

Important Notes

The for loop is useful when you need the position (index) of each item.

The foreach loop is simpler when you only need the item itself.

Both loops run in time proportional to the number of items (O(n)).

Summary

Use for to access items by index.

Use foreach to access items directly.

Both loops help you do something with every item in an array.

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