Array methods (Sort, Reverse, IndexOf) in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Array.Sort() method do in C#?

It arranges the elements of an array in ascending order (from smallest to largest).

beginner

How does Array.Reverse() affect an array?

It reverses the order of the elements in the array, so the last element becomes first and the first becomes last.

beginner

What does Array.IndexOf(array, value) return if the value is not found?

It returns -1 to indicate the value is not present in the array.

beginner

Can Array.Sort() be used on arrays of strings in C#?

Yes, it sorts strings alphabetically (lexicographically) in ascending order.

beginner

If you want to find the position of the first occurrence of a value in an array, which method do you use?

Use Array.IndexOf() to get the index of the first matching element.

What will Array.Sort() do to the array {3, 1, 4, 2}?

A Do nothing

B Reverse it to <code>{2, 4, 1, 3}</code>

C Return the index of 3

D Sort it to <code>{1, 2, 3, 4}</code>

Array.Sort() arranges elements in ascending order.

What does Array.Reverse() do to {"a", "b", "c"}?

A <code>{"c", "b", "a"}</code>

B <code>{"a", "b", "c"}</code>

C <code>{"b", "a", "c"}</code>

D <code>{"a", "c", "b"}</code>

It reverses the order of elements.

What does Array.IndexOf(array, value) return if value is not in the array?

A 1

B -1

C 0

D Throws an error

Returns -1 to show the value was not found.

Which method would you use to find the position of the number 5 in an array?

A Array.IndexOf()

B Array.Sort()

C Array.Reverse()

D Array.Clear()

Array.IndexOf() finds the index of a value.

After calling Array.Sort() on {"dog", "cat", "bird"}, what is the order?

A <code>{"bird", "dog", "cat"}</code>

B <code>{"dog", "cat", "bird"}</code>

C <code>{"bird", "cat", "dog"}</code>

D <code>{"cat", "dog", "bird"}</code>

Strings are sorted alphabetically in ascending order.

Explain how you would use Array.Sort(), Array.Reverse(), and Array.IndexOf() in a C# program.

Think about sorting a list, then reversing it, and searching for an item.

What happens if you try to find the index of a value that does not exist in the array using Array.IndexOf()?

Consider how to check if a value is missing.

Practice

1. What does the Array.Sort method do to an array in C#?

easy

A. It reverses the order of the items.

B. It arranges the items in ascending order.

C. It finds the index of a specific item.

D. It removes duplicate items from the array.

Solution

1. Step 1: Understand Array.Sort purpose

   The Array.Sort method organizes the elements of an array in ascending order by default.

2. Step 2: Compare with other methods

   Array.Reverse flips the array, and Array.IndexOf finds an element's position, so they do different things.

3. Final Answer:

   It arranges the items in ascending order. -> Option B

4. Quick Check:

   Array.Sort = ascending order [OK]

Hint: Sort means arrange from smallest to largest [OK]

Common Mistakes:

• Confusing Sort with Reverse
• Thinking Sort finds an element's position
• Assuming Sort removes duplicates

2. Which of the following is the correct way to reverse an array named numbers in C#?

easy

A. numbers.Reverse();

B. numbers.ReverseArray();

C. Array.Sort(numbers);

D. Array.Reverse(numbers);

Solution

1. Step 1: Identify correct method call syntax

   In C#, Array.Reverse is a static method that takes the array as an argument, so Array.Reverse(numbers); is correct.

2. Step 2: Check other options

   numbers.Reverse() is a LINQ extension returning IEnumerable, not reversing in place. Array.Sort sorts, not reverses. ReverseArray() does not exist.

3. Final Answer:

   Array.Reverse(numbers); -> Option D

4. Quick Check:

   Reverse array = Array.Reverse(array) [OK]

Hint: Use Array.Reverse(array) to flip order [OK]

Common Mistakes:

• Using instance method Reverse() which doesn't reverse in place
• Confusing Sort with Reverse
• Calling non-existent methods

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

string[] fruits = {"apple", "banana", "cherry"};
Array.Sort(fruits);
Array.Reverse(fruits);
Console.WriteLine(fruits[0]);

medium

A. cherry

B. banana

C. apple

D. Runtime error

Solution

1. Step 1: Sort the array

   After Array.Sort(fruits);, the array becomes ["apple", "banana", "cherry"] (already sorted).

2. Step 2: Reverse the array

   After Array.Reverse(fruits);, the array becomes ["cherry", "banana", "apple"].

3. Step 3: Print first element

   fruits[0] is now "cherry".

4. Final Answer:

   cherry -> Option A

5. Quick Check:

   Sort then Reverse = descending order [OK]

Hint: Sort then Reverse = descending order, first item is largest [OK]

Common Mistakes:

• Assuming array stays sorted ascending
• Confusing Reverse with Sort
• Expecting runtime error due to method calls

4. The following code throws an error. What is the mistake?

int[] nums = {3, 1, 4};
nums.Sort();
Console.WriteLine(nums[0]);

medium

A. Arrays don't have a Sort() instance method.

B. The array must be reversed before sorting.

C. You must use Console.WriteLine(nums.Sort()); instead.

D. The array must be converted to a list before sorting.

Solution

1. Step 1: Identify method call error

   Arrays in C# do not have an instance method called Sort(). Sorting is done via the static method Array.Sort(array).

2. Step 2: Understand correct usage

   The correct way is Array.Sort(nums); to sort the array in place.

3. Final Answer:

   Arrays don't have a Sort() instance method. -> Option A

4. Quick Check:

   Use Array.Sort(array), not array.Sort() [OK]

Hint: Array.Sort is static; no instance Sort() method [OK]

Common Mistakes:

• Calling Sort() directly on array instance
• Confusing List.Sort() with Array.Sort()
• Trying to print Sort() return value

5. Given the array int[] values = {5, 3, 8, 3, 9};, which code snippet finds the index of the first occurrence of the number 3 and then reverses the array?

hard

A. int index = Array.FindIndex(values, 3);
Array.Sort(values);

B. int index = values.IndexOf(3);
Array.Reverse(values);

C. int index = Array.IndexOf(values, 3);
Array.Reverse(values);

D. int index = Array.IndexOf(values, 3);
Array.Sort(values);

Solution

1. Step 1: Find index of first 3

   Array.IndexOf(values, 3) correctly returns the first index where 3 appears, which is 1.

2. Step 2: Reverse the array

   Array.Reverse(values); reverses the array in place as required.

3. Final Answer:

   int index = Array.IndexOf(values, 3); Array.Reverse(values); -> Option C

4. Quick Check:

   IndexOf + Reverse = int index = Array.IndexOf(values, 3); Array.Reverse(values); [OK]

Hint: Use Array.IndexOf(array, value) to find index [OK]

Common Mistakes:

• Using instance method IndexOf on array (not valid)
• Confusing FindIndex with IndexOf
• Sorting instead of reversing