Multidimensional arrays in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a multidimensional array in PHP?

A multidimensional array is an array that contains one or more arrays inside it. It is like a table or grid where each element can be accessed using multiple keys or indexes.

beginner

How do you access the element 'PHP' in this array?

$arr = [['HTML', 'CSS'], ['JavaScript', 'PHP']];

You access 'PHP' by using two indexes: $arr[1][1]. The first index selects the second array, and the second index selects the second element inside it.

intermediate

How to add a new element 'Python' to the second array in a multidimensional array?

Use $arr[1][] = 'Python'; to add 'Python' at the end of the second array inside the multidimensional array.

beginner

What will count($arr) return if $arr = [[1,2], [3,4], [5,6]];?

It will return 3 because count() counts the number of elements in the outer array, which has three inner arrays.

intermediate

How can you loop through all elements in a multidimensional array?

Use nested foreach loops: the outer loop goes through each inner array, and the inner loop goes through each element inside those arrays.

What does $arr[0][1] access in a multidimensional array?

A The first element of the second inner array

B The first element of the first inner array

C The second element of the first inner array

D The second element of the second inner array

$arr[0] selects the first inner array, and [1] selects its second element.

How do you add a new inner array to a multidimensional array $arr?

A <code>$arr[] = ['new', 'array'];</code>

B <code>$arr = ['new', 'array'];</code>

C <code>$arr[0][] = 'new';</code>

D <code>array_push($arr[0], ['new', 'array']);</code>

Using $arr[] = [...] adds a new inner array at the end of the outer array.

What will count($arr, COUNT_RECURSIVE) do?

A Count all elements including those in inner arrays

B Count only the outer array elements

C Count only the inner array elements

D Cause an error

Using COUNT_RECURSIVE counts all elements inside all nested arrays.

Which loop structure is best to iterate over a multidimensional array?

A While loop only

B Nested foreach loops

C Single for loop

D Do-while loop only

Nested foreach loops let you access each inner array and its elements easily.

What type of data structure is a multidimensional array similar to?

A Queue only

B Single list

C Stack only

D Table or grid

Multidimensional arrays are like tables with rows and columns.

Explain what a multidimensional array is and how you access its elements in PHP.

Think of it like a table with rows and columns.

Describe how to loop through all elements of a multidimensional array and why nested loops are needed.

Imagine opening each box inside a bigger box.

Practice

1. What is a multidimensional array in PHP?

easy

A. An array containing one or more arrays inside it

B. An array with only string keys

C. An array that can only hold numbers

D. An array that cannot be changed after creation

Solution

1. Step 1: Understand array nesting

   A multidimensional array is an array that contains other arrays as its elements.

2. Step 2: Identify the correct definition

   An array containing one or more arrays inside it correctly describes this nesting, while others describe unrelated properties.

3. Final Answer:

   An array containing one or more arrays inside it -> Option A

4. Quick Check:

   Multidimensional array = nested arrays [OK]

Hint: Think of arrays inside arrays like boxes within boxes [OK]

Common Mistakes:

• Confusing multidimensional arrays with simple arrays
• Thinking keys must be strings only
• Believing arrays cannot be changed

2. Which of the following is the correct way to access the element 'blue' in this array?

$colors = [['red', 'green'], ['blue', 'yellow']];

easy

A. $colors[1][0]

B. $colors[0][2]

C. $colors[2][1]

D. $colors[1][1]

Solution

1. Step 1: Identify the position of 'blue'

   'blue' is in the second inner array (index 1) and is the first element (index 0) there.

2. Step 2: Use correct indices to access

   Accessing $colors[1][0] retrieves 'blue'. Other options point to wrong indices or elements.

3. Final Answer:

   $colors[1][0] -> Option A

4. Quick Check:

   Second array, first element = 'blue' [OK]

Hint: Count arrays and elements starting at zero [OK]

Common Mistakes:

• Using wrong index numbers
• Confusing first and second arrays
• Trying to access non-existing elements

3. What will be the output of this PHP code?

$matrix = [[1, 2], [3, 4]];
echo $matrix[0][1] + $matrix[1][0];

medium

A. 9

B. 7

C. 5

D. Error

Solution

1. Step 1: Identify the values at given indices

   $matrix[0][1] is 2 and $matrix[1][0] is 3.

2. Step 2: Calculate the sum

   2 + 3 equals 5, so the output is 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   2 + 3 = 5 [OK]

Hint: Add values at given indices carefully [OK]

Common Mistakes:

• Mixing up indices
• Adding wrong elements
• Expecting string output

4. Find the error in this code snippet:

$arr = [[1, 2], [3, 4];
echo $arr[1][1];

medium

A. Missing closing bracket for the first array

B. Missing closing bracket for the outer array

C. Incorrect index used to access element

D. Missing semicolon after echo statement

Solution

1. Step 1: Check array syntax

   The array starts with [[ but only one closing bracket ] is present before the semicolon.

2. Step 2: Identify missing bracket

   The outer array requires a closing bracket ], so the code should end with ]];

3. Final Answer:

   Missing closing bracket for the outer array -> Option B

4. Quick Check:

   Outer array brackets must match [OK]

Hint: Count opening and closing brackets carefully [OK]

Common Mistakes:

• Confusing inner and outer brackets
• Ignoring syntax errors
• Assuming semicolon fixes bracket issues

5. Given this multidimensional array representing students and their scores:
<pre>$students = [
['name' => 'Alice', 'scores' => [85, 90]],
['name' => 'Bob', 'scores' => [78, 82]],
['name' => 'Charlie', 'scores' => [92, 88]]
];

Which code correctly calculates and prints the average score of Bob?

hard

A. $avg = array_sum($students[2]['scores']) / count($students[2]['scores']);echo $avg;

B. $avg = array_sum($students['Bob']['scores']) / count($students['Bob']['scores']);echo $avg;

C. $avg = array_sum($students[1]['score']) / count($students[1]['score']);echo $avg;

D. $avg = array_sum($students[1]['scores']) / count($students[1]['scores']);echo $avg;

Solution

1. Step 1: Locate Bob's data in the array

   Bob is at index 1, and his scores are under the key 'scores'.

2. Step 2: Calculate average correctly

   Use array_sum and count on $students[1]['scores'] to get average. $avg = array_sum($students[1]['scores']) / count($students[1]['scores']);
   echo $avg; does this correctly.

3. Final Answer:

   $avg = array_sum($students[1]['scores']) / count($students[1]['scores']); echo $avg; -> Option D

4. Quick Check:

   Index 1 and 'scores' key used correctly [OK]

Hint: Use numeric index for array, string keys for inner arrays [OK]

Common Mistakes:

• Using string key for outer array
• Misspelling 'scores' key
• Using wrong index for Bob