Multidimensional arrays in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested array access

What is the output of this PHP code?

PHP

<?php
$array = [[1, 2], [3, 4], [5, 6]];
echo $array[1][0];
?>

A 5

B 2

C 4

D 3

💡 Hint

Remember that arrays start at index 0.

The outer array has three elements: [0]=>[1,2], [1]=>[3,4], [2]=>[5,6]. Accessing $array[1] gives [3,4], and $array[1][0] is 3.

❓ Predict Output

intermediate

Count elements in multidimensional array

What is the output of this PHP code?

PHP

<?php
$array = [
  'fruits' => ['apple', 'banana'],
  'vegetables' => ['carrot', 'pea', 'corn']
];
echo count($array['vegetables']);
?>

A 5

B 2

C 3

D 1

💡 Hint

count() counts elements in the specified array.

The 'vegetables' array has three elements: 'carrot', 'pea', and 'corn'. So count($array['vegetables']) returns 3.

🔧 Debug

advanced

Identify the error in multidimensional array access

What error does this PHP code produce?

PHP

<?php
$array = [[1, 2], [3, 4]];
echo $array[2][0];
?>

A Undefined offset: 2

B Parse error: syntax error

C Fatal error: Cannot use [] for reading

D Notice: Undefined variable

💡 Hint

Check if the index exists before accessing.

The outer array has indexes 0 and 1 only. Accessing $array[2] causes an undefined offset error.

📝 Syntax

advanced

Which option correctly defines a multidimensional array?

Which of the following PHP code snippets correctly defines a multidimensional array with two rows and two columns?

A $array = [[1, 2], [3 4]];

B $array = [[1, 2], [3, 4]];

C $array = [1, 2; 3, 4];

D $array = array(1, 2, 3, 4);

💡 Hint

Look for proper array syntax and separators.

Option B uses correct PHP array syntax with nested arrays separated by commas. Option B uses semicolon inside array which is invalid. Option B is a flat array, not multidimensional. Option B misses a comma between 3 and 4.

🚀 Application

expert

Calculate sum of all elements in a multidimensional array

Given the multidimensional array below, what is the sum of all its integer elements?

PHP

<?php
$array = [
  [1, 2, 3],
  [4, 5],
  [6]
];
$sum = 0;
foreach ($array as $subarray) {
  foreach ($subarray as $value) {
    $sum += $value;
  }
}
echo $sum;
?>

A 21

B 20

C 18

D 19

💡 Hint

Add all numbers: 1+2+3+4+5+6.

The sum is 1+2+3+4+5+6 = 21.

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