Associative array creation in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of associative array creation with mixed keys

What is the output of the following PHP code?

PHP

<?php
$arr = ["a" => 1, 2 => "b", "3" => "c", 4 => "d"];
print_r($arr);
?>

A Array ( [0] => a [1] => 1 [2] => b [3] => c [4] => d )

B Array ( [a] => 1 [2] => b [3] => c [4] => d [0] => 0 )

C Array ( [a] => 1 [2] => b [3] => c [4] => d )

D Array ( [a] => 1 [2] => b [3] => c )

💡 Hint

Remember that numeric string keys are converted to integers in PHP arrays.

In PHP, string keys that are numeric strings like "3" are converted to integer keys. So the key "3" becomes 3. The array has keys 'a', 2, 3, and 4 with their respective values.

❓ Predict Output

intermediate

Count items in associative array created with array()

How many items does the following associative array contain?

PHP

<?php
$assoc = array('x' => 10, 'y' => 20, 'z' => 30);
echo count($assoc);
?>

A Error

B 0

C 1

D 3

💡 Hint

Count returns the number of key-value pairs in an array.

The array has three key-value pairs: 'x', 'y', and 'z'. So count returns 3.

🔧 Debug

advanced

Identify the error in associative array creation

What error does the following PHP code produce?

PHP

<?php
$arr = ["key1" => 100, "key2" 200, "key3" => 300];
print_r($arr);
?>

A Parse error: syntax error, unexpected '200' (missing =>)

B Notice: Undefined index 'key2'

C Warning: Array to string conversion

D No error, outputs the array

💡 Hint

Check the syntax for key-value pairs in arrays.

The key 'key2' is missing the => operator before its value 200, causing a syntax error.

🧠 Conceptual

advanced

Associative array key type behavior

Which statement about PHP associative array keys is TRUE?

A Null keys are converted to string 'null'

B Boolean true is converted to integer 1 as a key

C Arrays can be used as keys

D Float keys are stored as floats

💡 Hint

Think about how PHP converts different types to keys internally.

In PHP, boolean true is converted to integer 1 when used as an array key. Float keys are cast to integers. Null is cast to empty string. Arrays cannot be used as keys.

🚀 Application

expert

Create associative array from two lists

Given two arrays $keys = ['name', 'age', 'city'] and $values = ['Alice', 30, 'Paris'], which code correctly creates an associative array combining keys and values?

A $assoc = array_combine($keys, $values);

B $assoc = array_fill_keys($keys, $values);

C $assoc = array_map(fn($k, $v) => [$k => $v], $keys, $values);

D $assoc = array_merge($keys, $values);

💡 Hint

Look for a function that pairs keys and values into an associative array.

array_combine creates an associative array by using one array for keys and another for values. array_merge merges arrays sequentially. array_map returns an array of arrays, not a single associative array. array_fill_keys fills all keys with the same value.

Practice

1. Which of the following is the correct way to create an associative array in PHP with keys "name" and "age"?

easy

A. $person = ("name" => "Alice", "age" => 30);

B. $person = {"name": "Alice", "age": 30};

C. $person = array("Alice", 30);

D. $person = ["name" => "Alice", "age" => 30];

Solution

1. Step 1: Understand PHP associative array syntax

   Associative arrays use square brackets with key => value pairs.

2. Step 2: Identify correct syntax among options

   $person = ["name" => "Alice", "age" => 30]; uses square brackets and => correctly; others use invalid syntax.

3. Final Answer:

   $person = ["name" => "Alice", "age" => 30]; -> Option D

4. Quick Check:

   Associative array = key => value pairs [OK]

Hint: Use square brackets and => for associative arrays [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Using colon (:) instead of =>
• Creating indexed arrays instead of associative

2. Which of the following PHP code snippets will cause a syntax error when creating an associative array?

easy

A. $data = ["key1" : "value1", "key2" : "value2"];

B. $data = array("key1" => "value1", "key2" => "value2");

C. $data = ["key1" => "value1", "key2" => "value2"];

D. $data = ['key1' => 'value1', 'key2' => 'value2'];

Solution

1. Step 1: Check the syntax for key-value pairs

   PHP uses => to assign values to keys, not colon (:).

2. Step 2: Identify the invalid syntax

   $data = ["key1" : "value1", "key2" : "value2"]; uses colon (:) which is invalid in PHP associative arrays.

3. Final Answer:

   $data = ["key1" : "value1", "key2" : "value2"]; -> Option A

4. Quick Check:

   Use => not : for keys in PHP arrays [OK]

Hint: Remember PHP uses =>, not : for associative arrays [OK]

Common Mistakes:

• Using colon (:) instead of =>
• Mixing array() and [] syntax incorrectly
• Missing commas between pairs

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

$arr = ["fruit" => "apple", "color" => "red"];
echo $arr["color"];

medium

A. red

B. color

C. apple

D. fruit

Solution

1. Step 1: Understand array key access

   Accessing $arr["color"] returns the value for key "color".

2. Step 2: Identify the value for key "color"

   Value is "red", so echo outputs "red".

3. Final Answer:

   red -> Option A

4. Quick Check:

   Key "color" = "red" [OK]

Hint: Access values by keys with $array["key"] [OK]

Common Mistakes:

• Confusing keys and values
• Using wrong quotes or missing quotes
• Trying to access with dot notation

4. Identify the error in the following PHP code that tries to create an associative array:

$info = ["name" => "John", "age" = 25];

medium

A. Missing comma between elements

B. Keys should not be in quotes

C. Using = instead of => for key-value pair

D. Array must use array() syntax

Solution

1. Step 1: Check key-value assignment syntax

   PHP requires => to assign values to keys, not =.

2. Step 2: Locate incorrect assignment

   "age" = 25 uses = instead of =>, causing syntax error.

3. Final Answer:

   Using = instead of => for key-value pair -> Option C

4. Quick Check:

   Use => for key-value pairs [OK]

Hint: Use =>, not =, between key and value [OK]

Common Mistakes:

• Using single = instead of =>
• Omitting commas between pairs
• Not quoting string keys

5. You want to create an associative array in PHP that maps product names to their prices, but only include products with a price greater than 20. Which code correctly creates this filtered associative array?

hard

A. $products = ["apple" => 15, "banana" => 25, "cherry" => 30];
$filtered = [];
foreach ($products as $name, $price) {
if ($price > 20) {
$filtered[$name] = $price;
}
}

B. $products = ["apple" => 15, "banana" => 25, "cherry" => 30];
$filtered = [];
foreach ($products as $name => $price) {
if ($price > 20) {
$filtered[$name] = $price;
}
}

C. $products = ["apple" => 15, "banana" => 25, "cherry" => 30];
$filtered = array_filter($products, function($name, $price) {
return $price > 20;
});

D. $products = ["apple" => 15, "banana" => 25, "cherry" => 30];
$filtered = [];
foreach ($products as $price => $name) {
if ($price > 20) {
$filtered[$name] = $price;
}
}

Solution

1. Step 1: Understand foreach key-value syntax

   Use foreach ($array as $key => $value) to access keys and values correctly.

2. Step 2: Filter products with price > 20

   $products = ["apple" => 15, "banana" => 25, "cherry" => 30]; $filtered = []; foreach ($products as $name => $price) { if ($price > 20) { $filtered[$name] = $price; } } correctly checks price and assigns filtered items to new array.

3. Final Answer:

   The code using foreach ($products as $name => $price) -> Option B

4. Quick Check:

   foreach ($arr as $key => $value) for associative arrays [OK]

Hint: Use foreach ($arr as $key => $value) to access associative arrays [OK]

Common Mistakes:

• Swapping key and value variables in foreach
• Using incorrect foreach syntax with two variables separated by comma
• Misusing array_filter with wrong callback parameters