Array key and value extraction in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of array_keys with numeric keys

What is the output of the following PHP code?

PHP

<?php
$array = [10 => 'a', 20 => 'b', 30 => 'c'];
print_r(array_keys($array));
?>

A Array ( [0] => 10 [1] => 20 [2] => 30 )

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

C Array ( [10] => 0 [20] => 1 [30] => 2 )

D Array ( [0] => 0 [1] => 1 [2] => 2 )

💡 Hint

array_keys returns the keys of the array, not the values.

array_keys returns an array containing all the keys from the input array. Here, the keys are 10, 20, and 30.

❓ Predict Output

intermediate

Output of array_values with associative keys

What will this PHP code output?

PHP

<?php
$array = ['x' => 100, 'y' => 200, 'z' => 300];
print_r(array_values($array));
?>

A Array ( [0] => 100 [1] => 200 [2] => 300 )

B Array ( [0] => x [1] => y [2] => z )

C Array ( [x] => 100 [y] => 200 [z] => 300 )

D Array ( [100] => 0 [200] => 1 [300] => 2 )

💡 Hint

array_values returns the values of the array, ignoring keys.

array_values returns an array of all the values from the input array, reindexing them with numeric keys starting at 0.

❓ Predict Output

advanced

Result of array_keys with search value

What does this PHP code output?

PHP

<?php
$array = ['a' => 1, 'b' => 2, 'c' => 1, 'd' => 3];
print_r(array_keys($array, 1));
?>

A Array ( )

B Array ( [0] => a [1] => c )

C Array ( [a] => 1 [c] => 1 )

D Array ( [0] => 1 [1] => 1 )

💡 Hint

array_keys can accept a second parameter to filter keys by value.

array_keys with a second parameter returns all keys that have the specified value. Here, keys 'a' and 'c' have value 1.

❓ Predict Output

advanced

Output of array_values on mixed keys

What is the output of this PHP code?

PHP

<?php
$array = [5 => 'apple', 'color' => 'red', 10 => 'banana'];
print_r(array_values($array));
?>

A Array ( [apple] => 0 [red] => 1 [banana] => 2 )

B Array ( [5] => apple [color] => red [10] => banana )

C Array ( [0] => 5 [1] => color [2] => 10 )

D Array ( [0] => apple [1] => red [2] => banana )

💡 Hint

array_values returns values reindexed with numeric keys starting at 0.

array_values returns all values from the array ignoring original keys and reindexes them starting at 0.

❓ Predict Output

expert

Count of keys returned by array_keys with strict search

What is the output of this PHP code?

PHP

<?php
$array = [0 => '0', 1 => 0, 2 => false, 3 => null];
$result = array_keys($array, 0, true);
echo count($result);
?>

A 2

B 3

C 1

D 4

💡 Hint

The third parameter in array_keys enforces strict type comparison.

With strict=true, only keys with value exactly equal to integer 0 are returned. Only key 1 has value 0 as integer. Others differ in type.

Practice

1. Which PHP function is used to extract all the keys from an array?

easy

A. array_map()

B. array_values()

C. array_keys()

D. array_filter()

Solution

1. Step 1: Understand the function purpose

   array_keys() returns all keys from an array.

2. Step 2: Compare with other functions

   array_values() returns values, not keys; others do different tasks.

3. Final Answer:

   array_keys() -> Option C

4. Quick Check:

   Keys extraction = array_keys() [OK]

Hint: Keys come from array_keys(), values from array_values() [OK]

Common Mistakes:

• Confusing array_keys() with array_values()
• Using array_map() to get keys
• Trying to get keys with array_filter()

2. Which of the following is the correct syntax to get all values from an array $arr in PHP?

easy

A. array_values($arr);

B. array_keys($arr);

C. values_array($arr);

D. get_values($arr);

Solution

1. Step 1: Identify the correct function name

   The function to get values is array_values().

2. Step 2: Check syntax correctness

   Correct syntax is array_values($arr); with parentheses and variable inside.

3. Final Answer:

   array_values($arr); -> Option A

4. Quick Check:

   Values extraction = array_values() [OK]

Hint: Values come from array_values(), keys from array_keys() [OK]

Common Mistakes:

• Using array_keys() to get values
• Using non-existent functions like values_array()
• Missing parentheses in function call

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

$arr = ['a' => 1, 'b' => 2, 'c' => 3];
print_r(array_keys($arr));

medium

A. Array ( [0] => 1 [1] => 2 [2] => 3 )

B. Array ( [0] => 'a' [1] => 'b' [2] => 'c' )

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

D. Array ( [0] => a [1] => b [2] => c )

Solution

1. Step 1: Understand array_keys() output

   It returns an indexed array of all keys from the input array.

2. Step 2: Analyze given array keys

   Keys are strings 'a', 'b', 'c', so output is an indexed array with these strings.

3. Final Answer:

   Array ( [0] => a [1] => b [2] => c ) -> Option D

4. Quick Check:

   array_keys(['a'=>1,...]) = ['a','b','c'] [OK]

Hint: array_keys() returns keys as indexed array [OK]

Common Mistakes:

• Confusing keys with values in output
• Expecting associative array output
• Including quotes in output keys

4. Identify the error in this PHP code snippet:

$arr = ['x' => 10, 'y' => 20];
$keys = array_values($arr);
print_r($keys);

medium

A. array_values() should be array_keys() to get keys

B. Missing semicolon after array declaration

C. print_r() cannot print arrays

D. Variable $keys is not defined

Solution

1. Step 1: Understand the goal

   The code tries to get keys but uses array_values() which returns values.

2. Step 2: Correct function usage

   To get keys, use array_keys() instead of array_values().

3. Final Answer:

   array_values() should be array_keys() to get keys -> Option A

4. Quick Check:

   Keys need array_keys(), not array_values() [OK]

Hint: Use array_keys() for keys, array_values() for values [OK]

Common Mistakes:

• Mixing up array_keys() and array_values()
• Assuming print_r() can't print arrays
• Ignoring correct variable definitions

5. Given the array $data = ['apple' => 3, 'banana' => 0, 'cherry' => 5, 'date' => 0];, which PHP code correctly extracts only the keys with non-zero values?

hard

A. $keys = array_keys($data);

B. $keys = array_keys(array_filter($data));

C. $keys = array_values(array_filter($data));

D. $keys = array_filter(array_keys($data));

Solution

1. Step 1: Filter array to keep non-zero values

   array_filter($data) removes entries with zero values.

2. Step 2: Extract keys from filtered array

   array_keys() gets keys from the filtered array, giving only keys with non-zero values.

3. Final Answer:

   $keys = array_keys(array_filter($data)); -> Option B

4. Quick Check:

   Filter then keys = array_keys(array_filter()) [OK]

Hint: Filter first, then get keys with array_keys(array_filter()) [OK]

Common Mistakes:

• Using array_values() instead of array_keys() after filtering
• Not filtering before extracting keys
• Filtering keys instead of values