Array key and value extraction in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What function in PHP extracts all the keys from an array?

The array_keys() function extracts all the keys from an array and returns them as a new array.

beginner

How do you get all the values from an array in PHP?

Use the array_values() function to get all the values from an array as a new indexed array.

beginner

Given $arr = ['a' => 1, 'b' => 2];, what does array_keys($arr) return?

It returns ['a', 'b'], which are the keys of the array.

beginner

What will array_values(['x' => 10, 'y' => 20]) return?

It returns [10, 20], the values of the array re-indexed starting from 0.

intermediate

Can array_keys() be used to find keys for a specific value?

Yes, by passing the value as the second argument, array_keys($array, $value) returns all keys that have that value.

Which PHP function returns all keys from an array?

A array_keys()

B array_values()

C array_search()

D array_flip()

array_keys() returns all keys from an array.

What does array_values() return?

A All keys of an array

B The last value of an array

C All values of an array

D The first key of an array

array_values() returns all values of an array as a new indexed array.

How can you find all keys for a specific value in an array?

A array_values($array, $value)

B array_keys($array, $value)

C array_search($value, $array)

D array_flip($array)

Passing the value as the second argument to array_keys() returns all keys matching that value.

What will array_keys(['x' => 5, 'y' => 10]) return?

A ['x', 'y']

B [5, 10]

C ['5', '10']

D ['0', '1']

array_keys() returns the keys, which are 'x' and 'y'.

If you want only the values of an array without keys, which function do you use?

A array_keys()

B array_flip()

C array_search()

D array_values()

array_values() returns the values without keys.

Explain how to extract keys and values from an array in PHP.

Think about the two main functions for keys and values.

How can you find all keys associated with a specific value in a PHP array?

Use array_keys with a value parameter.

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