Array walk function in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the array_walk() function do in PHP?

It applies a user-defined function to every element of an array, allowing you to modify or use each element.

beginner

How do you define the callback function used in array_walk()?

The callback function takes at least one parameter: the value of the current element, passed by reference if you want to modify it. It can also take a second optional parameter for the key.

intermediate

What is the difference between array_walk() and array_map()?

array_walk() applies a function to each element and can modify the original array by reference. array_map() returns a new array with the results and does not modify the original array.

beginner

Can array_walk() modify the original array elements?

Yes, if the callback function uses the value parameter by reference, it can change the original array elements.

beginner

Write a simple example of array_walk() that appends '!' to each string in an array.

Example:<br><pre>function addExclamation(&$item, $key) {
  $item .= '!';
}
$array = ['Hi', 'Hello', 'Hey'];
array_walk($array, 'addExclamation');
// Result: ['Hi!', 'Hello!', 'Hey!']</pre>

What parameters does the callback function for array_walk() receive?

A Value and key of each array element

B Only the value of each element

C Only the key of each element

D The entire array

The callback function receives the value and key of each element to allow processing or modification.

Does array_walk() return a new array?

A No, it returns the original array

B Yes, always

C Yes, but only if the callback returns a value

D No, it returns true or false

array_walk() returns true on success or false on failure; it does not return a new array.

How can you modify the original array elements inside the callback?

A You cannot modify original elements

B By passing the value parameter by reference

C By using a global variable

D By returning the new value from the callback

Passing the value parameter by reference (&$item) allows modifying the original array elements.

Which function is better to create a new array with modified values without changing the original array?

A array_walk()

B array_filter()

C array_map()

D array_reduce()

array_map() returns a new array with modified values, leaving the original unchanged.

What happens if the callback function in array_walk() does not accept the key parameter?

A The key is ignored and only the value is passed

B The function receives the key only

C It causes an error

D The array is not processed

The callback can accept just the value parameter; the key is optional and ignored if not declared.

Explain how array_walk() works and how it can modify an array.

Think about how you can change each item in a list by visiting them one by one.

Compare array_walk() and array_map() in terms of modifying arrays.

Consider whether you want to change the original list or create a new one.

Practice

1. What does the PHP function array_walk do?

easy

A. It applies a user-defined function to every element of an array.

B. It sorts the array in ascending order.

C. It merges two arrays into one.

D. It removes duplicate values from an array.

Solution

1. Step 1: Understand the purpose of array_walk

   The function array_walk is designed to apply a callback function to each element of an array.

2. Step 2: Compare with other array functions

   Sorting, merging, or removing duplicates are done by other functions like sort, array_merge, or array_unique.

3. Final Answer:

   It applies a user-defined function to every element of an array. -> Option A

4. Quick Check:

   array_walk = apply function to each element [OK]

Hint: array_walk runs a function on each array item [OK]

Common Mistakes:

• Confusing array_walk with sorting functions
• Thinking it merges arrays
• Assuming it removes duplicates

2. Which of the following is the correct syntax to use array_walk with a callback function named modify on an array $arr?

easy

A. array_walk('modify', $arr);

B. array_walk(modify, $arr);

C. array_walk($arr, modify);

D. array_walk($arr, 'modify');

Solution

1. Step 1: Recall array_walk syntax

   The correct syntax is array_walk(array, callback), where callback is a string with the function name.

2. Step 2: Check the options

   Only array_walk($arr, 'modify'); matches the correct order and uses quotes around the callback function name.

3. Final Answer:

   array_walk($arr, 'modify'); -> Option D

4. Quick Check:

   array_walk(array, 'function') syntax [OK]

Hint: array_walk takes array first, then callback as string [OK]

Common Mistakes:

• Swapping the order of parameters
• Omitting quotes around callback name
• Passing callback without quotes

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

$numbers = [1, 2, 3];
array_walk($numbers, function(&$value) {
    $value *= 2;
});
print_r($numbers);

medium

A. Array ( [0] => 2 [1] => 4 [2] => 6 )

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

C. Array ( [0] => 0 [1] => 0 [2] => 0 )

D. Error: Cannot modify array elements

Solution

1. Step 1: Understand the callback function

   The callback multiplies each value by 2, and the value is passed by reference (&$value), so changes affect the original array.

2. Step 2: Apply changes to each element

   1*2=2, 2*2=4, 3*2=6, so the array becomes [2, 4, 6].

3. Final Answer:

   Array ( [0] => 2 [1] => 4 [2] => 6 ) -> Option A

4. Quick Check:

   Reference callback doubles values [OK]

Hint: Pass value by reference to modify array items [OK]

Common Mistakes:

• Not using & to modify original array
• Expecting original array unchanged
• Confusing output format

4. Identify the error in this code snippet:

$arr = [1, 2, 3];
array_walk($arr, function($val) {
    $val += 5;
});
print_r($arr);

medium

A. The array must be passed by reference to array_walk.

B. array_walk requires a named function, not an anonymous function.

C. Callback function parameter should be passed by reference to modify array.

D. print_r cannot print arrays modified by array_walk.

Solution

1. Step 1: Check how callback modifies array

   The callback parameter $val is passed by value, so changes inside do not affect original array.

2. Step 2: Correct usage for modification

   Passing &$val by reference allows modifying the original array elements.

3. Final Answer:

   Callback function parameter should be passed by reference to modify array. -> Option C

4. Quick Check:

   Use & in callback to change array items [OK]

Hint: Use & in callback parameter to modify array elements [OK]

Common Mistakes:

• Ignoring reference in callback parameter
• Thinking array_walk needs array by reference
• Believing anonymous functions are not allowed

5. Given the array $fruits = ['apple' => 2, 'banana' => 3, 'cherry' => 5];, how can you use array_walk to add 10 to each value only if the value is less than 4?

hard

A. array_walk($fruits, function($val) { if ($val < 4) $val += 10; });

B. array_walk($fruits, function(&$val) { if ($val < 4) $val += 10; });

C. array_walk($fruits, function(&$val, $key) { if ($key < 4) $val += 10; });

D. array_walk($fruits, function(&$val, $key) { if ($val > 4) $val += 10; });

Solution

1. Step 1: Identify condition and modification

   We want to add 10 to values less than 4, so check $val < 4 inside the callback.

2. Step 2: Ensure modification affects original array

   Pass $val by reference (&$val) to change original array values.

3. Step 3: Verify options

   array_walk($fruits, function(&$val) { if ($val < 4) $val += 10; }); correctly uses reference and condition. array_walk($fruits, function($val) { if ($val < 4) $val += 10; }); lacks reference, so no change. array_walk($fruits, function(&$val, $key) { if ($key < 4) $val += 10; }); checks key instead of value. array_walk($fruits, function(&$val, $key) { if ($val > 4) $val += 10; }); adds 10 when value is greater than 4, which is wrong.

4. Final Answer:

   array_walk($fruits, function(&$val) { if ($val < 4) $val += 10; }); -> Option B

5. Quick Check:

   Reference + condition on value < 4 [OK]

Hint: Use & and if condition inside callback to selectively modify [OK]

Common Mistakes:

• Not passing value by reference
• Checking key instead of value
• Using wrong comparison operator