Array walk function in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to apply a function to each element of the array using array_walk.

PHP

<?php
$fruits = ['apple', 'banana', 'cherry'];
function printFruit($item) {
    echo $item . "\n";
}
array_walk($fruits, [1]);
?>

Drag options to blanks, or click blank then click option'

AprintFruit

Bprint

Cecho

Darray_map

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'print' or 'echo' directly as the callback causes errors.

Passing 'array_map' instead of a callback function.

The array_walk function requires a callback function name to apply to each element. 'printFruit' is the correct callback.

2fill in blank

medium

Complete the code to modify each element of the array by appending ' fruit' using array_walk.

PHP

<?php
$fruits = ['apple', 'banana', 'cherry'];
function addFruitSuffix(&$item) {
    $item .= [1];
}
array_walk($fruits, 'addFruitSuffix');
print_r($fruits);
?>

Drag options to blanks, or click blank then click option'

A' fruit'

B"fruit"

C'fruit'

D" fruit"

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the space before 'fruit' causes words to join without space.

Using single quotes without space changes output formatting.

Double quotes " fruit" correctly append the string with a space to each element.

3fill in blank

hard

Fix the error in the code to correctly print keys and values using array_walk.

PHP

<?php
$colors = ['red' => '#FF0000', 'green' => '#00FF00', 'blue' => '#0000FF'];
function printColor($value, [1]) {
    echo "$key: $value\n";
}
array_walk($colors, 'printColor');
?>

Drag options to blanks, or click blank then click option'

Aindex

Bkey

Citem

Dcolor

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different variable name than 'key' causes undefined variable error.

Swapping the order of parameters causes wrong output.

The second parameter in the callback for array_walk is the key, so it must be named 'key' to match the echo statement.

4fill in blank

hard

Fill both blanks to create an array_walk callback that prints the key and value separated by a colon.

PHP

<?php
$items = ['pen' => 1.5, 'notebook' => 2.0, 'eraser' => 0.5];
function printItem([1], [2]) {
    echo "$key: $value\n";
}
array_walk($items, 'printItem');
?>

Drag options to blanks, or click blank then click option'

Avalue

Bitem

Ckey

Dprice

Attempts:

3 left

💡 Hint

Common Mistakes

Reversing the order of parameters.

Using variable names that don't match the echo statement.

The callback parameters must be value first, then key, matching the echo statement variables.

5fill in blank

hard

Fill all three blanks to create a callback that appends the key in parentheses to each value in the array.

PHP

<?php
$animals = ['cat' => 'meow', 'dog' => 'woof', 'cow' => 'moo'];
function appendKeyToValue(&[1], [2]) {
    $[1] .= ' (' . [2] . ')';
}
array_walk($animals, 'appendKeyToValue');
print_r($animals);
?>

Drag options to blanks, or click blank then click option'

Asound

Bkey

Cvalue

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable names causing undefined variable errors.

Not passing the first parameter by reference.

The first blank is the reference to the value variable 'sound', second is the key 'key', and the third is again the value variable 'sound' to append the key.

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