Array reduce function in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to calculate the sum of all numbers in the array using array_reduce.

PHP

<?php
$numbers = [1, 2, 3, 4];
$sum = array_reduce($numbers, [1]);
echo $sum;
?>

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

Afunction($carry, $item) { return $carry + $item; }

Bfunction($carry, $item) { return $carry * $item; }

Cfunction($carry, $item) { return $item - $carry; }

Dfunction($carry, $item) { return $carry / $item; }

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication instead of addition in the callback.

Forgetting to return the sum inside the callback.

The array_reduce function takes an array and a callback function. The callback combines elements. To sum, the callback adds $carry and $item.

2fill in blank

medium

Complete the code to calculate the product of all numbers in the array using array_reduce.

PHP

<?php
$numbers = [1, 2, 3, 4];
$product = array_reduce($numbers, [1], 1);
echo $product;
?>

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

Afunction($carry, $item) { return $carry + $item; }

Bfunction($carry, $item) { return $item / $carry; }

Cfunction($carry, $item) { return $carry - $item; }

Dfunction($carry, $item) { return $carry * $item; }

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of multiplication.

Not setting the initial value to 1.

To get the product of all numbers, the callback multiplies $carry by $item. The initial value is 1 to avoid zeroing the product.

3fill in blank

hard

Fix the error in the callback function to correctly concatenate all strings in the array.

PHP

<?php
$words = ['Hello', ' ', 'World', '!'];
$result = array_reduce($words, [1], '');
echo $result;
?>

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

Afunction($carry, $item) { return $carry . $item; }

Bfunction($carry, $item) { return $carry - $item; }

Cfunction($carry, $item) { return $item . $carry; }

Dfunction($carry, $item) { return $carry * $item; }

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication operators instead of concatenation.

Concatenating in the wrong order.

To join strings, the callback should concatenate $carry and $item using the dot operator ..

4fill in blank

hard

Fill both blanks to create a dictionary with words as keys and their lengths as values using array_reduce.

PHP

<?php
$words = ['apple', 'banana', 'cherry'];
$lengths = array_reduce($words, function($carry, $word) {
    $carry[[1]] = [2];
    return $carry;
}, []);
print_r($lengths);
?>

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

A$word

Bstrlen($word)

C$carry

Dcount($word)

Attempts:

3 left

💡 Hint

Common Mistakes

Using $carry as key or value incorrectly.

Using count() instead of strlen().

The key should be the word itself ($word), and the value should be its length using strlen($word).

5fill in blank

hard

Fill all three blanks to filter and sum only numbers greater than 10 using array_reduce.

PHP

<?php
$numbers = [5, 12, 7, 20, 3];
$sum = array_reduce($numbers, function($carry, $num) {
    if ($num [1] 10) {
        $carry [2]= $num;
    }
    return $carry;
}, [3]);
echo $sum;
?>

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

A>

B+

C0

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using less than operator instead of greater than.

Not initializing carry to zero.

Using wrong operator for addition.

The condition checks if the number is greater than 10 (>). If true, add it to $carry using +=. The initial value is 0.

Practice

1. What does the PHP function array_reduce do?

easy

A. It sorts the array elements in ascending order.

B. It combines all elements of an array into a single value using a callback function.

C. It filters elements of an array based on a condition.

D. It splits an array into smaller arrays.

Solution

1. Step 1: Understand the purpose of array_reduce

   The function takes an array and a callback to combine elements into one value.

2. Step 2: Compare with other array functions

   Unlike sorting or filtering, array_reduce accumulates values into one result.

3. Final Answer:

   It combines all elements of an array into a single value using a callback function. -> Option B

4. Quick Check:

   array_reduce = combine elements [OK]

Hint: Remember: array_reduce reduces array to one value [OK]

Common Mistakes:

• Confusing array_reduce with sorting functions
• Thinking it filters elements instead of combining
• Assuming it splits arrays into parts

2. Which of the following is the correct syntax to use array_reduce in PHP?

easy

A. array_reduce(mixed $initial = null, array $array, callable $callback)

B. array_reduce(callable $callback, array $array, mixed $initial = null)

C. array_reduce(array $array, mixed $initial = null, callable $callback)

D. array_reduce(array $array, callable $callback, mixed $initial = null)

Solution

1. Step 1: Recall the official function signature

   The correct order is array, callback, then optional initial value.

2. Step 2: Check each option's parameter order

   Only array_reduce(array $array, callable $callback, mixed $initial = null) matches the correct order and types.

3. Final Answer:

   array_reduce(array $array, callable $callback, mixed $initial = null) -> Option D

4. Quick Check:

   array first, callback second, initial last [OK]

Hint: Remember: array first, callback second, initial last [OK]

Common Mistakes:

• Swapping callback and array parameters
• Placing initial value before callback
• Misordering parameters causing errors

3. What is the output of the following PHP code?

$numbers = [1, 2, 3, 4];
$result = array_reduce($numbers, function($carry, $item) {
    return $carry + $item;
}, 0);
echo $result;

medium

A. 10

B. 24

C. 1234

D. Error

Solution

1. Step 1: Understand the callback function

   The callback adds the current item to the carry (accumulated sum).

2. Step 2: Calculate the sum step-by-step

   Starting with 0, add 1, then 2, then 3, then 4: 0+1=1, 1+2=3, 3+3=6, 6+4=10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Sum of 1+2+3+4 = 10 [OK]

Hint: Sum array with array_reduce and initial 0 [OK]

Common Mistakes:

• Forgetting initial value causes null carry
• Concatenating instead of adding numbers
• Expecting product instead of sum

4. Identify the issue in this PHP code using array_reduce:

$values = [];
$result = array_reduce($values, function($carry, $item) {
    return $carry + $item;
});
echo $result;

medium

A. Callback function has wrong parameter order.

B. array_reduce cannot be used on empty arrays.

C. Missing initial value causes $carry to be null.

D. The echo statement is missing parentheses.

Solution

1. Step 1: Check initial value usage

   No initial value is given, so $carry starts as null.

2. Step 2: Understand effect on empty array

   With empty array and no initial, array_reduce returns null without calling the callback.

3. Final Answer:

   Missing initial value causes $carry to be null. -> Option C

4. Quick Check:

   Initial value needed for empty arrays [OK]

Hint: Always provide initial value to avoid null carry [OK]

Common Mistakes:

• Assuming callback parameters are reversed
• Thinking array_reduce fails on empty arrays without initial
• Ignoring PHP syntax for echo

5. Given an array of strings ['apple', 'banana', 'cherry'], how can you use array_reduce to create a single string with all fruits separated by commas (e.g., 'apple,banana,cherry')?

hard

A. Use array_reduce with callback concatenating carry and item with a comma, initial value as empty string.

B. Use array_reduce with callback adding carry and item as numbers, initial value 0.

C. Use array_reduce with callback returning only the last item, no initial value.

D. Use array_reduce with callback multiplying carry and item, initial value 1.

Solution

1. Step 1: Understand the goal

   We want to join strings with commas into one string.

2. Step 2: Design the callback

   Callback should add a comma between carry and item, starting with empty string.

3. Step 3: Avoid leading comma

   Use a condition to avoid comma before first item or trim later.

4. Final Answer:

   Use array_reduce with callback concatenating carry and item with a comma, initial value as empty string. -> Option A

5. Quick Check:

   Concatenate strings with commas using array_reduce [OK]

Hint: Concatenate strings with commas, start with empty string [OK]

Common Mistakes:

• Adding strings as numbers causing errors
• Not providing initial value causing null carry
• Using multiplication on strings