Array reduce function in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP array_reduce example?

Consider the following PHP code using array_reduce. What will it output?

PHP

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

A 24

B 10

C 1234

D Error: Undefined variable

💡 Hint

Think about how array_reduce accumulates values starting from the initial value.

The function sums all numbers starting from 0, so 1+2+3+4 = 10.

❓ Predict Output

intermediate

What does this PHP code output when using array_reduce with strings?

Look at this PHP code that uses array_reduce to concatenate strings. What is the output?

PHP

<?php
$words = ['Hello', ' ', 'World', '!'];
$result = array_reduce($words, function($carry, $item) {
    return $carry . $item;
}, '');
echo $result;
?>

A Error: Cannot concatenate

B HelloWorld!

C Hello World!

D Array

💡 Hint

Remember that the dot operator . joins strings in PHP.

The function joins all array elements into one string, preserving spaces.

🔧 Debug

advanced

Why does this PHP array_reduce code cause an error?

Examine this PHP code snippet. Why does it cause an error?

PHP

<?php
$values = [2, 4, 6];
$result = array_reduce($values, function($carry, $item) {
    return $carry * $item;
});
echo $result;
?>

A Because the initial value for carry is missing, causing carry to be null on first call

B Because multiplication operator is invalid in PHP

C Because array_reduce requires a third argument to specify the array

D Because echo cannot output numbers

💡 Hint

Think about what happens if you don't provide the initial value for carry.

Without an initial value, carry is null on the first call, so multiplying null by a number causes unexpected behavior.

📝 Syntax

advanced

Which option shows the correct syntax for array_reduce with an initial value?

Choose the correct PHP syntax for using array_reduce with an initial value of 100.

A $result = array_reduce($arr, fn($carry, $item) => $carry + $item, 100);

B $result = array_reduce($arr, fn($carry, $item) => $carry + $item); 100;

C $result = array_reduce($arr, fn($carry, $item) => $carry + $item,);

D $result = array_reduce($arr, fn($carry, $item) => $carry + $item 100);

💡 Hint

Remember the third argument is the initial value and must be passed inside the parentheses.

Option A correctly passes the initial value as the third argument inside the function call.

🚀 Application

expert

How many items are in the resulting array after this array_reduce operation?

Given this PHP code, how many items does the resulting array contain?

PHP

<?php
$items = [1, 2, 3, 4];
$result = array_reduce($items, function($carry, $item) {
    if ($item % 2 === 0) {
        $carry[] = $item * 10;
    }
    return $carry;
}, []);
print_r($result);
?>

A 1

B 4

C 0

D 2

💡 Hint

Count how many even numbers are in the original array.

The code collects only even numbers multiplied by 10. The even numbers are 2 and 4, so the result has 2 items.