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.

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