How to Reduce an Array in Ruby: Syntax and Examples

In Ruby, you can use the reduce method to combine all elements of an array into a single value by applying a block or symbol operation. It takes an optional initial value and a block that specifies how to combine elements step-by-step.

Syntax

The reduce method is called on an array and can take an optional initial value and a block or a symbol representing the operation.

• array.reduce(initial) { |accumulator, element| ... }: Combines elements starting with initial.
• array.reduce { |accumulator, element| ... }: Combines elements starting with the first element as the initial accumulator.
• array.reduce(:operation): Uses a symbol like :+ to specify the operation.

ruby

array.reduce(initial) { |accumulator, element| block }
array.reduce { |accumulator, element| block }
array.reduce(:operation)

Example

This example shows how to sum all numbers in an array using reduce with and without an initial value, and how to multiply all elements.

ruby

numbers = [2, 4, 6]

# Sum without initial value
sum1 = numbers.reduce { |acc, n| acc + n }

# Sum with initial value 10
sum2 = numbers.reduce(10) { |acc, n| acc + n }

# Multiply all elements
product = numbers.reduce(1, :*)

puts "Sum without initial: #{sum1}"
puts "Sum with initial 10: #{sum2}"
puts "Product of elements: #{product}"

Output

Sum without initial: 12
Sum with initial 10: 22
Product of elements: 48

Common Pitfalls

One common mistake is forgetting the initial value when the array might be empty, which can cause errors or unexpected results. Another is mixing up the order of parameters in the block, which changes how elements combine.

Also, using reduce without a block or symbol will raise an error.

ruby

numbers = []

# This will raise an error because no initial value is given
# sum = numbers.reduce { |acc, n| acc + n } # Error

# Correct way: provide an initial value
sum = numbers.reduce(0) { |acc, n| acc + n }

puts "Sum with initial on empty array: #{sum}"

Output

Sum with initial on empty array: 0

Quick Reference

Usage	Description	Example
array.reduce { |acc, el| ... }	Combine elements using block, first element as initial	[1,2,3].reduce { |acc, n| acc + n } #=> 6
array.reduce(initial) { |acc, el| ... }	Combine elements starting with initial value	[1,2,3].reduce(10) { |acc, n| acc + n } #=> 16
array.reduce(:symbol)	Combine elements using symbol operation	[1,2,3,4].reduce(:*) #=> 24
array.reduce(initial, :symbol)	Combine elements starting with initial using symbol	[1,2,3].reduce(2, :*) #=> 12

Key Takeaways

Use reduce to combine array elements into one value with a block or symbol.

Always provide an initial value if the array might be empty to avoid errors.

The block takes two parameters: accumulator and current element, in that order.

You can use symbols like :+ or :* for common operations.

Without a block or symbol, reduce will raise an error.