How to Use Reduce in Ruby: Simple Guide with Examples

In Ruby, reduce is a method that combines all elements of an array into one value by applying a block or symbol operation. You start with an initial value (optional) and then apply the block to accumulate the result step-by-step.

Syntax

The reduce method can be used with or without an initial value. It takes a block where you define how to combine elements.

• Without initial value: The first element is used as the initial accumulator.
• With initial value: You provide a starting value for the accumulator.

ruby

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

Example

This example shows how to sum all numbers in an array using reduce. It starts with 0 and adds each element to the accumulator.

ruby

numbers = [1, 2, 3, 4, 5]
sum = numbers.reduce(0) { |total, num| total + num }
puts sum

Output

15

Common Pitfalls

One common mistake is forgetting the initial value, which can cause unexpected results if the array is empty. Another is mixing up the order of accumulator and element in the block.

Also, using reduce without a block but with a symbol requires careful syntax.

ruby

wrong = [1, 2, 3].reduce { |total, num| total + num } # wrong order
correct = [1, 2, 3].reduce { |total, num| total + num } # correct order

# Using symbol instead of block
sum = [1, 2, 3].reduce(:+)
puts sum

Output

6

Quick Reference

Usage	Description
array.reduce { |acc, elem| acc + elem }	Combine elements using block, first element as initial
array.reduce(initial) { |acc, elem| acc + elem }	Combine elements starting with initial value
array.reduce(:+)	Combine elements using symbol for addition
array.reduce(initial, :*)	Combine elements starting with initial using multiplication symbol

Key Takeaways

Use reduce to combine array elements into a single value by applying a block or symbol.

Always check if you need an initial value to avoid errors with empty arrays.

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

You can use symbols like :+ or :* with reduce for simple operations.

Remember the difference between reduce and inject—they are aliases in Ruby.