what does (&:even?) replace? can i use (&:) everywhere as iteraror? in ruby

The (&:even?) syntax in Ruby is a shorthand for calling the even? method on each element of an array or collection.

For example, suppose we have an array of numbers [1, 2, 3, 4, 5] and we want to select only the even numbers. We can achieve this using the select method and the (&:even?) syntax:

main.rb
numbers = [1, 2, 3, 4, 5]
even_numbers = numbers.select(&:even?)
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This is equivalent to:

main.rb
numbers = [1, 2, 3, 4, 5]
even_numbers = numbers.select { |number| number.even? }
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So, (&:even?) replaces the block that would be passed to the select method and calls the even? method on each element.

This shorthand syntax can only be used with methods that take a single argument and don't have any side-effects. So, you cannot use (&:) everywhere as an iterator. It is specific to certain methods that expect a block.

However, you can use it with other iterator methods like map, reduce, and each, as long as the method you are calling on each element satisfies the criteria mentioned above.

main.rb
numbers = [1, 2, 3, 4, 5]
squared_numbers = numbers.map(&:**) # equivalent to numbers.map { |number| number ** 2 }

sum = numbers.reduce(&:+) # equivalent to numbers.reduce { |sum, number| sum + number }

numbers.each(&:print) # equivalent to numbers.each { |number| number.print }
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In summary, (&:even?) is a shorthand syntax to call a method on each element of an array or collection, and it can be used with certain iterator methods that expect a block.