Is there a way to do this without eval?

I don't see eval used by others much in ruby, so I'm assuming this can be done without it. But I don't see how.

(1..9).each { |n|
    eval "user_#{n} = prefix << '_' &开发者_开发技巧lt;< user_#{n} if user_#{n}"
}

Rewrite your code using a hash instead of local variables and you can ditch eval:

(1..9).each { |n|
  user[n] = prefix << '_' << user[n] if user[n]
}

BTW, I think you didn't want to use << above as it will modify your prefix as well.

If you want to implement this as an array, you can try

users.map! do |user|
  user ? "#{prefix}_#{user}" : nil
end

assuming that users is already defined.

You can define a method dynamically using ruby's define_method.

You can replace eval-based statements with a define_method enclosed in a class_eval.

But, it depends on the situation. It might be overkill to replace eval with a dynamic method.

You need to provide more context for your question to get a better answer.