mapping elements of an array differently based on position

I want to map elements of an array such that all elements of the array are floats, except the first element wh开发者_如何学Pythonich is a string.

Anyone know how I can do this?

Tried this but doesn't work:

arr = arr.map { |e| e.to_i if e != arr.first }

Another solution is

[array.first] + array.drop(1).map &:to_f

This makes it clear that you want the first element separate from the rest, and you want the rest of the elements to be of type Float. Other options include

array.map { |element, index| index == 0 ? element : element.to_f }
array.map { |element| element == array.first ? element : element.to_f }

You can use a short ternary expression here:

a.map { |e| ( e == a.first ) ? e : e.to_f }

Another option (if you don't want to use ternary operators) is to do the following:

arr = arr.map { |e| (e == arr.first) && e || e.to_f}

This alternative is discussed here. A limitation with this method is that the first element in the array cannot be nil (or some other value that would evaluate false in a boolean evaluation), because if so, it will evaluate to the || expression and return e.to_f instead of just e.

Ruby 1.9 only?

arr = arr.map.with_index { |e, i| i.zero? ? e.to_s : e.to_f }

You can ask the objects themselves whether they're numbers.

"column heading".respond_to?(:to_int) # => false
3.1415926.respond_to?(:to_int) # => true

new_arr = arr.map do |string_or_float|
  if string_or_float.respond_to?(:to_int)
    string_or_float.to_int # Change from a float into an integer
  else
    string_or_float # Leave the string as-is
  end
end

respond_to?(:to_int) means "Can I call to_int on you?"

to_int is a method that only objects that are readily convertable to integers should have. Unlike to_i, which is "I'm not very much like an integer, but you can try to convert me into a integer", to_int means "I'm very much like an integer - convert me into an integer with full confidence!"