Magic First and Last Indicator in a Loop in Ruby/Rails?

Question

Ruby/Rails does lots of cool stuff when it comes to sugar for basic things, and I think there\'s a very common scenario that I was wondering if anyone has done a helper or s

Answer 1

There's no "do this the (first|last) time" syntax in Ruby. But if you're looking for succinctness, you could do this:

a.each_with_index do |x, i|
  print (i > 0 ? (i == a.length - 1 ? x*10 : x) : x+1)
end

The result is what you'd expect:

irb(main):001:0> a = Array.new(5,1)
=> [1, 1, 1, 1, 1]
irb(main):002:0> a.each_with_index do |x,i|
irb(main):003:1*   puts (i > 0 ? (i == a.length - 1 ? x*10 : x) : x+1)
irb(main):004:1> end
2
1
1
1
10

Answer 2

If the code for the first and last iteration has nothing in common with the code for the other iterations, you could also do:

do_something( a.first )
a[1..-2].each do |x|
  do_something_else( x )
end
do_something_else_else( a.last )

If the different cases have some code in common, your way is fine.

Answer 3

If you know the items in the array are unique (unlike this case), you can do this:

a = [1,2,3,4,5]

a.each_with_index do |x, i|
  if x == a.first
    print x+1
  elsif x == a.last
    print x*10
  else
    print x
  end
end

Answer 4

You could grab the first and last elements and process them differently, if you like.

first = array.shift
last = array.pop
process_first_one
array.each { |x| process_middle_bits }
process_last_one