How can I auto-increment a variable so each time it is used, it is incremented by one, starting with 0?
For example:
i = i+1 || 0
arr[i] = "foo"
arr[i] = "bar"
arr[i] = "foobar"
arr #=> ["foo","bar","foobar"]
I'm getting a NoMethodError undefined method '+' for nil:NilClass
A variable is just a name. It doesn't have behavior. If you want behavior, use a method:
def i
@i ||= -1
@i += 1
end
arr = []
arr[i] = 'foo'
arr[i] = 'bar'
arr[i] = 'foobar'
arr #=> ['foo', 'bar', 'foobar']
Alternatively:
_i = -1
define_method(:i) do
_i += 1
end
arr = []
arr[i] = 'foo'
arr[i] = 'bar'
arr[i] = 'foobar'
arr #=> ['foo', 'bar', 'foobar']
But really, what you have is just a very convoluted way of saying
arr = %w[foo bar foobar]
which is much clearer.
define_method is available unless you are calling it on a class or module. I just tried it out here in IRB. Also, if it does work it would affect the object you called it on.You can't. There is no way to associate variables with behaviors — in fact, doing anything with local variables besides just reading and setting them in the obvious way is nigh impossible in standard Ruby — and integers cannot change value.
However, if you are really looking to do something like this with an arrays, you can just use the << operator to push to the end of the array:
arr = []
arr << "foo"
arr << "bar"
arr << "foobar"
arr #=> ["foo","bar","foobar"]
The other answers are pretty good. I would just like to add a few ways that I would implement it.
You could use a Proc:
i = -1
next_i = Proc.new { i += 1 }
next_i.call # => 0
next_i.call # => 1
You could use an Enumerator:
ids = Enumerator.new { |y| i = -1; loop { y << (i+=1) } }
ids.next # => 0
ids.next # => 1
If it makes sense in your application for the ids to come from some larger object, you could define a method in that object like this:
def next_id
@i ||= -1
@i += 1
end
This seems like what you're trying to do, but I wouldn't recommend it:
ary = []
i = -1
ary[i += 1] = 'foo'
ary[i += 1] = 'bar'
ary # => ["foo", "bar"]
Instead, do the idiomatic and expected thing and assign to an array one of these ways:
ary = ['foo', 'bar']
ary = %w[foo bar]
ary # => ["foo", "bar"]
Or, use the << operator to append on the fly, as the other answers recommend.
ias an array index for assignment as in your example? If it is, why not just use the<<array append operator to fill the array?i = i+1 || 0. It's not idiomatic, nor is it particularly readable or maintainable. Instead do the normal thing and assigni = 0, then usei += 1wherever you need it to increment.