Infinite yields from an iterator.

Question

I'm trying to learn some ruby. Imagine I'm looping and doing a long running process, and in this process I want to get a spinner for as long as necessary.

So I could do:

a=['|','/','-','\\']
aNow=0
# ... skip setup a big loop
print a[aNow]
aNow += 1
aNow = 0 if aNow == a.length
# ... do next step of process
print "\b"

But I thought it'd be cleaner to do:

def spinChar
  a=['|','/','-','\\']
  a.cycle{|x| yield x}
end
# ... skip setup a big loop
print spinChar
# ... do next step of process
print "\b"

Of course the spinChar call wants a block. If I give it a block it'll hang indefinitely.

How can I get just the next yeild of this block?

Answer 1

Your code is a bit inside-out, if you'll pardon me saying so. :)

Why not:

class Spinner
  GLYPHS=['|','/','-','\\']
  def budge
    print "#{GLYPHS[@idx = ((@idx || 0) + 1) % GLYPHS.length]}\b"
  end
end

spinner = Spinner.new
spinner.budge
# do something
spinner.budge
spinner.budge
# do something else
spinner.budge

Now, if you want something like:

with_spinner do
  # do my big task here
end

...then you'd have to use multi-threading:

def with_spinner
  t = Thread.new do
    ['|','/','-','\\'].cycle { |c| print "#{c}\b" ; sleep(1) }
  end
  yield
  Thread.kill(t) # nasty but effective
end

Cheers, V.

Answer 2

I don't think you quite understand what yield does in Ruby. It doesn't return a value from a block — it passes a value to the block you've passed to the enclosing method.

I think you want something more like this:

def with_spinner
  a=['|','/','-','\\']
  a.cycle do |x|
    print x
    $stdout.flush # needed because standard out is usually buffered
    yield # this will call the do-block you pass to with_spinner
  end
end

with_spinner do
  #process here
  #break when done
end

Answer 3

Ruby's yield does not work in the way your example would like. But this might be a good place for a closure:

def spinner()
  state = ['|','/','-','\\']
  return proc { state.push(state.shift)[0] }
end

spin = spinner

# start working
print spin.call
# more work
print spin.call
# etc...

In practice I think this solution might be too "clever" for its own good, but understanding the idea of Procs could be useful anyhow.

Answer 4

I like all these suggestions, but I found the Generator in the standard library, and I think it's more along the lines of what I wanted to do:

spinChar=Generator.new{ |g|
  ['|','/','-','\\'].cycle{ |x|
    g.yield x
  }
}
#then
spinChar.next
#will indefinitly output the next character.

Plain array index increments with modulus on a frozen array seems to be fastest.

Vlad's thread is nifty but not exactly what I wanted. And in spinner class the one-line increment would be nicer if Ruby supported i++ like GLYPHS[@i++%GLYPHS.length]

Max's spinner closure with push shift seems a little intensive to me, but the resulting syntax is almost exactly like this Generator. At least I think that's a closure with proc in there.

Chuck's with_spinner is actually pretty close to what I wanted, but why break if you don't have to with a Generator as above.

Vadim, thanks for pointing out the generator would be slow.

"Here's a test of 50,000 spins:"
                   user       system     total       real
"array index"      0.050000   0.000000   0.050000 (  0.055520)
"spinner class"    0.100000   0.010000   0.110000 (  0.105594)
"spinner closure"  0.080000   0.030000   0.110000 (  0.116068)
"with_spinner"     0.070000   0.030000   0.100000 (  0.095915)
"generator"        6.710000   0.320000   7.030000 (  7.304569)

Answer 5

Once upon a time, I wrote an array. But it's not just an array, it's an array that has a pointer, so you can call next foreverrr! http://gist.github.com/55955

Pair this class with a simple iterator or loop and you are golden.

 a = Collection.new(:a, :b, :c)
 1000.times do |i|
   puts a.current
   a.next
 end