Lets imagine we’ve got an array of strings, and we want to print it out as a list of strings using printf. (If you’re complaining about my logic here, hold fire for just a second good sir/madam.)

So we start off with the array of strings, and then pass it to printf with the right amount of %s’s in the format string:

arr = ["one", "two", "three"]

printf "%s, %s, %s", arr
# ~> -:3:in `printf': too few arguments (ArgumentError)
# ~>     from -:3

Oh whoops, we’ve actually only passed "%s, %s, %s", ["one", "two", "three"] to printf. So of course it whinges about not getting enough arguments. Now how do we fix this, how do we pass an array with each element a seperate argument to a method?

We use the * of course! Just prefix the variable name with * and the method is passed each element as separate arguments, rather than the whole array as one arguement.

Going back to our printf example above, we simply insert one character (the lowly *) and end up with a string being outputted.

printf "%s, %s, %s", *arr
# >> one, two, three

Now I realise this is a partially stupid example, but it serves to explain the point I wanted to make. If you were complaining about my choice of printf earlier, here is the way I think most rubyists would solve this problem instead.

arr = ["one", "two", "three"]

print arr.join(" ")
# >> one two three

And if I wanted to be slightly cleverer with the printf version, and print out an array containing an unknown number of strings, but of a set width, then I could do the following. (NB: This is actually how I ran into this problem.)

arr = ["one", "two", "three"]

printf arr.map { "%6s" }.join, *arr
# >>    one   two three

And that is where the lowly * comes in.