Question

So, while working my way through “Scala for the Impatient” I found myself wondering: Can you use a Scala for loop without a sequence?

For example, there is an exercise in the book that asks you to build a counter object that cannot be incremented past Integer.MAX_VALUE. In order to test my solution, I wrote the following code:

var c = new Counter
for( i <- 0 to Integer.MAX_VALUE ) c.increment()

This throws an error: sequences cannot contain more than Int.MaxValue elements.
It seems to me that means that Scala is first allocating and populating a sequence object, with the values 0 through Integer.MaxValue, and then doing a foreach loop on that sequence object.

I realize that I could do this instead:

var c = new Counter
while(c.value < Integer.MAX_VALUE ) c.increment()

But is there any way to do a traditional C-style for loop with the for statement?

Answer 1

In fact, 0 to N does not actually populate anything with integers from 0 to N. It instead creates an instance of scala.collection.immutable.Range, which applies its methods to all the integers generated on the fly.

The error you ran into is only because you have to be able to fit the number of elements (whether they actually exist or not) into the positive part of an Int in order to maintain the contract for the length method. 1 to Int.MaxValue works fine, as does 0 until Int.MaxValue. And the latter is what your while loop is doing anyway (to includes the right endpoint, until omits it).

Anyway, since the Scala for is a very different (much more generic) creature than the C for, the short answer is no, you can’t do exactly the same thing. But you can probably do what you want with for (though maybe not as fast as you want, since there is some performance penalty).