algorithm - Scala for comprehension performance

Question

Why is

for (
  a <- 1 to 1000;
  b <- 1 to 1000 - a;
  c <- 1 to 1000 - a - b;
  if (a * a + b * b == c * c && a + b + c == 1000)
) println((a, b, c, a * b * c))

266 ms

slower then:

for (a <- 1 to 1000)
  for (b <- 1 to 1000 - a)
    for (c <- 1 to 1000 - a - b)
      if (a * a + b * b == c * c)
        if (a + b + c == 1000)
          println((a, b, c, a * b * c))

62 ms

If I understand correct this should be the same?

---

Solution after processing answers:

for (
  a <- 1 to 1000;
  b <- 1 to (1000 - a)
) {
  val c = (1000 - a - b)
  if (a * a + b * b == c * c)
    println((a, b, c, a * b * c))
}

9 ms

Answer 1

Your understanding is wrong.

This is what happens when the condition is in the loop body:

// this
for(x <- coll) if(condition) doSomething
// will translate to
coll.foreach{ x => if(condition) doSomething }

As opposed to when the condition is in the generator itself:

// this
for(x <- coll if(condition)) dosomething
// will translate to
coll.withFilter(x => condition).foreach{ x => dosomething }

You can look into The Scala Language Speci?cation 6.16 for more details.