Question

I don’t know how and why this piece of code works:

// postorder dfs
Iterator< Index<String<char> >, TopDown<ParentLink<Postorder> > >::Type
     myIterator(myIndex);
while (goDown(myIterator));
for (; !atEnd(myIterator); goNext(myIterator)) 
// do something with myIterator ( Traverse Through (Suffix)-tree )

It’s an example from seqan and the interface is described here: API

1. How can the while affect the for-loop?
2. Why isn’t the for loop initialized?

Answer 1

You’ve run into the fun parts of C++ – using language constructs in syntactically valid but difficult-for-human-parsing techniques.

while (goDown(myIterator));

This will “goDown(myIterator)” until it returns false. Then it will continue onto the for loop. It’s looping over nothing – but that’s okay, because the function goDown is doing work.

for (; !atEnd(myIterator); goNext(myIterator)) 

This doesn’t initialize anything, but tests that it’s not atEnd(myIterator) – while it’s not, it will goNext(myIterator). It could also be written as a while loop to make it slghtly easier to understand:

while(!atEnd(myIterator)) 
{
    goNext(myIterator));
}

So the code will:

1. Go down until goDown() returns false – this means goDown is modifying myIterator each time
2. goNext() until it is atEnd()