What does lockstep mean in programming?

Question

I have been reading Effective Java on

Item 46: Prefer for-each loops to traditional for loops

In the part where are mentioned the cases when is iterator/for loop needed isntead of for-each loop, there is this point:

Parallel iteration—If you need to traverse multiple collections in parallel, then you need explicit control over the iterator or index variable, so that all iterators or index variables can be advanced in lockstep.

Now, I understand what explicit control over iterator/index variable mean (not controller by for each loop). But I could not understand the meaning of lockstep in this sense. I tried to google it and found an article on Wikipedia which states:

Lockstep systems are fault-tolerant computer systems that run the same set of operations at the same time in parallel.

This I understand as having aditional instance of for example server for fail-over That's ok. But I fail to fully understand what could be the exact meaning in the context of iterating over collection in programming.

Answer 1

In this context, the meaning is more like the military marching.

Or, when one operation advances, other operations advances/follows with it.

Or more specifically, if you want to iterate over two collections, you cannot easily the foreach construct:

for (Item i : list1) { //only allows you to iterate over 1 list.

}

Iterate over 2 collections )

Iterator iter1 = list1.iterator();
Iterator iter2 = list2.iterator();
while (iter1.hasNext() && iter2.hasNext()){
    Item a = iter1.next();
    Item b = iter2.next();
    doSomething(a, b);
}

i.e. while iterating list1, iterating list2 follows with it - "in lockstep"