How would I get the minVal without a loop and using only foldLeft?

问题

So, I have gotten a bit of an understanding of functional programming but for this I cannot use a loop and instead have to use the foldLeft or foldRight function that I wrote, on order to get the minVal.

static <U,V> V foldLeft(V e, Iterable<U>l, BiFunction<V,U,V> f){
for(U u:l) {
    e = f.apply(e, u);
}
return e;
}

static <U,V> V foldRight(V e, Iterable<U>l, BiFunction<U,V,V> f){
   for(U u:l) {
      e = f.apply(u, e);
   }
return e;

I now have to write a minVal:

//(5) Use minVal to calculate the minimum of a List of 
//       Integers
static <U> U minVal(Iterable<U> l, Comparator<U> c){
// write using fold.  No other loops permitted. 
List<U> temp = new ArrayList<U>();
l.forEach(temp::add);
return temp.stream().min(c).get(); //Not sure if this actually works yet
}

I have attempted to write this and test it, but I am now stuck also on ow I would test the minVal:

List<Integer> numList = new ArrayList<>();

    numList.add(5);
    numList.add(10);
    numList.add(15);
    numList.add(20);
    numList.add(22);
    numList.add(1);


    System.out.println(minVal(numList, 0)); //What would I place as the 
                                            //comparable argument

The above, of course, is giving me an error. I have read up on Comparators in Lambda but do not understand how to implement this in the test(or the print statement).

Any help/explanation is appreciated! P.S. Please let me know if I am missing any information, I tried to be as thorough as possible.

回答1:

You can define your minVal using foldLeft as :

static <U> U minVal(Iterable<U> l, Comparator<U> c) {
    // write using fold.  No other loops permitted.
    return foldLeft(l.iterator().next(), l, (u, u2) -> c.compare(u, u2) < 0 ? u : u2);
    // check for l.iterator().hasNext() or else define the behaviour
}

and then invoke it using the Comparator<Integer defined as:

List<Integer> numList = List.of(5, 10, 15, 20, 22, 1);
System.out.println(minVal(numList, Integer::compare));

来源：https://stackoverflow.com/questions/59924750/how-would-i-get-the-minval-without-a-loop-and-using-only-foldleft

标签

java

oop

lambda

functional-programming