我无法用 Stream
替换包含递归调用的 for 循环。该代码是关于在已知该数字的质因数时生成给定数字的适当除数。算法取自here , 它在图像中被描述。这是我用于演示目的的代码的一部分,它是可运行的:
public class Demo {
private static void generateDivisorsTraditional(int start, long lastFactor, Multiset<Long> primeFactors, Set<Long> divisors) {
for (int i = start; i < primeFactors.elementSet().size(); i++) {
long prime = Iterables.get(primeFactors.elementSet(), i);
int count = primeFactors.count(prime);
++start;
for (int c = 0; c <= count; c++) {
long factor = ArithmeticUtils.pow(prime, c);
divisors.add(lastFactor * factor);
generateDivisorsTraditional(start, lastFactor * factor, primeFactors, divisors);
}
}
}
private static void generateDivisorsStream(int start, long lastFactor, Multiset<Long> primeFactors, Set<Long> divisors) {
IntStream.range(start, primeFactors.elementSet().size())
.forEach((int i) -> {
long prime = Iterables.get(primeFactors.elementSet(), i);
int count = primeFactors.count(prime);
final int begin = start + 1;
IntStream.range(0, count + 1)
.forEach((int c) -> {
long factor = ArithmeticUtils.pow(prime, c);
divisors.add(lastFactor * factor);
generateDivisorsStream(begin, lastFactor * factor, primeFactors, divisors);
});
});
}
private static void testTraditional(Multiset<Long> primeFactors) {
Set<Long> divisors = new TreeSet<>();
generateDivisorsTraditional(0, 1, primeFactors, divisors);
System.out.println("Traditional=> " + divisors);
}
private static void testStream(Multiset<Long> primeFactors) {
Set<Long> divisors = new TreeSet<>();
generateDivisorsStream(0, 1, primeFactors, divisors);
System.out.println("Stream=> " + divisors);
}
private static void testStream1(Multiset<Long> primeFactors) {
Set<Long> divisors = new TreeSet<>();
new Inner().generateDivisorsStream(1, primeFactors, divisors);
System.out.println("Stream1=> " + divisors);
}
public static void main(String[] args) {
System.out.println("Test number: 10");
Multiset<Long> primeFactors = TreeMultiset.create();
primeFactors.add(2L);
primeFactors.add(5L);
testTraditional(primeFactors);
testStream(primeFactors);
testStream1(primeFactors);
System.out.println();
System.out.println("Test number: 90");
primeFactors = TreeMultiset.create();
primeFactors.add(2L);
primeFactors.add(3L);
primeFactors.add(3L);
primeFactors.add(5L);
testTraditional(primeFactors);
testStream(primeFactors);
testStream1(primeFactors);
}
private static class Inner {
private int start = 0;
private void generateDivisorsStream(long lastFactor, Multiset<Long> primeFactors, Set<Long> divisors) {
IntStream.range(start, primeFactors.elementSet().size())
.forEach((int i) -> {
long prime = Iterables.get(primeFactors.elementSet(), i);
int count = primeFactors.count(prime);
++start;
IntStream.range(0, count + 1)
.forEach((int c) -> {
long factor = ArithmeticUtils.pow(prime, c);
divisors.add(lastFactor * factor);
generateDivisorsStream(lastFactor * factor, primeFactors, divisors);
});
});
}
}
}
它生成的输出是:
Test number: 10
Traditional=> [1, 2, 5, 10]
Stream=> [1, 2, 5, 10, 25]
Stream1=> [1, 2, 5]
Test number: 90
Traditional=> [1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90]
Stream=> [1, 2, 3, 5, 6, 9, 10, 15, 18, 25, 27, 30, 45, 50, 75, 81, 90, 125, 135, 225, 405]
Stream1=> [1, 2, 3, 5, 9]
顾名思义,generateDivisorsTraditional
方法使用传统的 for 循环,在其中我递归调用了相同的方法。 generateDivisorsStream
方法使用 IntStream.range()
来模拟 for 循环。
我怀疑 ++start;
和 final int begin = start + 1;
的 generateDivisorsTraditional
和 generateDivisorsStream
分别做了一些不同。我也尝试在 generateDivisorsTraditional
中使用 final int begin = start + 1;
而不是 ++start;
并发现它已经开始产生错误的结果。我在内部类 Inner
中还有另一个变体,它也会产生错误的输出。
我想知道为什么这不是它应该的行为方式?我犯了什么错误?
最佳答案
我认为有一些错误:
private static void generateDivisorsTraditional(int start, long lastFactor, Multiset<Long> primeFactors, Set<Long> divisors) {
for (int i = start; i < primeFactors.elementSet().size(); i++) {
long prime = Iterables.get(primeFactors.elementSet(), i);
int count = primeFactors.count(prime);
// ++start; remove it
for (int c = 0; c <= count; c++) {
long factor = ArithmeticUtils.pow(prime, c);
divisors.add(lastFactor * factor);
generateDivisorsTraditional(i+1, lastFactor * factor, primeFactors, divisors); // replaced start -> i+1
}
}
}
private static void generateDivisorsStream(int start, long lastFactor, Multiset<Long> primeFactors, Set<Long> divisors) {
IntStream.range(start, primeFactors.elementSet().size())
.forEach((int i) -> {
long prime = Iterables.get(primeFactors.elementSet(), i);
int count = primeFactors.count(prime);
IntStream.range(0, count + 1)
.forEach((int c) -> {
long factor = ArithmeticUtils.pow(prime, c);
divisors.add(lastFactor * factor);
generateDivisorsStream(i+1, lastFactor * factor, primeFactors, divisors);
});
});
}
public static void main(String[] args) {
Multiset<Long> m = HashMultiset.create();
m.add(1L, 1);
m.add(2L, 1);
m.add(5L, 1);
Set<Long> divisors = new HashSet<>();
generateDivisorsTraditional(1, 1, m, divisors);
System.out.println("Traditional=> "+divisors);
divisors = new HashSet<>();
generateDivisorsStream(1, 1, m, divisors);
System.out.println("Stream=> "+divisors);
}
它打印:
Traditional=> [1, 2, 5, 10]
Stream=> [1, 2, 5, 10]
关于java - 用 Stream 替换 for 循环中的递归,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/39085147/