获取地图键的数组 [英] Get array of Map's keys
本文介绍了获取地图键的数组的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
我正在尝试以Python为基础学习Java,所以请多多包涵.
I am trying to learn Java with a Python basis, so please bear with me.
我正在实现Eratosthenes的Sieve方法(我在Python中有一个方法;试图将其转换为Java):
I am implementing a Sieve of Eratosthenes method (I have one in Python; trying to convert it to Java):
def prevPrimes(n):
"""Generates a list of primes up to 'n'"""
primes_dict = {i : True for i in range(3, n + 1, 2)}
for i in primes_dict:
if primes_dict[i]:
num = i
while (num * i <= n):
primes_dict[num*i] = False
num += 2
primes_dict[2] = True
return [num for num in primes_dict if primes_dict[num]]
这是我尝试将其转换为Java:
This is my attempt to convert it to Java:
import java.util.*;
public class Sieve {
public static void sieve(int n){
System.out.println(n);
Map primes = new HashMap();
for(int x = 0; x < n+1; x++){
primes.put(x, true);
}
Set primeKeys = primes.keySet();
int[] keys = toArray(primeKeys); // attempt to convert the set to an array
System.out.println(primesKeys); // the conversion does not work
for(int x: keys){
System.out.println(x);
}
// still have more to add
System.out.println(primes);
}
}
我得到的错误是它找不到方法toArray(java.util.Set)
.我该如何解决?
The error I get is that it cannot find the method toArray(java.util.Set)
. How can I fix this?
推荐答案
首先,使用泛型:
Map<Integer, Boolean> map = new HashMap<Integer, Boolean>();
Set<Integer> keys = map.keySet();
第二,要将集合转换为数组,可以使用toArray(T[] a)
:
Second, to convert the set to an array, you can use toArray(T[] a)
:
Integer[] array = keys.toArray(new Integer[keys.size()]);
,如果要使用int
而不是Integer
,请遍历每个元素:
and if you want int
instead of Integer
, then iterate over each element:
int[] array = new int[keys.size()];
int index = 0;
for(Integer element : keys) array[index++] = element.intValue();
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