Java 数组中的素数
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Prime numbers in array
提问by user3214606
I need to write a function that recieve from the user a number(n), and the function return an array with all the prime numbers until the user number(n). I know how to write the function that check if number is prime, but i dont know how to enter the numbers to an array.. for example if the user entered 53, it will return [2 ,3 ,5 ,7 ,11 ,13 ,17 ,19 ,23 ,29 ,31 ,37 ,41 ,43 ,47 ,53]. I forgot to tell that the language is java.. my bad!
我需要编写一个从用户那里接收一个数字（n）的函数，该函数返回一个包含所有素数的数组，直到用户编号（n）。我知道如何编写检查数字是否为素数的函数，但我不知道如何将数字输入到数组中。例如，如果用户输入 53，它将返回 [2 ,3 ,5 ,7 ,11 , 13,17,19,23,29,31,37,41,43,47,53]。我忘了说语言是 java .. 我的错！
回答by barak manos
Here is one way to do it (you did not specify language, so I'm using Python):
这是一种方法（您没有指定语言，所以我使用的是 Python）：
def GetPrimes(n):
array = []
for i in range(2,n+1):
if IsPrime(i):
array.append(i)
return array
def IsPrime(n):
for i in range(2,n/2+1):
if n % i == 0:
return False
return True
print GetPrimes(53)
回答by Dmitry Bychenko
Usually, you can'tchange array length, so you should use the appropriate
collection where you can add items; that is List<>in C#, ArrayList<>in Java etc.
通常，您无法更改数组长度，因此您应该使用适当的集合来添加项目；这是List<>在C＃，ArrayList<>Java中等等。
Technically, you can implement separate IsPrimemethod and then check the integers in 2..maxrange, but here it can just be inlined.
从技术上讲，您可以实现单独的IsPrime方法，然后检查2..max范围内的整数，但在这里它可以内联。
Possible code in C#
C#中可能的代码
public static int[] GetPrimes(int max) {
if (max <= 1)
return new int[0];
else if (max == 2) // <- let's process the only even prime (2) separately
return new int[] { 2 };
List<int> primes = new List<int>() { 2, 3 };
// Sieve through odd numbers:
for (int i = 5; i <= max; i += 2) {
int sqrt = (int) Math.Sqrt(i + 1);
Boolean isPrime = true;
// There's no need to check if (i % primes[0] == 0):
// primes[0] == 2, and loop is for odd numbers - 5, 7, 9,...
for (int j = 1; primes[j] <= sqrt; ++j)
if ((i % primes[j]) == 0) {
isPrime = false;
break;
}
if (isPrime)
primes.Add(i);
}
return primes.ToArray();
}
// ....
// Test will return the string
// "2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53"
String result = String.Join(", ", GetPrimes(53).Select(x => x.ToString()));
回答by Tushar Thakur
Here is the code
这是代码
private ArrayList getPrimeNumbers(int number)
{
ArrayList primeNumbers = new ArrayList();
for (int i = 0; i <= number; i++)
{
if(isPrime(i))
{
primeNumbers.add(i);
}
}
return primeNumbers;
}
private boolean isPrime(int n)
{
for(int i=2;i<n;i++)
{
if(n%i==0)
return false;
}
return true;
}
