[英]Sieve of eratosthenes not working C++
So I am trying to make a C++ program to generate all prime numbers till a certain point but for some reason, it prints all numbers as non-primes after 2.所以我试图制作一个 C++ 程序来生成所有素数,直到某个点,但由于某种原因,它在 2 之后将所有数字打印为非素数。
int A[1000000];
void sieve(int till)
{
for(int i = 2; i < till; i++)
{
if(A[i] == 0)
{
for(int j = i*i; j < till; j+=i)
{
A[j - 1] = 1;
//printf("%i is NOT prime\n", ij);
}
}
}
}
and in the main i do:主要是我做的:
int N;
scanf("%i", &N);
sieve(N);
but when I try to debug it says:但是当我尝试调试时,它说:
NOT PRIME 0
NOT PRIME 1
NOT PRIME 2
PRIME 3
NOT PRIME 4
PRIME 5
NOT PRIME 6
PRIME 7
NOT PRIME 8
PRIME 9
NOT PRIME 10
PRIME 11
NOT PRIME 12
NOT PRIME 13
Can anybody figure what am I doing wrong?有人能弄清楚我做错了什么吗?
EDIT: The A int table is automatically intialized to 0s as it is outside the main class.编辑:A int 表自动初始化为 0,因为它在主类之外。
Very easy.好简单。 We have a few trivial case:我们有几个小案例:
So, basically we should not calculate the even numbers.所以,基本上我们不应该计算偶数。 We are interested in only odd numbers.我们只对奇数感兴趣。
We will mark all non prime numbers as 1.我们将所有非质数标记为 1。
int A[1000000];
void sieve(int till)
{
// mark all even numbers (except 2) as not prime
for(int i = 4; i < till; i += 2)
{
A[i] = 1;
}
// now calculate not primes for only odd numbers
for(int i = 3; i < till; i += 2 )
{
if(A[i] == 0)
{
for(int j = i*i; j < till; j+=(i+i))
{
A[j] = 1;
}
}
}
}
int main()
{
int N;
scanf("%i", &N);
sieve(N);
for(int i = 2; i < N; i++)
{
if(A[i] == 1) printf("%d is not prime\n", i);
else printf("%d is prime\n", i);
}
return 0;
}
There are many issues with your code.你的代码有很多问题。
int A[1000000];
You haven't initialized the array.你还没有初始化数组。 you should initialize A[0]=1
, A[1]=1
and all other indexes to 0
.您应该将A[0]=1
、 A[1]=1
和所有其他索引初始化为0
。
for(int j = i*i; j < till; j+=i)
i*i
may lead to integer overflow. i*i
可能会导致整数溢出。 you should probably use j=i+i
to decrease your chances of overflow.您可能应该使用j=i+i
来减少溢出的机会。
A[j - 1] = 1;
Not sure you are using proper array indexes.不确定您使用的是正确的数组索引。
This line:这一行:
A[j - 1] = 1;
Should be:应该:
A[j] = 1;
So, now should works well:所以,现在应该运行良好:
#include<stdio.h>
int A[1000000];
void sieve(int till)
{
for(int i = 2; i < till; i++)
{
if(A[i] == 0)
{
for(int j = i*i; j < till; j+=i)
{
A[j] = 1;
}
}
}
}
int main()
{
int N,i;
scanf("%i", &N);
for (i = 0;i < N;i++)
A[i] = 0;
sieve(N);
for (i = 2;i < N;i++)
if (A[i])
printf("%i is not prime\n",i);
else
printf("%i is prime\n",i);
}
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