SPOJ PRIME1: RTE; Is there a method to avoid dynamic array?

Question

I read about Sieve of Eratosthenes algorithm and tried to implement it, the code is getting compiled without any errors but I am getting blank output. Here is the code:-

#include <stdio.h>
#include <stdlib.h>
#define limit 100000
int main()
{
int prime[limit];
int i,j,t;
int n,m;
scanf("%d",&t);
while(t--)
{
scanf("%d%d",&m,&n);
for(i=0;i<=n;i++)
{
    prime[i]=1;
}
prime[0]=0;
prime[1]=0;
for(i=2;i<=n;i++)
{
    if(prime[i]==1)
    {    for(j=2;i*j<=n;j++)
              prime[i*j]=0;
    }
}
for(i=m;i<=n;i++)
{
    if(prime[i]!=0)
        printf("%d\n",i);
}
}


return 0;
}

score 1 · Accepted Answer · answered Dec 12 '15 at 14:47

1

The problem is the size of your sieve array: it has to be large enough to stretch to the highest number that you wish to process, not for the number of items that you wish to find.

Your implementation is suitable for finding prime numbers up to 100, not for the first 100 primes. Since prime number 100 is 541, you need to change the size to 542:

int prime[542];

You also need to make the distinction between n-the-number-of-primes and n-the-highest-prime in your code. I would recommend keeping n for the count of primes that you wish to find, and use #define SIZE 542 for the size of the array. Make sure to use < SIZE instead of <= SIZE when accessing array elements.

answered Dec 12 '15 at 14:47

Sergey Kalinichenko

714,442
84
1,110
1,523

Yes, but I am just testing my implementation with a default no. say 100, later I will increase the size of array, but even for 100 there is a blank output – Aniket Sanadhya Dec 12 '15 at 14:59
@AniketSanadhya 100 produces undefined behavior. please try the fix. – Sergey Kalinichenko Dec 12 '15 at 15:06
It should not give blank output. Should give a list of '1'. Unless the prime[i*j] = 0 for i*j==100 overwrites n. – marom Dec 12 '15 at 15:06
@dasblinkenlight I am stuck again :( – Aniket Sanadhya Dec 13 '15 at 12:43
@AniketSanadhya Move `int prime[limit];` outside `main`. – Sergey Kalinichenko Dec 13 '15 at 12:53
@dasblinkenlight placed it before main, still getting RTE :( – Aniket Sanadhya Dec 13 '15 at 12:57
@AniketSanadhya The limit is probably too low then. If you need up to 100,000 primes, the limit needs to be 1,299,827. Also use `char` instead of `int` to save memory. – Sergey Kalinichenko Dec 13 '15 at 13:15

score 0 · Answer 2 · answered Dec 12 '15 at 14:53

0

One important thing: you do not have to print prime[i] (which is 1) but i. This way you get a much more interesting output...

answered Dec 12 '15 at 14:53

marom

5,064
10
14

To get the limit give a look at https://en.wikipedia.org/wiki/Prime-counting_function – marom Dec 13 '15 at 19:46

SPOJ PRIME1: RTE; Is there a method to avoid dynamic array?

2 Answers2