3
\$\begingroup\$

I'm trying to solve the SPOJ primes problem:

Peter wants to generate some prime numbers for his cryptosystem. Help him! Your task is to generate all prime numbers between two given numbers!

Input

The input begins with the number t of test cases in a single line (t <= 10). In each of the next t lines there are two numbers m and n (1 <= m <= n <= 1000000000, n-m<=100000) separated by a space.

Output

For every test case print all prime numbers p such that m <= p <= n, one number per line, test cases separated by an empty line.

Example

Input:

2
1 10
3 5

Output:

2
3
5
7

3
5

Warning: large Input/Output data, be careful with certain languages (though most should be OK if the algorithm is well designed)

Due to the warning, I use a segmented sieve of Eratosthenes, having found the primes between 1 and 32000 with a normal sieve:

#include <stdio.h>

int find_primes(int begin, int end)
{

    int i;
    int y = 0;
    int z = 0;
    int primes[3402] = {3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97,101,103,107,109,113,127,131,137,139,149,151,157,163,167,173,179,181,191,193,197,199,211,223,227,229,233,239,241,251,257,263,269,271,277,281,283,293,307,311,313,317,331,337,347,349,353,359,367,373,379,383,389,397,401,409,419,421,431,433,439,443,449,457,461,463,467,479,487,491,499,503,509,521,523,541,547,557,563,569,571,577,587,593,599,601,607,613,617,619,631,641,643,647,653,659,661,673,677,683,691,701,709,719,727,733,739,743,751,757,761,769,773,787,797,809,811,821,823,827,829,839,853,857,859,863,877,881,883,887,907,911,919,929,937,941,947,953,967,971,977,983,991,997,1009,1013,1019,1021,1031,1033,1039,1049,1051,1061,1063,1069,1087,1091,1093,1097,1103,1109,1117,1123,1129,1151,1153,1163,1171,1181,1187,1193,1201,1213,1217,1223,1229,1231,1237,1249,1259,1277,1279,1283,1289,1291,1297,1301,1303,1307,1319,1321,1327,1361,1367,1373,1381,1399,1409,1423,1427,1429,1433,1439,1447,1451,1453,1459,1471,1481,1483,1487,1489,1493,1499,1511,1523,1531,1543,1549,1553,1559,1567,1571,1579,1583,1597,1601,1607,1609,1613,1619,1621,1627,1637,1657,1663,1667,1669,1693,1697,1699,1709,1721,1723,1733,1741,1747,1753,1759,1777,1783,1787,1789,1801,1811,1823,1831,1847,1861,1867,1871,1873,1877,1879,1889,1901,1907,1913,1931,1933,1949,1951,1973,1979,1987,1993,1997,1999,2003,2011,2017,2027,2029,2039,2053,2063,2069,2081,2083,2087,2089,2099,2111,2113,2129,2131,2137,2141,2143,2153,2161,2179,2203,2207,2213,2221,2237,2239,2243,2251,2267,2269,2273,2281,2287,2293,2297,2309,2311,2333,2339,2341,2347,2351,2357,2371,2377,2381,2383,2389,2393,2399,2411,2417,2423,2437,2441,2447,2459,2467,2473,2477,2503,2521,2531,2539,2543,2549,2551,2557,2579,2591,2593,2609,2617,2621,2633,2647,2657,2659,2663,2671,2677,2683,2687,2689,2693,2699,2707,2711,2713,2719,2729,2731,2741,2749,2753,2767,2777,2789,2791,2797,2801,2803,2819,2833,2837,2843,2851,2857,2861,2879,2887,2897,2903,2909,2917,2927,2939,2953,2957,2963,2969,2971,2999,3001,3011,3019,3023,3037,3041,3049,3061,3067,3079,3083,3089,3109,3119,3121,3137,3163,3167,3169,3181,3187,3191,3203,3209,3217,3221,3229,3251,3253,3257,3259,3271,3299,3301,3307,3313,3319,3323,3329,3331,3343,3347,3359,3361,3371,3373,3389,3391,3407,3413,3433,3449,3457,3461,3463,3467,3469,3491,3499,3511,3517,3527,3529,3533,3539,3541,3547,3557,3559,3571,3581,3583,3593,3607,3613,3617,3623,3631,3637,3643,3659,3671,3673,3677,3691,3697,3701,3709,3719,3727,3733,3739,3761,3767,3769,3779,3793,3797,3803,3821,3823,3833,3847,3851,3853,3863,3877,3881,3889,3907,3911,3917,3919,3923,3929,3931,3943,3947,3967,3989,4001,4003,4007,4013,4019,4021,4027,4049,4051,4057,4073,4079,4091,4093,4099,4111,4127,4129,4133,4139,4153,4157,4159,4177,4201,4211,4217,4219,4229,4231,4241,4243,4253,4259,4261,4271,4273,4283,4289,4297,4327,4337,4339,4349,4357,4363,4373,4391,4397,4409,4421,4423,4441,4447,4451,4457,4463,4481,4483,4493,4507,4513,4517,4519,4523,4547,4549,4561,4567,4583,4591,4597,4603,4621,4637,4639,4643,4649,4651,4657,4663,4673,4679,4691,4703,4721,4723,4729,4733,4751,4759,4783,4787,4789,4793,4799,4801,4813,4817,4831,4861,4871,4877,4889,4903,4909,4919,4931,4933,4937,4943,4951,4957,4967,4969,4973,4987,4993,4999,5003,5009,5011,5021,5023,5039,5051,5059,5077,5081,5087,5099,5101,5107,5113,5119,5147,5153,5167,5171,5179,5189,5197,5209,5227,5231,5233,5237,5261,5273,5279,5281,5297,5303,5309,5323,5333,5347,5351,5381,5387,5393,5399,5407,5413,5417,5419,5431,5437,5441,5443,5449,5471,5477,5479,5483,5501,5503,5507,5519,5521,5527,5531,5557,5563,5569,5573,5581,5591,5623,5639,5641,5647,5651,5653,5657,5659,5669,5683,5689,5693,5701,5711,5717,5737,5741,5743,5749,5779,5783,5791,5801,5807,5813,5821,5827,5839,5843,5849,5851,5857,5861,5867,5869,5879,5881,5897,5903,5923,5927,5939,5953,5981,5987,6007,6011,6029,6037,6043,6047,6053,6067,6073,6079,6089,6091,6101,6113,6121,6131,6133,6143,6151,6163,6173,6197,6199,6203,6211,6217,6221,6229,6247,6257,6263,6269,6271,6277,6287,6299,6301,6311,6317,6323,6329,6337,6343,6353,6359,6361,6367,6373,6379,6389,6397,6421,6427,6449,6451,6469,6473,6481,6491,6521,6529,6547,6551,6553,6563,6569,6571,6577,6581,6599,6607,6619,6637,6653,6659,6661,6673,6679,6689,6691,6701,6703,6709,6719,6733,6737,6761,6763,6779,6781,6791,6793,6803,6823,6827,6829,6833,6841,6857,6863,6869,6871,6883,6899,6907,6911,6917,6947,6949,6959,6961,6967,6971,6977,6983,6991,6997,7001,7013,7019,7027,7039,7043,7057,7069,7079,7103,7109,7121,7127,7129,7151,7159,7177,7187,7193,7207,7211,7213,7219,7229,7237,7243,7247,7253,7283,7297,7307,7309,7321,7331,7333,7349,7351,7369,7393,7411,7417,7433,7451,7457,7459,7477,7481,7487,7489,7499,7507,7517,7523,7529,7537,7541,7547,7549,7559,7561,7573,7577,7583,7589,7591,7603,7607,7621,7639,7643,7649,7669,7673,7681,7687,7691,7699,7703,7717,7723,7727,7741,7753,7757,7759,7789,7793,7817,7823,7829,7841,7853,7867,7873,7877,7879,7883,7901,7907,7919,7927,7933,7937,7949,7951,7963,7993,8009,8011,8017,8039,8053,8059,8069,8081,8087,8089,8093,8101,8111,8117,8123,8147,8161,8167,8171,8179,8191,8209,8219,8221,8231,8233,8237,8243,8263,8269,8273,8287,829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//List of primes to use

    if(begin % 2 == 0)
    {
        begin += 1;     //If begin is even, we start with begin + 1

    }
    for(i = begin; i <= end; i += 2)    //Check half the numbers, to save time(I hope)
        {
            for(z = 0; z <= 3399; z++)
            {
                if(i % primes[z] == 0)
                {
                    y = 1;          //y is a switch
                    break;
                }
            }
            if(y == 0)
            {
                printf("%d\n", i);
            }
            y = 0;

        }

    printf("\n");
    return 0;

}
int main(void)      //The main() function, takes input and calls the find_primes() function
{

    int times;
    scanf("%d", &times);
    int i;

    for(i = 0; i < times; i++)
        {
            int input_1, input_2;
            scanf("%d %d", &input_1, &input_2);
            find_primes(input_1, input_2);

        }
    return 0;

}

Although it's very fast in some tests that I did, I still get a time limit exceeded error. How can I make it faster?

\$\endgroup\$
  • \$\begingroup\$ Is it allowed for you to use a pre-computed list of primes? Since it is something that computer can compute, it seems to me that it should rather be generated at run-time. \$\endgroup\$ – ThoAppelsin Aug 2 '14 at 12:25
  • \$\begingroup\$ Yes, it is, it saves time, and it's easy to use brute force and find them. \$\endgroup\$ – Antoni4040 Aug 2 '14 at 12:31
  • 1
    \$\begingroup\$ This code here does not do any sieve-of-eratosthenes work... just saying. \$\endgroup\$ – rolfl Aug 2 '14 at 12:41
  • 2
    \$\begingroup\$ I rolled back your edit, we have a policy on code review regarding follow ups to your questions. You should read up on How to proceed after you have some answers. \$\endgroup\$ – rolfl Aug 2 '14 at 14:58
  • \$\begingroup\$ That's strange. Can I add an answer with the new code? I don't want anyone venturing here to see the old one... \$\endgroup\$ – Antoni4040 Aug 2 '14 at 15:09
2
\$\begingroup\$

Your solution is, in fact, conceptually quite clever. There are some style issues, some bugs, and some optimizations...

So ... Magic numbers

You have the prepared list of primes, and you declare the array as length 3402. Why specify the length when if you leave it blank, the compiler will do it for you? Anyway, when I counted, there are 3401....

int primes[] = {3,5,......};

Then, in your loop, you set the limit at 3399 ... why? Well, element 3399 is prime 31607 which is the largest prime who's square (999002449) is less than 1000000000... OK.

So, you have magic numbers.... I would define them and move on... having them as constant numbers in the code is confusing.

Bugs

Your code does not successfully output 2, ever. You need to work on that edge case.

Variables

The names are all wrong, except the array primes.

I would use p instead of i, as it is not a classic i++ for loop.

y and z are meaningless.... use a variable name that makes sense, like isprime and i

Performance

There are two things I can suggest to improve performance....

  1. If begin or end are less than primes[3400] then you can do a binary-search on primes and then just output those primes while they fall within the primes array, and between begin and end. No need for calculations.....

  2. Your inner z loop scanning primes should terminate when primes[z] > sqrt(i). At the moment, if i is 31, you are scanning all 3400 primes to see if they are factors.... ouch.

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  • \$\begingroup\$ Thanks for your answer, I had suspected the last point, see the new code. \$\endgroup\$ – Antoni4040 Aug 2 '14 at 14:55
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Don't ever leave your variables uninitialized.

int main(void)  
{

    int times;
    scanf("%d", &times);
    int i;

    for(i = 0; i < times; i++)

here times is not initialized. If scanf for some reason couldn't parse its input, you will be left with times in unknown state. And then you loop according to its value.

Always initialize your variables with a suitable value:

    int times = 0;

If there's no suitable value to use, examine the return code of scanf to make an appropriate decision.

As for the algorithm, I would like to reiterate that this is not a "segmented Sieve of Eratosthenes" but rather just trial division over a range. See a related answer on SO.

Sieve is much faster: when I tried it, my sieve–based solution for this SPOJ problem ran 38x faster than a trial-division–based one.

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