Java vs C (Run time) EDIT: Added Code

Question

I just wrote a program that finds all the prime numbers with an upper bound.

The algorithm: Sieve of Eratosthenes.

Wrote it both in C and Java. The upper bound is 666014.

For some reason C gives the result in more than 2.5 seconds, while Java does it in like half a second.

Details:

The array in C is of type char
The array in Java is of type boolean

C IDE: CodeBlocks

Java IDE: IntellijIdea Community Edition

C code:

#include <stdio.h>

int main() {

    int n = 666013;
    int i;
    int k;

    char a[n];

    for (i = 2; i <= n; i++)
        a[i] = 0;

    for (i = 2; i <= n; i++)
         if ( a[i] == 0 )
         {
            printf("%d\n", i);
            for (k = i + i; k <= n; k += i)
                 a[k] = 1;
         }

    return 0;

}

Java code:

package primes;

public class Prime {

    public static void main(String[] args) {
        long starttime = System.nanoTime();
        final int MAXN = 666013;
        boolean a[] = new boolean[MAXN];

        for (int i = 2; i < a.length; i++)
            a[i] = true;

        for (int i = 2; i < a.length; i++)
            if (a[i])
            {
                System.out.println(i);
                System.out.printf("");
                for (int j = i + i; j < a.length; j += i) {
                    a[j] = false;
                }
            }

        System.out.println(System.nanoTime() - starttime);

    }
}

Last Edit: used System.nanoTime() Java gives 0.35 seconds

The C algorithm cannot be any faster. What is the reason Java is faster here?

If you're going to do cross-testing then it's smarter to use the same datatypes — Juxhin, Sep 02 '14 at 18:29
In what sense are you talking about compilation time, as per your title? — Jon Skeet, Sep 02 '14 at 18:30
So, you are asking, why does Java compile faster, than C compiles (and links?)? — hyde, Sep 02 '14 at 18:30
Would be helpful if you shared some code samples of the Java version and C version. Only mentioning what arrays you are using is not very helpful, as you are not even specifying what the arrays are used for. — Gary Drocella, Sep 02 '14 at 18:32
`For some reason C gives the result in more than 2.5 seconds` So are you telling us that your `C` coding skills are not up to par? — PaulMcKenzie, Sep 02 '14 at 18:33
How are you using an array of Booleans to get prime numbers? And as for the chars - are you spelling out the numbers like "two", etc etc? — doctorlove, Sep 02 '14 at 18:33
"gives the result" suggests runtime to me, not compile time. What would IDE have to do with anything? — duffymo, Sep 02 '14 at 18:34

score 1 · Answer 1 · answered Sep 02 '14 at 18:34

1

I'm going to bet that you're actually asking about run-time even though you listed compilation time in your question header. If you're trying to time compilation, its really apples to oranges, especially if you're doing it in IDEs instead of using command line.

If you're comparing a Java application to a C++ application, there's a lot of things you must consider.

You should use the same data types.
You MUST make sure your Java code isn't triggering extra boxing.
You should read for both languages and find how to use the most accurate low level system timer. System.currentTimeMillis() is not very accurate in Java, for example.
You should make sure your time is measured prior to putting it to output so that output differences in the two languages don't get measured in the test.
Are you running the same architecture in both languages? (32 or 64 bit).

Frankly, your assessment is very code dependent. Without showing code people can't help you. There are many, many things you could do to affect timings in even the simplest code.

answered Sep 02 '14 at 18:34

John Humphreys

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Just for the note, System.currentTimeMillis() is more accurate than nanoTime() – joey rohan Sep 02 '14 at 18:38
Why is that out of curiosity? I thought I remembered something from Joshua Bloch's effective Java saying that currentTimeMillis() was inaccurate, but I don't have the book at work to check at the minute. I probably remembered the text wrong. – John Humphreys Sep 02 '14 at 18:55
This link might be useful : http://stackoverflow.com/questions/351565/system-currenttimemillis-vs-system-nanotime – joey rohan Sep 02 '14 at 18:58
And that book is opened in another tab.Cheers:) – joey rohan Sep 02 '14 at 19:00
1

Ah, nice link :) The 28 vote comment on the answer helps a lot. And it's a great book, I bought it because it was modeled on Effective C++ which was awesome too. :D – John Humphreys Sep 02 '14 at 19:16

Java vs C (Run time) EDIT: Added Code

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