cout == slow (prime numbers)

This is a discussion on cout == slow (prime numbers) within the C++ Programming forums, part of the General Programming Boards category; i've been working on a prime number program on and off since last year and i thought everyone (who doesn't ...

  1. #1
    Registered User heat511's Avatar
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    cout == slow (prime numbers)

    i've been working on a prime number program on and off since last year and i thought everyone (who doesn't know this already) how slow cout really is. for example when running my program and couting all the primes between 1 and 10 million my program completed in 125.5 seconds whereas w/ cout commented out, the program ran in 17.7 seconds!!!

    also, if any1 knows how fast direct access files are (in a vague comparision to whatever you can think of...) it would be nice. i am planning on implementing them to hold the found prime numbers.
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  2. #2
    and the hat of int overfl Salem's Avatar
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    Store the whole lot in one big char array, and fwrite() the whole array in one go at the end
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    Registered User heat511's Avatar
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    LOL

    that char would have to be... *counts fingers* very very large. when i ran it to 25 million earlier today i found (well i can't remember exactly) i think 1.7 million primes... each of those is anywhere from 1 to 7 digits... that's means i would have to have something like this.

    Code:
    char primes[7000000];
    which seems pretty ridiculous... not to mention using that much ram will slow down the program and i could overflow the variable and lose all the primes i just found
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    Does the 17.7 sec run include writing them to a file? If no, try ofstream. If yes:

    Is this what you're trying to make faster? If so, you could try fopen(), fprintf().

    And if you don't care about portability, for windows there are CreateFile(), WriteFile(), etc. which are supposed to be fast.

  5. #5
    Cat without Hat CornedBee's Avatar
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    File access or console write access is slow no matter what method you use. Yes, cout /ofstream are slower than raw solutions like fputs or WriteFile, but only marginally so, the real speed problem is the actual access, especially with files.
    Not using endl for example should speed up the streams quite a bit.

    For the char array: 7 MB aren't ridiculous at all. Any modern game uses far more than that, and prime number seekers are memory intensive, nobody will complain. But you should allocate the thing via new, not on the stack. SGI's rope class might be a good choice too (I believe STLport contains it too).
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  6. #6
    and the hat of int overfl Salem's Avatar
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    Well you don't have to store all 7MB at once (but you know already how much space it will take from the file you've already written right)

    Besides, how much space do all your primes take up, excluding the textual representation of them?

    Create a buffer of whatever size you feel happy with (or just experiment). When the buffer is full, you write it out and start over.

    > i could overflow the variable and lose all the primes i just found
    Only if your code was so hopelessly bug ridden as to write over memory it didn't own.
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    wait, so if cout is slow, what is the fast solution that reported the quick 17.7 seconds?

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    Registered User major_small's Avatar
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    ^ no output?

    what about writing them into an array of unsigned long ints? or a two dimensional array?
    Code:
    char array[numberofprimes][longestprimelength];
    finding prime numbers is a pretty heavy operation... there's no way around it... either eat memory, slow down with constant outputs, a mix of both, or no ouput at all...
    Last edited by major_small; 12-17-2003 at 11:10 PM.
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    *******argv[] - hu? darksaidin's Avatar
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    The *print* functions are actually faster I believe. I rember I tested this a while ago. Though of course, both cout and the *print* thingys are slower than no output at all.

    Why don't you just store the primes in an array of some bin-type, maybe int64_t. I doubt you could produce enough primes to use up your memory

    Does you program find primes based on probability or does it actually find all primes ?
    [code]

    your code here....

    [/code]

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    Registered User heat511's Avatar
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    thanks for the responses guys

    at the moment there is no output because of the huge speed difference... (well aside from the gettime output at the end)

    the program actually finds all the primes and doesn't just guess which numbers are primes

    so are creatfile() and writefile() faster than direct access?

    basically at this point im hoping to make a program that runs fast (albeit slower than at the moment) that uses files so i have some sort of hardcopy of the primes found.
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  11. #11
    S Sang-drax's Avatar
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    Actually, in an optimized implementation, cout is way faster than printf.
    cout does the type-checking at compile-time. printf on the other hand parses a format string at run-time. This is *much* slower.

    The reason cout is slower on some implementations of C++ is because the programmers have been lazy (cout uses printf internally of something like that).
    Last edited by Sang-drax : Tomorrow at 02:21 AM. Reason: Time travelling

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    You could try write() from ofstream. I just ran a program which wrote 10,000,000 ints to a file using write(), and it took 5-7 seconds. This would be considerably more ints than the primes between 0 and 10,000,000.

    Here's what I used to write one int:
    Code:
    out.write(reinterpret_cast<char *> (&num), sizeof(num));
    One would guess write() from ifstream or the c-style fwrite() would be comparable in speed.

    If this isn't fast enough, then try writing a block at a time, as Salem suggested.

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    I just used fwrite() in combination with a buffer to write 10,000,000 ints, and it took 2 seconds. Without a buffer, it's about the same speed as ofstream's write().

    So with a buffer, fwrite() appears faster than ofstream's write(). Without a buffer, they are about the same speed.

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    Registered User major_small's Avatar
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    I would use a buffer... if you have the memory availible, use as much of it as you can/need (unless there's a reason you don't want), and then dump the buffer into a file and maybe output it to the screen at the end of the run...
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    Registered User heat511's Avatar
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    hmmmmm

    im afraid what you guys are talking about is over my head at the moment. i only know the fstream header file ofstream and direct access (also inhe fstream header file)...

    (note: i ran the program with my school comp left on to find all the primes between 1 and 1 billion (10 digits) last night and it took 4 hours 40ish minutes to complete... meaning im nowhere near factoring a 200 digit number for the http://www.rsasecurity.com/rsalabs/c...ges/index.html contest lol)
    Last edited by heat511; 12-19-2003 at 02:40 PM.
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