optimizing loop (frequency counting)... HELP

This is a discussion on optimizing loop (frequency counting)... HELP within the C Programming forums, part of the General Programming Boards category; okay. i am trying to make this loop run as quick as possible. i am trying to collect all occurances ...

  1. #1
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    optimizing loop (frequency counting)... HELP

    okay. i am trying to make this loop run as quick as possible.

    i am trying to collect all occurances of each ASCII character and put into table. i don't really care if ASCII char value equals it's position in the table.

    Code:
    for (;;) {
    	c = fgetc(input_file);
    
    	/* If symbol is already in table then increase frequency
    	 * by one, otherwise put symbol into the table.
    	 */
    	for (i = 0; i < 256; i++) {
    		if(c == i) {
    			if (symbol[i].achar == c) {
    				symbol[i].count++;
    				break;
    			}
    			symbol[i].achar = c;
    		}
    	}
    
    	nc++;
    
    	if (feof(input_file))
    		break;
    }
    "No, I am not wise, but I am a lover of wisdom." --Pythagoras

  2. #2
    & the hat of GPL slaying Thantos's Avatar
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    Why it's bad to use feof() to control a loop

    My suggestion:
    Code:
    char freq[256];
    /* Initalize the array so everything is 0 */
    int c;
    for (;;)
    {
      c = fgetc(input_file);
      if ( c == EOF )
        break;
      freq[c]++;
    }
    easist way IMO if you aren't worried about wasted space.

  3. #3
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    nope... no performance improvement from that. anything else? maybe hash tables, linked lists?
    "No, I am not wise, but I am a lover of wisdom." --Pythagoras

  4. #4
    & the hat of GPL slaying Thantos's Avatar
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    To get the most improvement you need to load as much of the file into memory as possible and then do it. Most of the loop's time is from reading the harddrive.

    Example:

    Code:
    #include <stdio.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <fcntl.h>
    #include <unistd.h>
    
    void init (int *, int);
    
    int main (void)
    {
      int size_read, fid, count;
      char buffer[1048576];
      int freq[256];
    
    
      init(freq, sizeof freq/sizeof freq[0]);
    
      fid=open("test.txt", O_RDONLY);
      if ( fid == -1 )
      {
        puts("Could not open file");
        return 1;
      }
    
      while ( (size_read = read(fid, buffer, sizeof buffer)) != 0 )
      {
        for(count=0; count<size_read; count++)
          freq[(int)buffer[count]]++;
      }
    
      for (count=0; count<sizeof freq/sizeof freq[0]; count++)
        printf("ASCII %3d:  %3d\n", count, freq[count]);
      return 0;
    }
    
    void init (int *x, int y)
    {
      int count;
      for (count=0; count < y; count++)
        x[count] = 0;
    }
    Last edited by Thantos; 05-20-2004 at 10:30 PM.

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    okay... overnight i looked at the loop, and came up with this. running it with the old code on a 65MB file took 10 mintues (on a 233Mhz Cyrix).

    now, on the same machine i get 1.5 minutes with this:

    Code:
    for (;;) {
    	c = fgetc(input_file);
    
    	if (feof(input_file))
    		break;
    
    	if (symbol[c].count > 0) {
    		symbol[c].count++;
    		continue;
    	}
    	symbol[c].count++;
    
    	nc++;
    }
    haven't tried loading into fread(), but i think this is the best it's gunna get. but if anyone has any other suggestions... bring it on.
    "No, I am not wise, but I am a lover of wisdom." --Pythagoras

  6. #6
    and the hat of wrongness Salem's Avatar
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    Well calling feof() every time is gonna hurt
    Especially since fgetc also returns EOF
    Code:
    int c;
    while ( (c = fgetc(input_file)) != EOF ) {
    	if (symbol[c].count > 0) {
    		symbol[c].count++;
    		continue;
    	}
    	symbol[c].count++;
    
    	nc++;
    }
    Using fread() with
    char buff[BUFSIZ];
    is likely to be a better bet
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  7. #7
    End Of Line Hammer's Avatar
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    Slightly improved:
    Code:
    int c;
    while ( (c = fgetc(input_file)) != EOF ) {
    	if (symbol[c].count++ > 0) {
    		continue;
    	}
    	nc++;
    }
    Seeing as you increment whatever the result of the test, might as well only access that variable once.
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

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    whoa! good stuff...
    "No, I am not wise, but I am a lover of wisdom." --Pythagoras

  9. #9
    Registered User Codeplug's Avatar
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    Skip the nc++ as well, since that's known by the file size (unless it's a text file with DOS newlines).

    Other suggestions:
    - use your OS's "raw" file I/O API
    - read into a memory-page sized buffer at a time

    gg
    Last edited by Codeplug; 05-21-2004 at 02:51 PM.

  10. #10
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    >>Skip the nc++ as well, since that's known by the file size (unless it's a text file with DOS newlines).<<
    Based on the snippet last posted, nc looks like its counting the number of different characters in the file, not the total file size.
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

  11. #11
    Registered User Codeplug's Avatar
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    Allow me to continue my OS specific suggestions (in order to redeem myself)....

    Under Windows and using the Win32 API, you could open the file with FILE_FLAG_NO_BUFFERING and FILE_FLAG_OVERLAPPED. Use a buffer size that is some multiple of the hard disk's sector size, but less than or equal to the systems memory page size. While the OS is working on a read operation, crunch the data of the previous read operation.

    You can achieve similar results using asynchronous I/O under *nix (or a couple of threads, one to read and one to crunch).

    gg

  12. #12
    & the hat of GPL slaying Thantos's Avatar
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    Using the method I posted on a 25 meg file it took about 6.5 seconds to read and print off the results.

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    i was omitting what "nc" was for...

    [CODE}
    while ( (c = fgetc(input_file)) != EOF ) {
    symbol[i].position = symbol[i].position + nc; /* this is what the nc is for */
    if (symbol[c].count++ > 0) {
    continue;
    }
    nc++;
    }
    [/CODE]
    "No, I am not wise, but I am a lover of wisdom." --Pythagoras

  14. #14
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    oops.

    Code:
    while ( (c = fgetc(input_file)) != EOF ) {
    	symbol[c].position = symbol[c].position + nc;
    	if (symbol[c].count++ > 0) {
    		continue;
    	}
    	nc++;
    }
    "No, I am not wise, but I am a lover of wisdom." --Pythagoras

  15. #15
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    if just counting frequency... i got it down to just this!!!

    Code:
    while ((c = fgetc(input_file)) != EOF ) {
    	symbol[c].count++;
    }
    "No, I am not wise, but I am a lover of wisdom." --Pythagoras

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