Shrinking/trimming an array?

This is a discussion on Shrinking/trimming an array? within the C Programming forums, part of the General Programming Boards category; I'm trying to cut data from the beginning and end of an array, but my idea isn't working. The first ...

  1. #1
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    Shrinking/trimming an array?

    I'm trying to cut data from the beginning and end of an array, but my idea isn't working. The first memcpy line compiles but crashes the program. I can't figure out the right way to do it. This array pointer crap always confused me. It should be possible to memcpy from a location in the array though. Any ideas?

    Code:
    	unsigned char *tmpResults;
        if (!(tmpResults = (unsigned char*)malloc((RamInfo.ResHigh - RamInfo.ResLow)/Search.Size/8 ))) {
            sprintf(ErrTxt, "Unable to allocate shrink results memory () -- Error %u", GetLastError());
            MessageBox(NULL, ErrTxt, "Error", MB_OK);
            goto RUN_SEARCH_ERROR;
        }
        memcpy(&tmpResults, &RamInfo.Results[RamInfo.ResLow >> 3], (RamInfo.ResHigh - RamInfo.ResLow)/Search.Size/8);
    	free(RamInfo.Results); RamInfo.Results = NULL;
        if (!(RamInfo.Results = (unsigned char*)malloc((RamInfo.ResHigh - RamInfo.ResLow)/Search.Size/8))) {
    		sprintf(ErrTxt, "Unable to allocate results memory (DO_SEARCH_CMD) -- Error %u", GetLastError());
            MessageBox(NULL, ErrTxt, "Error", MB_OK); goto RUN_SEARCH_END;
        }
        memcpy(&RamInfo.Results, &tmpResults, (RamInfo.ResHigh - RamInfo.ResLow)/Search.Size/8);
        free(tmpResults);

  2. #2
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    That's a problem with complex code in general, and especially when it involves things like pointers. Even if you get it to work right by really focusing on it, what will happen in 3 years time when you want to modify the code a bit?

    If it's like hell now, it will be like hell X 2, after you've been away from it for a few years.

    Try and make your code as simple and intuitive as feasible, and do use index numbers for working in arrays. Modern compilers and hardware are heavily optimized for them, and they ARE definitely standard C. You'll never run into a compiler that doesn't support them.

  3. #3
    and the hat of int overfl Salem's Avatar
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    Well it would help if you had some vertical white space (blank lines) and decent indentation to start with.

    > memcpy(&tmpResults, &RamInfo.Results[RamInfo.ResLow >> 3], (RamInfo.ResHigh - RamInfo.ResLow)/Search.Size/8);
    FFS, create some extra temporary variables to store the intermediate results.
    Just look at how many times you've copy/pasted RamInfo.ResHigh - RamInfo.ResLow)/Search.Size/8 for example.

    If you're trying to get to AAACC from AAABBBBBBBBCC, then 4 variables called say aStart, aLen, cStart, cLen will help you figure it out.

    Oh, and just in case you're trying to copy memory within the same block, use memmove() instead of memcpy.
    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.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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    Quote Originally Posted by Salem View Post
    Well it would help if you had some vertical white space (blank lines) and decent indentation to start with.

    > memcpy(&tmpResults, &RamInfo.Results[RamInfo.ResLow >> 3], (RamInfo.ResHigh - RamInfo.ResLow)/Search.Size/8);
    FFS, create some extra temporary variables to store the intermediate results.
    Just look at how many times you've copy/pasted RamInfo.ResHigh - RamInfo.ResLow)/Search.Size/8 for example.

    If you're trying to get to AAACC from AAABBBBBBBBCC, then 4 variables called say aStart, aLen, cStart, cLen will help you figure it out.

    Oh, and just in case you're trying to copy memory within the same block, use memmove() instead of memcpy.
    No, 4 vars won't help me figure anything out. I know what I'm trying to get (BBBBBBBBBB by your example, but mines not a string it's data). What I don't know is how to copymem from the middle of the array.

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    Post some hard to make work, examples of your data, I'd like to study them a bit.

    The program below, produces this output. Yes, it's from a string, but it could be non string data, with a few tweaks:

    Code:
    Buffer is: Hello world
    Buffer is: Hello*world
    Buffer is: Hell**world
    Buffer is: Hell***orld
    Buffer is: Hel****orld
    Buffer is: Hel*****rld
    Buffer is: He******rld
    Buffer is: He*******ld
    Buffer is: H********ld
    Buffer is: H*********d
    Buffer is: **********d
    Buffer is: ***********
    
    
    			    press enter when ready
    Code:
    #include <stdio.h>
    #include <string.h> //or mem.h
    
    int main(void)
    {
       char buffer[] = "Hello world";
       int i, len;
       len = strlen(buffer);
    
       for(i=len-1;i>-1;i--) {
         printf("Buffer is: %s\n", buffer);
         memset(buffer+(i/2),'*',len-i);  //also try buffer+i, etc.
       }
       printf("Buffer is: %s\n", buffer);
       printf("\n\n\t\t\t    press enter when ready");
       (void) getchar();
       return 0;
    }

  6. #6
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    Quote Originally Posted by Adak View Post
    Post some hard to make work, examples of your data, I'd like to study them a bit.

    The program below, produces this output. Yes, it's from a string, but it could be non string data, with a few tweaks:

    Code:
    Buffer is: Hello world
    Buffer is: Hello*world
    Buffer is: Hell**world
    Buffer is: Hell***orld
    Buffer is: Hel****orld
    Buffer is: Hel*****rld
    Buffer is: He******rld
    Buffer is: He*******ld
    Buffer is: H********ld
    Buffer is: H*********d
    Buffer is: **********d
    Buffer is: ***********
    
    
    			    press enter when ready
    Code:
    #include <stdio.h>
    #include <string.h> //or mem.h
    
    int main(void)
    {
       char buffer[] = "Hello world";
       int i, len;
       len = strlen(buffer);
    
       for(i=len-1;i>-1;i--) {
         printf("Buffer is: %s\n", buffer);
         memset(buffer+(i/2),'*',len-i);  //also try buffer+i, etc.
       }
       printf("Buffer is: %s\n", buffer);
       printf("\n\n\t\t\t    press enter when ready");
       (void) getchar();
       return 0;
    }
    You're missing the point completely. If I wanted to play around with loops, I'd do a function for it. I simply want to know the proper syntax to send copymem a pointer to the middle of the array (i.e. array[blah]) as the source to copy from, assuming it can be done, because copymem should be faster. I'm trying to save only the data from the middle of the array based on the first and last result of my search (RamInfo.ResLow and high) dynamically shrinking my search area after each search. Actually, the way you did the first argument for memset probably tells me what I needed to know.
    Last edited by Viper187; 03-20-2011 at 09:06 AM.

  7. #7
    and the hat of int overfl Salem's Avatar
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    It doesn't matter whether it's a string or data - do the maths.

    > What I don't know is how to copymem from the middle of the array.
    Code:
    char buff[100];
    int bStart = 10;
    int bLen = 20;
    memcpy( destination, &buff[bStart], bLen );
    Figuring out bStart and bLen is up to you, but assuming you can do that, the rest is easy.
    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.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  8. #8
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    Works now. I really hate pointers sometimes. Been a year or more since I bothered to program anything. Trying to pick up where I left off on an old project.

    Code:
    	unsigned char *tmpResults; u32 NewResSize = (RamInfo.ResHigh - RamInfo.ResLow)/Search.Size/8;
        if (!(tmpResults = (unsigned char*)malloc(NewResSize))) {
            ShowError("Unable to allocate shrink results memory () -- Error %u", GetLastError());
            goto RUN_SEARCH_ERROR;
        }
        memcpy(tmpResults, &RamInfo.Results[(RamInfo.ResLow/Search.Size) >> 3], NewResSize);
    	free(RamInfo.Results); RamInfo.Results = NULL;
        if (!(RamInfo.Results = (unsigned char*)malloc(NewResSize))) {
    		ShowError("Unable to allocate results memory (DO_SEARCH_CMD) -- Error %u", GetLastError());
            goto RUN_SEARCH_END;
        }
        memcpy(RamInfo.Results, tmpResults, NewResSize);
        ShowError("Original: %X\nNew: %X\nResLow: %X", RamInfo.NewResultsInfo.DumpSize/Search.Size/8, NewResSize, (RamInfo.ResLow/Search.Size)>>3);
        free(tmpResults);
        RamInfo.NewResultsInfo.DumpSize = RamInfo.ResHigh - RamInfo.ResLow;
    Last edited by Viper187; 03-20-2011 at 06:32 PM.

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