FILE pointer

This is a discussion on FILE pointer within the C Programming forums, part of the General Programming Boards category; Hi, I have to find the ‘position’ of ‘23’ in file a1.txt . At the corresponding ‘ position ’ in ...

  1. #1
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    FILE pointer

    Hi,
    I have to find the ‘position’ of ‘23’ in file a1.txt. At the corresponding ‘position
    in a2.txt, I want to add 500 to member value. I am able to pick up the values from both a1.txt and a2.txt but the problem comes when I a trying to replace the value in a2.txt by adding 500 to the corresponding member value. The value is not getting printed in a2.txt.

    I am using gcc compiler. Kindly help..

    a1.txt:
    11
    12
    13
    ....
    23
    24
    25

    a2.txt:
    61
    62
    63
    ...
    73
    74
    75

    Code:
    #include<stdio.h>
    int main()
    {
    
    FILE *fp,*fp1;
    int i,ch;
    long n;
    
    fp1=fopen("a1.txt","r");
    
    while(fscanf(fp1,"%d",&i) == 1)
    
    {
       
         if(i==23)
        {
            n=ftell(fp1);
            printf("%ld\n",n);
            break;
        }
    
    
    }
    
    fclose(fp1);
    
    n=n-2;                                      //offset starts with '0'
    fp=fopen("a2.txt","rw");
    
    fseek(fp,n,0);        
    fscanf(fp,"%d",&i);
    
    fprintf(fp,"%d\n",i+5);
    fclose(fp);
    
    return 0;
    
    }
    Last edited by Sanchari Sircar; 10-26-2012 at 12:47 PM.

  2. #2
    SAMARAS std10093's Avatar
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    i think you haven't understand the offset thing.If first search for 11 then you will get n equal to 2.Why?
    Because the pointer of file was moved by scanf until it reads the first number,which in this case is two digit number,so it will move from zero position to the second one (it will be increased by two).If you had a n-digit number it would be increased by n.Then if you search for 12,then fscanf will read the first two digit number,then the newline character and then the number 12(two digit),so it will increase the pointer by (2+1+2)...Hope this helps

  3. #3
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    I got that part...I am facing problem when i am trying to print i+500 instead of i in the second file. Its not getting printed at all. fp is correctly pointing at 73...I checked it.

  4. #4
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    look at the manual for fopen: fopen(3): stream open functions - Linux man page. Where do you see "rw" as a valid mode? You have to pick a valid one for what you want to do. You want text mode, read and write.

    EDIT: Also, you should check the return value of fopen. If it fails, print a sensible error (use the perror function) and exit.

  5. #5
    and the hat of wrongness Salem's Avatar
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    Well if your file looks like
    23\n
    24\n

    And you seek to the start of 23 and write 525, then the file is going to look like
    523\n
    4\n

    If you're changing the length of any line, you're going to have to rewrite the file.
    There is no magic "insert text here" option at this level.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  6. #6
    SAMARAS std10093's Avatar
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    By looking at reference fopen - C++ Reference i would suggest another approach which is more costly but easy to achieve.Find the position as you do.Then go to the second file and open it with "r" .I think what you are using for it does not exist.Then read this data in an array.Then close the file.Then modify this array (go to the position you want and increase the value).Then open again the very same file with "w".Now you are overwriting what is in the file,so just go and write there the array.Then of course close it again

  7. #7
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    Thanks..

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