deleting file contents

[Schol-R-LEA]

If you are working with very large files, or are dealing with large numbers of insertions and deletions, you may be better off 'cancelling' records without removing them. For example, if your fields are simple newline delimited strings, you can fill the record to be removed with newlines; you would then write your read and search functions to ignore successive newlines. You would still want to periodically copy the data to a fresh file to compact the data, but you would avoid the overhead of moving the data around with every single deletion.

This is especially useful when you are dealing with indexed data, because you don't need to update the indices as often, and there would be little impact on per record read time (because you would be seeking to a fixed position). This is how many database systems work, in fact, though most will also try to 'recover' cancelled space while running as well.

[rich_m]

there seems to be a bug in this coding (not suprising), well this is my function to delete the first 8 lines from a file if the file contains more than 8 lines . well it works, but at the end of both the files a dummy variable seems to be formed (somthing like a ':y' ), i have no idea wat to do bout it?? down below is my function

Code: Select all

//delete lines from the file if file contains >=8 lines
void trim_file(char *url2,char *turl2) 
{

          int rk=0;???//counter, to find no. of line
          char ch;???//to store each char from file
          FILE *pt,*ptr;//"pt" is the file^,"ptr" is the temp file^


//COPING NECESSARY DATA

 ?????????pt=fopen(url2,"r");
?????????ptr=fopen(turl2,"w");
?????????rk=0;
?????????do
?????????{
????????????ch=fgetc(pt);
????????????if(ch=='\n')
???????????????rk++;
????????????if(rk>8)
????????????{
???????????????putc(ch,ptr);
????????????}

?????????}while(ch!=EOF);
?????????fclose(ptr);
?????????fclose(pt);

//TRANSFERING FROM TEMP FILE


?????????ptr=fopen(turl2,"r");
?????????pt=fopen(url2,"w");
?????????do
?????????{
????????????ch=fgetc(ptr);
????????????putc(ch,pt);
?????????}while(ch!=EOF);

?????????fclose(ptr);
?????????fclose(pt);
}

[rich_m]

???

[AR]

To take a guess:

Code: Select all

do
{
   ch=fgetc(ptr);
   putc(ch,pt);
}while(ch!=EOF);

This code is flawed, if you read an EOF, it then writes the EOF out to the other file before exiting the loop, this bug is in both loops. It would be better set up like this:

Code: Select all

ch=fgetc(ptr);
while(ch!=EOF)
{
   putc(ch,pt);
   ch=fgetc(ptr);
}

[rich_m]

shucks.........how stupid of me to misuse the do{}while; :-\

:)thanks a lot ar.

[rich_m]

the program works but does'nt serve the purpose it deletes all except the last line of the file ???

[rm]

whew.............got it workin,

Code: Select all

pt=fopen(url2,"r");
         ptr=fopen(turl2,"w");
         rk=0;
         while(rk<8)
         {

            ch=fgetc(pt);
            if(ch=='\n')
            {
               rk++;
               ch=fgetc(pt);
            }
         }
                 while(ch!=EOF)
               {

               putc(ch,ptr);
               ch=fgetc(pt);
               }
         fclose(ptr);
         fclose(pt);

         ptr=fopen(turl2,"r");
         pt=fopen(url2,"w");
         while((ch=fgetc(ptr))!=EOF)
         {
            putc(ch,pt);
         }

         fclose(ptr);
         fclose(pt);