The best way to delete a record from a file?

This is a discussion on The best way to delete a record from a file? within the C Programming forums, part of the General Programming Boards category; Hi ALL, Lets say I have a file name "cars.bin". Each record in the file is a struct (made from ...

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    The best way to delete a record from a file?

    Hi ALL,

    Lets say I have a file name "cars.bin".
    Each record in the file is a struct (made from int, char [20], int).
    Lets say I have 10000 records,
    struct 1, struct 2, struct 3, struct ....., struct 10000.

    Now, If I want to delete record #400, how should I do it??
    The only thing I could think of is to:
    1. Create a new file
    2. Copy all the records from old file to new file (except of the #400)
    3. Delete old file
    4. Rename new file to old file name.

    This seem like a slow poor solution, is there any other way to do it?

    Many thanks
    Salvador

  2. #2
    Jack of many languages Dino's Avatar
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    Since all the structures are the same size, you could open the file, seek to structure 10,000, copy it to structure #400 position, then resize the file to be smaller by one structure.

    This would be very fast.

    Todd

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    You could have a marker that says "deleted" in each record [e.g. the first int being a negative number, just as an example]. Then when you are reading the records into your application, check the "deleted" marker and skip those that are deleted.

    If you want to, you could have a "compress" mechanism, so that when you exit the applicaiton, if you counted more than X% deleted records, THEN you rewrite the whole file without the deleted records.

    Or, if you want to re-use deleted records, you could keep a linked list of records that have been deleted, e.g. using the second int to indicate which record is the next free one, so when you add new records, you overwrite the first record in that list.

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    hummm... I probably would copy struct401 to struct400 then struct402 to struct140 and so on...
    I have no much experience with handling files but if the structs were linked with pointers is could be easier and faster. Unfurtunately in the case of files i don't think this is like this.
    Mac OS 10.6 Snow Leopard : Darwin

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    Quote Originally Posted by Todd Burch View Post
    Since all the structures are the same size, you could open the file, seek to structure 10,000, copy it to structure #400 position, then resize the file to be smaller by one structure.

    This would be very fast.

    Todd
    Although resizing files is only supported by some OS's.

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    Jack of many languages Dino's Avatar
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    This dilemma is why databases were invented.

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    Yes, and most databases don't ACTUALLY delete things, they just mark it as "deleted", and re-use as needed.

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    Cheers!

    How do I resize a file?

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    Dr Dipshi++ mike_g's Avatar
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    Since all the structures are the same size, you could open the file, seek to structure 10,000, copy it to structure #400 position, then resize the file to be smaller by one structure.

    This would be very fast.

    Todd
    I don't get exactly how this would work. Would you not need to move the structure instead of copy it?

    Say each entry is a line of text, then to actually delete it you would have to copy all the following text over it, then resize the file. Or am I missing something here?

    Personally I like matsp's suggestion, which sounds easy to do.

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    Jack of many languages Dino's Avatar
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    On unix, you would use truncate().

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    Jack of many languages Dino's Avatar
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    Quote Originally Posted by mike_g View Post
    I don't get exactly how this would work. Would you not need to move the structure instead of copy it?

    Say each entry is a line of text, then to actually delete it you would have to copy all the following text over it, then resize the file. Or am I missing something here?

    Personally I like matsp's suggestion, which sounds easy to do.
    move==copy. Same thing.

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    Dr Dipshi++ mike_g's Avatar
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    Oh yeah, actually I get it now (i think). You could just swap the entry to be deleted with the last entry then truncate it. So yeah both ways would be fast

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    I will adapt mat's suggestion too if the file is sorted. (thank you)
    and Todd's if not (thank you too)
    I still don't understand how can I resize a file size on Windows....

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    Jack of many languages Dino's Avatar
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    On Windows, _chsize().

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    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by mike_g View Post
    Oh yeah, actually I get it now (i think). You could just swap the entry to be deleted with the last entry then truncate it. So yeah both ways would be fast
    But then the records would be out-of-order. Record 1 ... 400, 900, 401 ... 899.
    The best and easiest way is to just use a flag to indicate if the record is valid or deleted. You can then write new data there later or just move data down if the records needs to be in order.
    Again, if they don't need to be in order, mike_g's suggestion works, as well.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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