Thread: Bubblesort with unknown size struct read from a bin file

Hello folks, can you help me please?

I have a binary file, and I want to pass these infos to a TXT ordered file.

I can pass the data without problems if I don' try to order: I created a vector of struct of size 2, and I go reading the bin file, if my count of lines, get to the number 2, I write the data on the txt, then I put my count as zero again, and using this way, I can make it work without actually knowing how many lines my bin file has...

Well, the problem is: by this method, I'm constantly deleting my structs, so how can I order the data?

PS: I dont know how to allocate vector dinamically, so for now I want to understand how could I use a bubble sort to order these data from my bin file and then pass to a txt file, if I dont know how long this file is.

Thanks in advance,

Do you know the maximum number of records in your binary file?
How about defining your array to have that many elements.

Use malloc to allocate the vector dynamically. Examples abound on the internet, but you might have more luck searching for "dynamic array C" (vecotr is perhaps more of a C++ term). If you feel like getting fancy, you can malloc some memory, then use realloc to expand it if you run out before you're done reading the file.

Also, you could use some fseek and ftell "trickery" (it's not that tricky or tough) to figure out the size of the file, and divide that by the size of the struct. That may not be 100% accurate, since binary files may have some extra "junk" data at the end, and I can't remember how ftell works off the top of my head. I'll check and get back to you.

EDIT: I was right, the fseek bit isn't guaranteed to work if you try to fseek using SEEK_END*. A slower, but guaranteed alternative is to read through the binary file one struct at a time, storing the data in a "dummy" variable, and counting as you go. Then malloc enough space, rewind the file to the beginning, and read it all in to your newly allocated array.

* The relevant part of the standard if anybody cares is:

Originally Posted by C99 7.19.9.2p3

...A binary stream need not meaningfully support fseek calls with a whence value of SEEK_END.

If you can't malloc what you need, and you can't afford to allocate stupidly large arrays to cover "all" eventualities, then you need something like this
External sorting - Wikipedia, the free encyclopedia

Originally Posted by frank1

I dont know how to allocate vector dinamically, so for now I want to understand how could I use a bubble sort to order these data from my bin file and then pass to a txt file, if I dont know how long this file is.

I will note that this statement does not mean you CAN'T use malloc(). It means you don't know how.

I suggest some effort in learning about malloc() will be more useful than trying to find an alternative solution. Yes, there are alternative solutions, but they are even more advanced (i.e. difficult to use for your scenario) than malloc().