Thread: Reading file from the bottom up

I have a file that goes like

Code:

++++07/17/2016++++Memo: wdawd
retire (Chase Savings): 20
investment (Chase College): 70
Chase total: 90
savings (IQ Savings): 70
spending(IQ Checking): 40
IQ total: 110
Total: 200
++++END++++

but I was wondering how I can read the file from the end to the beginning and copy to string. Like start at end and go to total and IQ and so on

Originally Posted by Salem

Read the whole file into an array of strings, then index the array backwards.

Or use ftell/fgets to read the whole file forward to get an array of file positions marking the start of each line.
Then index that array backwards to allow you to fseek/fgets each line in turn, but in reverse order.

I don't fully understand the fseek part, can you show me an example code?

Originally Posted by Guest

screwlu, are you familiar with std::getline? I would use that to read each line into a std::string and push it onto a std::vector<std::string>, storing all lines. Afterwards, you could iterate backwards over that vector to process your data.

If you literally want to reverse the file, not just the line order, then you could read the whole file into a single std::string and use its reverse iterators.

does that use up a lot of ram?

Originally Posted by screwlu

does that use up a lot of ram?

Depends on how big your data is and how you want to process it. Is memory use even a concern for you in this case? There are always tradeoffs to be made between 1) easily understandable code, 2) memory consumption, 3) speed, but unless you're short on 2), I would focus on 1) and then 3).

You can read one buffer-full of data from the end of the file, process that, then read the next buffer-full from before that, etc.

A sketch of the idea (without error handling and with many details still to work out) :

Code:

char buf[BUFSIZ];

fseek(f, 0, SEEK_END);
long pos = ftell(f);
if (pos == 0) /* file is empty */ ;

pos -= pos % BUFSIZ; // make pos a multiple of BUFSIZ

while (pos >= 0) {
    fseek(f, pos, SEEK_SET);
    size_t cnt = fread(buf, 1, BUFSIZ, f);

    // process buf

    pos -= BUFSIZ;
}