Reading 64 bit Integer File

Question

I am trying to learn C++ by writing some code by my own and very new in this field.

Currently, I am trying to read and write a 64 bit integer file. I write 64 bit integer file in following way:

ofstream odt;
odt.open("example.dat");
for (uint64_t i = 0 ; i < 10000000 ; i++)
odt << i ;

Can anybody help me how to read that 64 bit integer file (one by one) ? So, far examples I have found, that reads line by line, not one by one integer.

Edit:

ofstream odt;
odt.open("example.dat");
for (uint64_t i = 0 ; i < 100 ; i++)
odt << i ;

odt.flush() ;

ifstream idt;
idt.open("example.dat");
uint64_t cur;
while( idt >> cur ) {
  cout << cur ;
}

I give above code for an example. I have added neccessary exception handling with that. — user1838343, Nov 24 '12 at 09:57
If you're going to use text, separate your writes with whitespace between each, or you'll quickly find it impossible to read them back as you wrote them out. — WhozCraig, Nov 24 '12 at 09:59
@WhozCraig Is it not possible to raed it back by 64 bit chunck ? — user1838343, Nov 24 '12 at 10:01
Your integers are being written as *text*, not as 64-bit chunks. That is normal behaviour when writing to a file using `ofstream`. If you want to write the actual binary bits to the file, you'll have to use something other than `<<`. — Greg Hewgill, Nov 24 '12 at 10:03
@GregHewgill, if I want to write and read the above, what should I do thus file size will be less and I can do read write faster ? — user1838343, Nov 24 '12 at 10:06
It is entirely possible to write your values in byte-form (8bytes per value). If you want portability you'll need to ensure to account endian (big or small) configuration. But as Greg said, you'll need to use the lower-level raw functions. — WhozCraig, Nov 24 '12 at 10:07
You really need to get a good book on C++ and read it - you can't learn a programming language properly through guesswork + trial and error. — Paul R, Nov 24 '12 at 10:08
@user1838343: a good book is the best pointer you'll ever get, by far. — Paul R, Nov 24 '12 at 10:17

WhozCraig · Accepted Answer · 2012-11-25T05:11:28.910

3

If you must use a text file, you need something to delineate the separation of formatted values. spaces for example:

ofstream odt;
odt.open("example.dat");
for (uint64_t i = 0 ; i < 100 ; i++)
    odt << i << ' ';

odt.flush() ;

ifstream idt;
idt.open("example.dat");
uint64_t cur;
while( idt >> cur )
    cout << cur << ' ';

That being said, I would strongly advise you use lower level iostream methods (write(), read()) and write these in binary.

Sample using read/write and binary data (is there a 64-bit htonl/ntohl equiv btw??)

ofstream odt;
odt.open("example.dat", ios::out|ios::binary);
for (uint64_t i = 0 ; i < 100 ; i++)
{
    uint32_t hval = htonl((i >> 32) & 0xFFFFFFFF);
    uint32_t lval = htonl(i & 0xFFFFFFFF);
    odt.write((const char*)&hval, sizeof(hval));
    odt.write((const char*)&lval, sizeof(lval));
}

odt.flush();
odt.close();

ifstream idt;
idt.open("example.dat", ios::in|ios::binary);
uint64_t cur;
while( idt )
{
    uint32_t val[2] = {0};
    if (idt.read((char*)val, sizeof(val)))
    {
        cur = (uint64_t)ntohl(val[0]) << 32 | (uint64_t)ntohl(val[1]);
        cout << cur << ' ';
    }
}
idt.close();

edited Nov 25 '12 at 05:11

answered Nov 24 '12 at 12:08

WhozCraig

65,258
11
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thanks a lot. I am searching google for an hour to find a nice (fast) way to read/write integers to/from a file (need not be a text file) like in 64 bit chunks. But, I haven't find any nice example. What method are you stating, can you give a small example on that ? Sorry, I have very limited idea on C++.. – user1838343 Nov 24 '12 at 12:15
@user1838343 if you're unfamiliar with C++ you should go with this for now, but read about the different methods of stream object, specifically the `ostream::write()` method and the `istream::read()` method. It will come to you over time. – WhozCraig Nov 24 '12 at 12:51
There is a slightly easier hack then your current method, its faster to. – Garret Gang May 10 '19 at 21:51
There exists compiler specific ones for it, ``__builtin_bswap64`` for gcc/clang, ``_byteswap_uint64`` for msvc – visu Feb 18 '21 at 12:21

score 0 · Answer 2 · answered Nov 24 '12 at 10:51

0

You mean something like this?

ifstream idt;
idt.open("example.dat");
uint64_t cur;
while( idt>>cur ) {
  // process cur
}

answered Nov 24 '12 at 10:51

Rontogiannis Aristofanis

8,883
8
41
58

Reading 64 bit Integer File

2 Answers2