Reading 64 bit Integer File

4
votes

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 ;
}
2
c++file-iouint64
I give above code for an example. I have added neccessary exception handling with that. – user1838343
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
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
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
@user1838343: a good book is the best pointer you'll ever get, by far. – Paul R

2 Answers

3
votes

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();
0
votes

You mean something like this?

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