How to copy 2 bytes from a file an put into result variable?

[Leannie]

Hi! I don't know if I am even wording this right but here goes. I have to write a programme to read two data files into C++, then ouput as one file in the same format.

The two files are files of 10 bit data. Each 10-bit recorded value is represented by a 16 bit word, of which the top 6 bits are not in use.

I have managed to read in the files using "ifstream inFile(. . . . .)"

I want to ouput the result as ((data1-data2)/(data1+data2))*1000.

I have tried using "inFile.get()" but that only takes 1 byte from the file and puts it into my result variable. Hence I end up with every value as 00. I want to take the first byte and times by 256 (100 in hexidecimal) and add the 2nd byte on.

My problem is how do I copy 2 bytes of data?

I know it is something to do with byte swapping, lobyte and high byte and I want C=A*256+B where every A is 00. But I have no idea how to do this?

Any help would be great,
thanks in advance :)

 

[Mark44]

When you use the ifstream constructor, in what mode are you opening the file? It seems to me you should be opening it in binary mode, not in text mode.

 

[Leannie]

Hi! I am opening it in binary format as below, I do the same with both files so do this twice.

ifstream inFile1("h:\\data1.dat",ios::in,ios::binary);

I don't know how write code to multiply the first byte by 256 and add on the second byte though. I want to place this new value in my result variable so that I can output my results as required.

Thanks

 

[Mark44]

You need to read two bytes from the file, and then separate these into high and low bytes.

Let's suppose you have read a value into an unsigned short int varible named input_data. This data type should be two bytes or 16 bits, but you should check that.
To get the low byte and high byte, use the bitwise "and" operator, &, with suitable bit masks.
lo_byte = input_data & 0xFF;
hi_byte = input_data & 0xFF00;

The first statement above puts the low 8 bits into lo_byte. The second statement puts the high 8 bits into hi_byte.

To get back the number, do this:
number = 256 * hi_byte + lo_byte;

 

[Leannie]

Thanks very much! I will try that as soon as I go into uni tomorrow :D Glad you knew what I meant, I'm rubbish at wording things. Thanks again, think that's exactly what I'm looking for :)