What is the meaning of this pData[1+2*i]<<8|pData[2+2*i] C++ syntax?

what is the meqaning of pData[1+2*i]<<8|pData[2+2*i] where pData[ ] is the array containing BYTE data? I have the following function in the main function

{
..........
....
BYTE Receivebuff[2048];
..
ReceiveWa开发者_JAVA技巧vePacket(&Receivebuff[i], nNextStep);
....
...
..
}

Where Receivebuff is the array of type BYTE.

ReceiveWavePacket(BYTE * pData, UINT nSize)
{
 CString strTest;
 for(int i = 0 ; i < 60 ; i++)
 {
  strTest.Format("%d\n",(USHORT)(pData[1+2*i]<<8|pData[2+2*i]));
  m_edStatData.SetWindowTextA(strTest);
          }
}

I want to know the meaning of ",(USHORT)(pData[1+2*i]<<8|pData[2+2*i]).

Can any body please help me?

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This seems to be code for synthesizing a 16-bit value out of two eight-bit values. If you'll note, the math has the form

(a << 8) | b

For suitable a and b. This first part, (a << 8), takes the eight bits in a and shifts them up eight positions, giving a 16- bit value whose first eight bits are the bits from a and whose second eight bits are all zero. Applying the bitwise OR operator between this new value and the value of b creates a new sixteen-bit value whose first eight bits are the bits of a (because zero-extending b for the OR step leaves these bits intact) and whose lower eight bits are the bits of b, since ORing zero bits with the bits of b yields b.

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I think @templatetypedef is correct, it does look like code that creates a 16-bit value by logical OR'ing two 8 bit values. ( since it's OR'ing two elements from your array of BYTE's ).

Logical OR'ing of two bits basically means, if either of the bits is 1 then the result is 1 ("this bit OR that bit").

As a further example, take a look at this function that takes a pointer to a char (that is 8 bits in size), and returns an integer ( in this implementation that means 32 bits).

int Read32(const char *pcData)
{
    return ( (pcData[3]<<24) & 0xff | pcData[2]<<16 | pcData[1]<<8 | pcData[0]);
}

If pcData is a pointer to an array of char - then it takes the 3rd char it finds and it shifts it by 24 bits:

e.g.

if pcData[3] was 10110001 then it is now

10110001000000000000000000000000

It takes this 32 bit value and subsequently OR's it with pcData[2], shifted by 16 bits - which means that if pcData[2] is 11111111 then the 32 bit value is now:

10110001111111110000000000000000

and so on with pcData[1] and pcData[0].

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@templatetypedef's answer is correct, but there is another interesting thing going on here:

(pData[1+2*i]<<8|pData[2+2*i])

i goes from 0 to 59, so this will process 60 words, starting with pData[1] (1+2*0 == 1).
This means that the first byte in the array is never processed, which seems odd. Why isn't this the more natural pData[2*i]<<8|pData[2*i+1]?

One possibility: Word data can be stored in a byte stream 2 ways: the word 0xAA11 can be stored as 0xAA 0x11 or 0x11 0xAA. Imagine it is the latter.

For the words : 0xAA11 0xBB22 0xCC33 ...
The byte stream will be 11 AA 22 BB 33 CC ...

Parsing with the 'natural' method would give 0x11AA 0x22BB..., which is obviously wrong.

This code will print 0xAA22 0xBB33, 0xCC44 ...., which will probably pass a quick-look sanity check, but is actually totally incorrect.

I hope the extra +1 wasn't added to "fix" the endianness issue.