Adding Bytes (Combining 2 x 8bits into 16bits)

philspitler

Hi, I'm reading data from a text file and want two join to bytes together and read the value as an INT.

So the first byte I read is

0x02

and the 2nd byte I read is

0x58

I would like to convert 0x0258 into an int which would be 600

If I just add them together it doesn't work out because it is simply adding the digits together instead of making a longer digit. I hope this makes some kind of sense.

Here is what I have,

    byte b1 = b[15];
    byte b2 = b[16];

    println (hex(b1));
    println (hex(b2));

    numPixels = byte(b1) + byte(b2);
    println(numPixels);

I'm sure it's pretty simple but searching the web isn't getting me very far.

Thanks.

Phil

TfGuy44

Here's one way. I'm sure you could do it with addition and bitshift operators (<<) too, but this is the most clear to me.

byte b1 = byte(unhex("be"));
byte b2 = byte(unhex("ef"));

println (hex(b1));
println (hex(b2));

int cow = int(unhex(hex(b1) + hex(b2)));
println(hex(cow));

philspitler

That worked great, thanks

Phil

kfrajer

Another easy way:

byte b1 = 0x2;
byte b2 = 0x58;

int numPixels = (b1<<8) + (b2);
println(numPixels);

https://processing.org/reference/leftshift.html

Kf

philspitler

Excellent, thanks.

Phil

prince_polka

Like kfrajer's but will also work for negative bytes.

int numPixels = ((b1 << 8)&0xff00) | (b2&0xff);  

philspitler

Actually I get different results if I use the reply from kfrajer.

Here is my code with the results from each method.

  byte b1 = led[15];
  byte b2 = led[16];

  numPixels_v1 =  (unhex(hex(b1) + hex(b2)));
  numPixels_v2 = (b1<<8) + (b2);
  numPixels_v3 = ((b1 << 8)&0xff00) | (b2&0xff);  


  println (numPixels_v1); // returns a value of 1200
  println (numPixels_v2); // returns a value of 944 
  println (numPixels_v3); // returns a value of 1200

I have a solution but curious what the difference is.

Thanks.

Phil

prince_polka

The difference happens because of negative bytes. For example -1 is 0xff as a byte but 0xffffffff as an int.
So it you want -1 + -1 to be 0xffff, then you have to mask it.
The + operator should work the same as the | operator as the masks don't overlap, but I switched to | just to be sure.