Comparing part of a Byte

Projects Programming
1

Hey guys,

I'm trying to compare the first number of a byte, instead of checking the whole byte to reduce code.
The situation is as follow:

I'm comparing an incoming message (byte array) to another byte array.
I need to know if the first number in the 4th byte is a 2. It doesn't matter wat the next number in that byte is.
Usually the byte will be 0x20, 0x21, 0x22 or 0x23

Is there a cleaner way to check this, rather than:

byte indicatorDirection = packet[4];

 if (indicatorDirection == 0x20 || indicatorDirection == 0x21 || indicatorDirection == 0x22 || indicatorDirection == 0x23) {

}

I've been thinking I could also just work with it as a decimal, since the value would then always be between 32 and 35, but I just wanted to know if it's possible to 'extract' the first part of the byte.

all help appreciated!

2

If ((IndicatorDirection & 0xF0)==0x20)

Bitwise and with 0xF0 to mask off the low half of the byte.

Or

If ((IndicatorDirection >> 4)==0x02)

Rightshift 4 bits (losing the low 4 bits) and compare with the target value similarly rightshifted.

The bitwise and bitshift operators are incredibly useful in embedded applications, for exactly this kind of thing, learn and use them.

3

0x20 is 32 in decimal, and 0x23 is 35. 0x2F is 47. So you can try

if (32 <= indicatorDirection && indicatorDirection <= 47)
4

Thanks for the insanely quick response guys!

I now remember about bitshifting which I learned during a programming lesson many years ago. Thanks!

Will test which solution required less memory.

5

Comparing bitwise with & 0xF0 requires least space.

Comparing in decimal requires 4 more bytes in sketch.

Comparing by rightshifting 4 bits requires and additional 12 more bytes.