Question

I have a byte array of 32 bytes where the first 4 bits of each byte (0 to 3) represent the set or unset state of a number between 1 and 128. For example, If I am given the number 3, I need to set bit 2 in the first byte in the array. If I am given the number 9, I need to set bit 0 of the third byte in the array. The problem I have is finding a sensible way to do this in C#. I’m sure there must be a simple way to do it mathematically but so far haven’t been able to find a method. Whilst I scratch my head on this one I thought I’d see if anyone can give some advice.

——— Update ——————-

Based on the answers given, I have produced the following function. This does exactly what I need. I may not have made it clear in my question what I needed but enough advice was given for me to find the right code.

// outputNumber = number passed into this function

byte[] bytes = new byte[32];

int bit = (outputNumber - 1) % 4;

byte byteSetting = (byte)(1 << bit);

bytes[(outputNumber - 1) / 4] |= byteSetting;

Answer 1

int byt = bitNumber / 4; // You could do byt = bitNumber >> 2
int bit = bitNumber % 4; // You could do bit = bitNumber & 3

bytes[byt] |= (byte)(1 << bit);

Where bytes is your byte array.

To reset it:

bytes[byt] &= (byte)(byte.MaxValue ^ (1 << bit));

To read the value of a byte:

var res = bytes[byt] & (byte)(1 << bit)

(if you are interested, ^ is the xor operator)