Add a number to a byte array?

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This sounds so easy to do but I (probably stupidly) can't workout how to do it in VB.net.

I have a byte array of 32 bytes and all I want to do is add a byte to it.

eg.

010.......011101011000001

plus 00000001 becomes

010.......011101011000010

What I have tried:

I first thought I could just add to the last byte in the array, but at times it will need to be carried over into the next byte.

I also tried converting it into an Integer, but 32 bytes can often exceed the largest Integer limits.

Posted 11-Dec-17 8:00am

codetowns

Updated 11-Dec-17 20:22pm

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Afzaal Ahmad Zeeshan 11-Dec-17 14:23pm

So? What is the question?

001 + 001 = 010 in binary, where 1 is carried over to the next addition and so on. Consider reading a bit about the binary number system and arithmetics for it.

codetowns 11-Dec-17 14:59pm

Yes, but what happens when 11111111 has 1 added to it? The subsequent byte would then need to be added to.

Is the only solutions to iterate each byte until there is no contribution from the last one?

Richard MacCutchan 11-Dec-17 15:23pm

Create a new array one byte larger, and copy the old to the new.

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OriginalGriff · Accepted Answer · 2017-12-11T20:22:00

Solution 1

I assume you are writing a "binary" data type of some form: if so, the simplest solution is to handle it in a loop and do the math yourself:

byte[] inp = new byte[32] { 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 1, 0, 1, 1, 0, 0, 0, 0, 0, 1 };
byte b = 1;
for (int i = 31; i >= 0; i--)
    {
    b += inp[i];
    inp[i] = (byte)(b % 2);
    b = (byte)(b / 2);
    }

Quote:
Sorry the dots were supposed to show total width as up to 8*32=256 bits. I didn't explain myself well enough.

Ah... details are important!
That makes it simpler:

C#

byte[] inp = new byte[32] { ... };
int carry  = 1;   // Value to add on.
for (int i = 31; i >= 0; i--)
    {
    carry += inp[i];
    inp[i] = (byte)carry;
    carry >>= 8;
    }

Posted 11-Dec-17 20:22pm

OriginalGriff

Updated 13-Dec-17 0:01am

v2

Comments

codetowns 12-Dec-17 11:46am

Thanks. My C# is poor but I think I understand the idea - I have to loop it and add it manually.

OriginalGriff 12-Dec-17 11:53am

Yes - but is there a good reason why you are doing this? It's a bit of a cack-handed way to do binary yourself! :laugh:

codetowns 12-Dec-17 12:23pm

Cack-handed is right! It revolves around the fact that I have a 32 Byte array which I have to add a byte to. I'm surprised .NET doesn't let me do it in one line!

OriginalGriff 12-Dec-17 12:35pm

Yeah - but why do you have 32 bytes to store 32 bits? Why not store them in an integer (32 bits) and cut out the middle man?

That's why .NET doesn't do anything for you here - it's a strange (and very inefficient) way to store info.

codetowns 12-Dec-17 14:38pm

No, it's 32 bytes with 8 bits in each byte.

OriginalGriff 13-Dec-17 4:47am

Then what you are describing isn't right: adding a byte containing one to an array of bytes containing 010.......011101011000001 does not give you an array of bytes containing 010.......011101011000010 - it gives you an array of bytes containing 010.......011101011000002

codetowns 13-Dec-17 5:51am

Sorry the dots were supposed to show total width as up to 8*32=256 bits. I didn't explain myself well enough.

OriginalGriff 13-Dec-17 6:01am

Answer updated.