C#: Converting 24-bit integer into a three byte array?

Question

So basically here is what I have. The user gives an integer and I'm converting it into 3 bytes.

int number = 167123;

byte[0] = (int)number / 65536;
byte[1] = (int)number / 256;
byte[2] = (int)number;

stream.Position = 0x503;   
stream.WriteByte((byte)byte[2]);
stream.WriteByte((byte)byte[1]);
stream.WriteByte((byte)byte[0]);

(Note: I'm cycling through the byte array backwards on purpose at the end.)

When I check the value later it works as intended. Now I'm looking hard at the code and trying the calculation by hand and I'm not getting the right answer. What am I doing wrong? How is this working? And what is Visual C# writing into the third byte when it casts 167123 as a "byte"?

just saying you are casting a byte as a byte which is unnecesary. Also is the question what is happening behind the scenes? — Nomad101, Apr 28 '13 at 23:10
After some more testing my question is mostly, what is happening when an integer larger than 255 is cast as (byte)? For example, when 167123 is cast as (byte) it is changed to 211. — Binarynova, Apr 28 '13 at 23:25

score 2 · Accepted Answer · answered Apr 28 '13 at 23:20

The reason this works is because the assignment of the int value to the byte truncates the value. This may be why your math isn't working out - you're not truncating.

Essentially what you're doing by dividing is bitshifting. Your code is the same as this:

byte[0] = (int)number >> 16;
byte[1] = (int)number >> 8;
byte[2] = (int)number;

To make your manual math work, do the math, then convert it to binary, and chop off anything above the last 8 digits. That's the number you're assigning to the byte array.

One example:

byte[1] = (int)number / 256;

This is 167123 / 256 = 652. In binary, this is 001010001100. Now, truncate everything above the size of a byte (8 bits), and you have 10001100, which is 140 in decimal. This is what is assigned to this byte array index.

score 1 · Answer 2 · edited May 23 '17 at 12:04

1

Try to use this method instead: BitConvet.GetBytes(int). More about this problem is also available in this question.

edited May 23 '17 at 12:04

Community

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answered Apr 28 '13 at 23:17

filipko

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C#: Converting 24-bit integer into a three byte array?

2 Answers2