Extracting N-bit signed integer from a byte array

Question

I have a byte array of variable size (3-5). I need to extract N-bit signed integer stored as two's complement having a range of -2^(N-1) + 1 to +2^(N-1) – 1 How can I do that?

Vulpes · Answer

Well in .NET for Windows signed integers are stored in little endian two's complement format in any case so as long as the last byte in the array is the most significant then a four byte array will map directly to an Int32. However the three and five byte arrays will need some manipulation to find out what number they represent as there are no corresponding integral types in .NET. Incidentally the range you mentioned is for one's complement format. For two's complement the range is: -2^(N-1) to +2^(N-1) – 1 Subject to all that I think the following should work though I'd test it thoroughly: using System; class Test { static void Main() { byte[] ba = {0 0 0 128}; Console.WriteLine(GetInteger(ba)); byte[] ba2 = {255 255 255 127}; Console.WriteLine(GetInteger(ba2)); Console.WriteLine(); byte[] ba3 = {0 0 128}; Console.WriteLine(GetInteger(ba3)); byte[] ba4 = {255 255 127}; Console.WriteLine(GetInteger(ba4)); Console.WriteLine(); byte[] ba5 = {0 0 0 0 128}; Console.WriteLine(GetInteger(ba5)); byte[] ba6 = {255 255 255 255 127}; Console.WriteLine(GetInteger(ba6)); Console.ReadKey(); } static long GetInteger(byte[] ba) { switch(ba.Length) { case 4 : return (long)BitConverter.ToInt32(ba 0); case 3 : byte[] temp3 = {ba[0] ba[1] ba[2] 0}; if(temp3[2] >= 128) temp3[3] = 255; return (long)BitConverter.ToInt32(temp3 0); case 5 : byte[] temp5 = {ba[0] ba[1] ba[2] ba[3] ba[4] 0 0 0}; if(temp5[4] >= 128) { for(int i = 5; i } return BitConverter.ToInt64(temp5 0); default : throw new ArgumentException(Byte array must have length 3 4 or 5); } } } The output which should represent the minimum and maximum values of the 4 3 and 5 byte numbers is : -2147483648 2147483647 -8388608 8388607 -549755813888 549755813887