Extracting representative bits for an integer and doing the reverse.

I have an integer value and want to extract all its representative bits in two-bit blocks; each block consists of two bits. Reversely, I want to build an integer value from given two-bit blocks. What is the more optimized code performing these tasks?

Answer 1

[Digitalsd]

For Positive integer Values,By the use of bitwise operations, and,or,shift:

 % Decomposition:
 a=4235; % any Positive Integer <= 65535 : Up to 2byte.
 a1=a;
 block=[0,0,0,0,0,0,0,0];
 for k=1:8
    block(k) = bitand(a1, 3);
    a1 = bitshift(a1, -2);
 end
 % Reversely: The Composition
 % We have the resulted block.
 b=0; % Integer
 for k=1:8
    b = bitshift(b, +2);   
    b = bitor(b,block(9-k));
 end
 comp=isequal(a,b);

Answer 2

[Image Analyst]

% Say, for 171 = AB = 10101011.
str = dec2bin(171)
theBlocks = reshape(str, [2 4])'

In the command window:

str =
10101011
theBlocks =
10
10
10
11

To reconstruct from theBlocks:

reconstructedString = reshape(theBlocks', [1 8])
reconstructedInteger = bin2dec(reconstructedString)

In the command window:

reconstructedString =
10101011
reconstructedInteger =
   171

Is that what you mean?

[Digitalsd]

OK, Here the blocks are in string type, How I can convert them to integer type. Finally, they should be 0, 1, 2 or 3 values. And the reverse. Regards

[Walter Roberson]

subtract '0' (the string which is the character for the digit 0) to convert to integer type.

Multiply the first of each pair by 2 and add the second of each pair, to get 0, 1, 2, or 3.

[Jan]

Please post example data in Matlab syntax. "Blocks" and "string type", "integer type", "0,1,2,3 values" and "reverse" is not exactly clear, such that an answer needs too much guessing.

[Digitalsd]

Exactly, I need to convert any integer value to smaller integer values belonging to the group {0,1,2,3}.

[Walter Roberson]

Any integer? What about negative integers?

[Image Analyst]

How about smallNumber = mod(yourBigNumber, 3). That will give 0-3 for any integer. I totally agree with Jan that you're not being clear about what you want, hence my answer now being totally different than what I first gave. Why don't you give a question that can be answered the very first time without us guessing and trying numerous times to help you?

[Walter Roberson]

I'm pretty sure that the poster is trying to do base 4 encoding / decoding.

[Digitalsd]

The answer of image analyst was close to what I wanted but it gives the smaller elements in string type: ‘00’,’01’,’10’,’11’. Instead I need these elements as integer: 0, 1, 2, and 3. In other words I want to decompose the integer to smaller representative integers each has two bits, and from these small integers I can reconstruct the original big integer. To Walter Roberson, it is positive integer. Anyway, I’ve written and tested a code doing exactly what I need,it is done using the bitwise operations, and,or,shift:

% Decomposition:

a=4235; % any Integer <= 65535 : Up to 2byte.
a1=a;
block=[0,0,0,0,0,0,0,0];

for k=1:8

   block(k) = bitand(a1, 3);
   a1 = bitshift(a1, -2);

end

% Reversely: The Composition:
% We have the resulted block.

 b=0; % Integer

for k=1:8

   b = bitshift(b, +2);   
   b = bitor(b,block(9-k));

end

 comp=isequal(a,b);

Best Regards