Looping through bytes of a variable

I'm doing a genetic algorithm with real parameter values. Say that I have two candidate solutions:

x(1) = 1.2345678
x(2) = 8.7654321

And I want to perform a crossover. I pick a random location along the length of the variables using randi and get, say, 3. Then I want to switch the values before the 3rd byte such that my output would be:

y(1) = 1.2654321
y(2) = 8.734567

How would you program this?

[Guillaume]

Is there a confusion here and are you looking at swapping digits in a decimal representation rather than bytes? Swapping bytes will result in numbers that bear absolutely no resemblance to the original numbers. For example, if you swap the 8th bytes of your two numbers, you'll end with:

>>format longg
>>x = [1.2345678; 8.7654321]
x =

                 1.2345678
                 8.7654321  

>>y = reshape(typecast(x, 'uint8'), 8, []);  %swapping 8th byte
>>y(8, [2 1]) = y(8, [1 2]);
>>newx = typecast(y(:), 'double')
newx =  

             80908.6353408
      0.000133749879455566

Answer 1

[Guillaume]

From your example, I'm not sure that you actual want to swap the bytes of the number, but actually want to swap the decimal digits of the numbers. Swapping bytes, = 8 bits of the number represented in binary, makes absolutely no sense to me.

Assuming you want to swap digits after the decimal:

x = [1.2345678; 8.7653421];
decimaltoswap = 2;

y = x - mod(x, 10^(1-decimaltoswap)) + mod(x([2 1]), 10^(1-decimaltoswap))

Answer 2

[Walter Roberson]

y8 = reshape( typecast(y, 'uint8'), 8, []);

Now you can cross-over on the rows.

Afterwards you can typecast(y8, 'double')

[Caleb]

Is there any way that I can loop through the values byte-by-byte. Something more like:

nBytes = 8;
CutLocation = randi([1,nBytes]);
for i = 1:nBytes
  if i < CutLocation  
      y1(i) = x1(i);
      y2(i) = x2(i);
  else
      y1(i) = x2(i);
      y2(i) = x1(i);
  end
end

Obviously, i would have to be looping through bytes instead of an index of a vector, but hopefully you get the idea.

[Walter Roberson]

x12 = reshape( typecast(x, 'uint8'), 8, []);
x1 = x12(:,1);
x2 = x12(:,2);

Followed by your loop.

Or skip the loop and do

x12 = reshape( typecast(x, 'uint8'), 8, []);
x12(CutLocation:end, [1 2]) = x12(CutLocation:end, [2 1]);
y12 = typecast(x12, 'double');

The reconstructed values will be y12(1) and y12(2)