MATLAB: Efficient way to get MSB position of a double variable

bitgetbitshiftmsb

I am currently using the following equation to get the index of the most significant bit (MSB) of an integer value stored in a double variable.

xMsbIdx=ceil(log2(abs(x+1)))

I would like to know if there is a more computationally efficient way of doing it.

Edit for clarification propose

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My guess is you're trying to get the index of the most significant bit of an integer, not a floating point double (where a number is divided in two parts, mantissa and exponent, so not sure what you'd call the msb).

To get that value:

xMsb = nextpow2(x);

is simpler but probably not as fast as your algorithm. You can look at the code for nextpow2, it is implemented as m file.

Related Solutions

MATLAB: How binary floating point to real decimal number representation

Yes, as Stephen said, be careful about the difference between binary numbers and IEEE storage. You can easily see the IEEE representation of a double number in matlab with:

>> n = 1.9844
>> dec2bin(typecast(n, 'uint64'), 64);  %typecast to uint64 doesn't change the bits, so we can extract the bit pattern
ans = 
    '0011111111111111110000000001101000110110111000101110101100011100'

As for your question, the computer is never aware of the decimal representation of the number, and doesn't care about it. All math is done with the binary representation. Which is why people sometimes are surprised by the result they get (see why 0.3-0.2-0.1 is not 0) because the binary math differs from the decima math. It's only for display to the user/programmer that the numbers are converted to their textual decimal representation. This is done by well established code routine, if you want to know more search for the source code of fprintf (in C). Similarly, the textual decimal representation of numbers you enter is immediately converted to the binary representation.

MATLAB: The mantissa of a floating point number??

It's the fractional part of a real number.

http://mathworld.wolfram.com/Mantissa.html

In MATLAB, you can calculate the mantissa of x as

>> x - floor(x)

There are lots of places to read about floating point numbers and their representation. Here's one:

http://www.mathworks.com/company/newsletters/news_notes/pdf/Fall96Cleve.pdf

Please see James Tursa's answer for a cautionary note about the two different definitions of mantissa! Since you asked about binary form, I expect you mean the one from the Cleve Moler article, not the Mathworld article definition (or the formula I gave).

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MATLAB: How binary floating point to real decimal number representation

MATLAB: The mantissa of a floating point number??

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