MATLAB: Are there any functions for taking the most/least significant digits from a given value

MATLABrandom number generatorsignificant-digits

Title kinda says it all.

If there isn't a function, can anyone think of a way of writing one to do it efficiently? I'm generating long streams of outputs (experimenting with random number generator implementations) and don't want to bog the whole thing down running a ton of weird arithmetic just to lop a few digits off one end of the output.

Thanks in advance

Will

Best Answer

If I'm correctly understanding your question you would like to extract some digits from a number.

The mathematical approach is simply based on multiplication/division by powers of 10 and then using REM or MOD you can extract the number you are interested in.

>> A = 123.456;
>> rem(A,1)
ans =
      0.4560
>> rem(A,10)
ans =
      3.4560

You can then rescale this number appropriately to your needs.

Another way to work on the position is simply to convert the number into a string with NUM2STR and then index into the string to extract the desired value. String conversion has an additional computational cost but may be easier if you just need to operate on digits. With STR2DOUBLE you can convert the value back if you need to do additional arithmetic operations.

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MATLAB: Extracting a specific digit from a float w/o floating-point implementation rounding issues

Some people will incorrectly tell you that this is not possible, but in reality there is a simple and efficient solution:

>> N = 130404809.288;
>> mod(round(10000*N),10)
ans =  0
>> N = 130404809.2882;
>> mod(round(10000*N),10)
ans =  2
>> N = 136147309.434;
>> mod(round(10000*N),10)
ans =  0

This relies on your statement that the values have "with either three or four digits to the right of the decimal point", i.e. that the fifth digit is zero (the sixth and any further digits, including any floating point error, are totally irrelevant). This means that after multiplying by 10000 (NOT by 1000 as you tried) a simple round will remove all floating point error from the number:

xyz.abcd0err % input
xyzabcd.0err % *10000
xyzabcd      % round
      d      % mod 10

You were almost there, just off by a factor of ten.

MATLAB: Convert int to string without using int2str()

Of course, it's possible. How do you write a number? You write down each decimal digit one by one. Just do the same: Determine each decimal digit of your integer and convert it to a character. A very crude way of doing it is:

%only work for unsigned integer. it's trivial to test for sign beforehand and work on the absolute value
str = '';
while number > 0
   digit = mod(number, 10);
   str = [digit + '0', str]; %digit + '0' convert an integer between 0-9 into the corresponding character
   number = floor(number / 10);
end

Best Answer

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MATLAB: Extracting a specific digit from a float w/o floating-point implementation rounding issues

MATLAB: Convert int to string without using int2str()

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