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int2str returns the wrong result

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Kevin
Kevin on 8 Jul 2015
Commented: Kevin on 8 Jul 2015
>> int2str(73167176531330624919225119674426574742355349194934)
ans =
73167176531330625585928818171611107149744948903936
Anyone know what's going on here? I'm guessing the number is too because when I evaluate the difference between the input and output, the answer is 0.
What causes Matlab to fill in nonsense numbers though?
  1 Comment
Stephen
Stephen on 8 Jul 2015
double numbers, the default in MATLAB and many other programming languages, only support between 15 and 17 significant figures, the rest of those digits are meaningless.

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Accepted Answer

James Tursa
James Tursa on 8 Jul 2015
Edited: James Tursa on 8 Jul 2015
Your original number has too many digits to store in a double precision number. E.g.,
num2strexact(73167176531330624919225119674426574742355349194934)
ans =
7.3167176531330625585928818171611107149744948903936e49
So what you are seeing as output is exactly what is being stored (i.e., the exact decimal conversion of the binary bit pattern being stored), and there aren't enough mantissa bits in double precision to store more precision.
In fact, the nearest numbers in the double precision set to your original number are:
>> x = 73167176531330624919225119674426574742355349194934
>> num2strexact(x)
ans =
7.3167176531330625585928818171611107149744948903936e49
>> num2strexact(x+eps(x))
ans =
7.3167176531330635970522535241266364210737607344128e49
>> num2strexact(x-eps(x))
ans =
7.3167176531330615201335101101955850088752290463744e49
  5 Comments
Kevin
Kevin on 8 Jul 2015
Yes, I figured pasting into a file would probably be the proper thing to do but I was hoping there might be some shortcut.

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