int2str returns the wrong result

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Kevin 2015-7-8

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/229081-int2str-returns-the-wrong-result

评论： Kevin 2015-7-8

>> 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?

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Stephen23 2015-7-8

编辑：Stephen23 2015-7-8

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.

http://www.mathworks.com/help/matlab/matlab_prog/floating-point-numbers.html

https://en.wikipedia.org/wiki/Double-precision_floating-point_format

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Answer 1

James Tursa 2015-7-8

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/229081-int2str-returns-the-wrong-result#answer_185442

编辑：James Tursa 2015-7-8

在 MATLAB Online 中打开

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

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James Tursa 2015-7-8

Well, you can't do a simple copy-paste since MATLAB will assume a double as you found out. So that means a bit more caution on your part. E.g., read as a string directly from the source, or copy-paste with manually adding the apostrophes, or copy-paste into a file and then read the file, etc.

Kevin 2015-7-8