int2str returns the wrong result

2015 7 8
1 个回答
回答已采纳
更新时间：2015 7 8
8 次查看（30 天）

0 个投票

>> 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 个评论
显示 -1更早的评论 隐藏 -1更早的评论

Stephen23
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

请先登录，再进行评论。

请先登录，再回答此问题。

 采纳的回答

James Tursa
James Tursa 2015-7-8
编辑：James Tursa 2015-7-8

1 个投票

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 个评论
显示 3更早的评论 隐藏 3更早的评论

Guillaume
Guillaume 2015-7-8
It has too many digits to store it in any of the core matlab data types! It needs at least 166 bits to represent it as an integer.
You will have to defer to the Symbolic Math Toolbox, or java.BigInteger
James Tursa
James Tursa 2015-7-8
Or vpi in the FEX by John D'Errico.
Kevin
Kevin 2015-7-8
编辑：Kevin 2015-7-8
Thank you for the reply, that does make sense.
But if one is pasting in a large block of numbers, what are the options for defining it as a string besides manually typing in apostrophes.
Perhaps I'll include the problem I am solving to make this clearer: https://projecteuler.net/problem=8
My goal was to create a 1-dimensional array of integers in which each element is a number from that big block of 1000 digits. I figured I could do this simply if the digits were a continuous string and I could then go character by character with str2num (or str2double).
James Tursa
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
Kevin 2015-7-8
Yes, I figured pasting into a file would probably be the proper thing to do but I was hoping there might be some shortcut.

请先登录，再进行评论。

更多回答（0 个）

类别

在 帮助中心 和 File Exchange 中查找有关 Logical 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by