Find the number of digits after the decimal point in a number, please help!

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SB 2012-11-2

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https://ww2.mathworks.cn/matlabcentral/answers/52590-find-the-number-of-digits-after-the-decimal-point-in-a-number-please-help

回答： Chris Taylor 2023-7-26

Say you have a number, x, and you want to find the number of digits after the decimal point. I know that in matlab, one way to do it would be to convert the number to a string and find the length from after the decimal point, but I think num2str has issues. How would you find the number of digits after the decimal point in a number then (the question is complicated by the fact that the numbers are floating point).

    %
function y = decimalpoint(x)
x = abs(x); %in case of negative numbers
y = 0;
while (floor(x)~=x)
y = y+1;
x = x*10;
end

This code doesn't work for some cases, such as decimalpoint(11.111). How would I make the code work for all cases? I think recursion could be useful too, but probably not necessary.

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Daniel Shub 2012-11-2

This is essentially a duplicate of http://www.mathworks.com/matlabcentral/answers/52068-machine-precision-problem-in-code

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采纳的回答

Azzi Abdelmalek 2012-11-2

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https://ww2.mathworks.cn/matlabcentral/answers/52590-find-the-number-of-digits-after-the-decimal-point-in-a-number-please-help#answer_64099

编辑：Azzi Abdelmalek 2012-11-2

在 MATLAB Online 中打开

x=11.111
x = abs(x); %in case of negative numbers
n=0
while (floor(x*10^n)~=x*10^n)
n=n+1
end

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Azzi Abdelmalek 2012-11-2

编辑：Azzi Abdelmalek 2012-11-2

在 MATLAB Online 中打开

 x=11.111*(1-eps)
 x*10^4=1.1111e+05
 floor(x*10^4)=111109

and

x*10^5= 1111100

I know it has something to do with the way Matlab codes floting point numbers. Errors caused by quantification

Daniel Shub 2012-11-2

The point is that MATLAB cannot represent any of these numbers exactly. Therefore the number of decimal places is poorly defined.

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更多回答（1 个）

Chris Taylor 2023-7-26

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/52590-find-the-number-of-digits-after-the-decimal-point-in-a-number-please-help#answer_1279017

在 MATLAB Online 中打开

a = 11.111;
b = numdecpoints(a);
function f = numdecpoints(x)
i = -1;
X = 0;
while X ~= x
    i = i + 1;
    X = round(x, i);
end
f = i;
end

This seems to work, making use of MATLAB's round function. Not sure if it is the most efficient but seems to work for me. Simply compare the output of your original number and the number rounded to i decimal places. When the values match, i is the number of demical places in the orginal number.