sum() missing for certain values

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Anton
Anton 2023-2-20
评论： Anton 2023-2-21
Hello,
I have an issue which i do not understand using the basic function sum(). I have an array with 2 million values consisting of doubles from 0 to 1 with 3 decimals (rounded). I want to display the distribution by counting every possible output except zero and one using the sum function (0.001-0.999). I tried to calculate it with and without a loop but im allways missing the sum for certain values either way. Example code below.
Vector=rand(2000000,1);
Vector=round(Vector,3,"decimals");
numbers=0.001:0.001:0.999;
b=1;
distribution2=double.empty(999,0);
for x=0.001:0.001:0.999
distribution2(b)=sum(Vector==x);
b=b+1;
end
figure()
plot(numbers,distribution2)
title('distribution2 (with loop)')
distribution(:)=sum(Vector==numbers);
figure()
plot(numbers,distribution)
title('distribution')
As u can see I get a feedback with different gaps using the calculation methods as shown above. I checked the gaps (e.g. 0.7) by using the sum function inside the command window and got back a positive feedback
>> sum(Vector==0.7)
ans =
2009
Can someone explain to me why I am having issues calculating the sum for all numbers and why I am getting two different results depending on how I am trying to calculate it?
Thank you and nice Regards
Anton
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Stephen23
Stephen23 2023-2-20
编辑：Stephen23 2023-2-20
Ran in:
"I'm aware that matlab is not able to display exactly 0.7 due to binary flaoting numbers"
MATLAB certainly can "display exactly 0.7", even binary floating point. Lets try it right now:
format short G
0.7
ans =
0.7
"i was not aware that matlab is using different construction for 0.7 inside command window and inside code"
I have never heard that MATLAB uses a "different construction for 0.7 inside command window and inside code", and I strongly doubt that such a "different construction" exists due solely to where the value is defined. I tried it in R2018a:
The two values are exactly the same.
The links I posted earlier introduce floating point number behavior. David Goldberg's article is highly recommended.
Anton
Anton 2023-2-21
Thank you for clarificaton. I'll read the article.

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采纳的回答

KSSV
KSSV 2023-2-20
Don't use
sum(Vector==x)
use
tol = 0.0001 ; % set your acceptable tolerance
sum(abs(Vector-x)<tol) ;

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