Strange behavior of if statement

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martino corrà
martino corrà 2021-4-15
评论： martino corrà 2021-4-15
i have this very simple code that should return 'ans = 1' when run, since the sum of d is 1. However it returns the error message specified in the 'else' section instead, as if d didn't add up to 1. What's going on?
d = [.4 .1 .1 .1 .1 .1 .1];
if sum(d) == 1
sum(d)
else
error('d doesnt add up to 1')
end

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采纳的回答

Rik
Rik 2021-4-15
Ran in:
Welcome to floating point integers.
Matlab stores these numbers in a binary representation with a finite precision. Similar to what would happen if you only store 4 decimals and do this: (1/3)*3=0.3333*3=0.9999.
You should probably compare to a tolerance:
d = [.4 .1 .1 .1 .1 .1 .1];
if abs( sum(d)-1 ) <= 2*eps
sum(d)
else
error('d doesnt add up to 1')
end
ans = 1.0000
  1 个评论
martino corrà
martino corrà 2021-4-15
ah I see, thank you soo soo much, i've been trying to figure out what was going on for hours! It seemed like matlab was mocking me! now i understand. Thanks a lot!

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更多回答（1 个）

DGM
DGM 2021-4-15
编辑：DGM 2021-4-15
It's correct. The sum isn't 1. It's approximately 1.
d = [0.4 0.1 0.1 0.1 0.1 0.1 0.1]; % leading zeros for readability, pls
sum(d) % that sure looks like 1
sum(d)-1 % but you just don't have the display resolution to see the error
gives
ans =
1.0000
ans =
-1.1102e-16
Beware simple equality tests when using floating point numbers. Test to within a tolerance.
tol=1E-12;
if abs(sum(d)-1)<tol
sum(d)
else
error('d doesnt add up to 1')
end

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