logical less than thinks values are the same?

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David Pesetsky
David Pesetsky 2018-3-31
评论： Walter Roberson 2018-4-1
I have wracked my brain over this one. It must be a dumb mistake.
if scalarMin < 1.0
fprintf('*scalarMin %f.\n',scalarMin);
end
scalarMin is exactly 1.000000, but it insists on printing anyway. Just want an output when it's actually <1.
What could it be?
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John D'Errico
John D'Errico 2018-3-31
format long g
x = 1 - eps
x =
1
It sure looks like 1. But is it?
sprintf('%0.55f',x)
ans =
'0.9999999999999997779553950749686919152736663818359375000'
No, it is not 1. And MATLAB confirms that fact.
x < 1
ans =
logical
1

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回答（1 个）

John D'Errico
John D'Errico 2018-3-31
编辑：John D'Errico 2018-3-31
You don't seem to believe what we are telling you. Here is 1.
num2hex(1)
ans =
'3ff0000000000000'
That is 1. Creating something slightly smaller than 1? Try this:
num2hex(1-eps)
ans =
'3feffffffffffffe'
Just because a number displays at the command line as looking like 1, it need not be exactly 1.
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John D'Errico
John D'Errico 2018-4-1
编辑：John D'Errico 2018-4-1
Just a teeny tiny, itsy bitsy amount less than 1. But not 1.
sprintf('%0.55f',hex2num('3fefffffffffffff'))
ans =
'0.9999999999999998889776975374843459576368331909179687500'
As much as it looks like 1,
format long g
hex2num('3fefffffffffffff')
ans =
1
What is the saying? Close is only good in horseshoes and hand grenades.

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