Improper negative number representation
显示 更早的评论
I have encountered a very strange issue. I was having trouble with an if statement that was comparing two numbers. Although the numbers were apparently identical, the equality was seen as false. I dug a little deeper and I found this extremely simple minimal working example. Why is this happening? This doesn't make any sense!
>> format long
>> A = -6*1e-9
A =
-6.000000000000001e-09
2 个评论
Vittorio Picco
2020-11-11
David Hill
2020-11-11
Floating point numbers! There is lots of matlab documentation on floating point numbers, you just need to search for it.
采纳的回答
James Tursa
2020-11-11
编辑：James Tursa
2020-11-11
IEEE double precision floating point cannot represent the number -6e-9 exactly. What you are seeing is the decimal representation of the closest binary floating point bit pattern that IEEE double can represent. This is normal behavior for binary floating point systems. See this link for more discussion:
The "exact" decimal conversion of the binary floating point numbers stored for the numbers in your examples:
>> num2strexact(-1*1e-9)
ans =
'-1.0000000000000000622815914577798564188970686927859787829220294952392578125e-9'
>> num2strexact(-2*1e-9)
ans =
'-2.000000000000000124563182915559712837794137385571957565844058990478515625e-9'
>> num2strexact(-3*1e-9)
ans =
'-3.0000000000000003936399275115964879745433790958486497402191162109375e-9'
>> num2strexact(-4*1e-9)
ans =
'-4.00000000000000024912636583111942567558827477114391513168811798095703125e-9'
>> num2strexact(-5*1e-9)
ans =
'-5.0000000000000001046128041506423633766331704464391805231571197509765625e-9'
>> num2strexact(-6*1e-9)
ans =
'-6.000000000000000787279855023192975949086758191697299480438232421875e-9'
>> num2strexact(-7*1e-9)
ans =
'-7.00000000000000064276629334271591365013165386699256487190723419189453125e-9'
>> num2strexact(-8*1e-9)
ans =
'-8.0000000000000004982527316622388513511765495422878302633762359619140625e-9'
>> num2strexact(-9*1e-9)
ans =
'-9.0000000000000011809197825347894639236301372875459492206573486328125e-9'
2 个评论
Vittorio Picco
2020-11-11
James Tursa
2020-11-11
Good choice. When working with floating point comparisons it is best to use tolerances unless you know for sure that the numbers being compared are exact integers.
更多回答（0 个）
类别
在 帮助中心 和 File Exchange 中查找有关 Physics 的更多信息
产品
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!
Translated by 
