problem with low infinite number

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Trop Trader
Trop Trader 2024-3-3
移动：John D'Errico 2024-3-6
Ediff(8,35)
ans =
7.4525e+03
profit(8,35)
ans =
7.4525e+03
bu=minus(profit,Ediff);
bu(8,35)
ans =
1.8190e-12
Why does subtracting 2 equal numbers produce a very small number?

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回答（2 个）

the cyclist
the cyclist 2024-3-3
编辑：the cyclist 2024-3-3
Ran in:
Because their floating-point representations are not exactly equal.
You can see that they are not quite equal:
load("matlab_Ediff.mat","Ediff");
load("matlab_profit.mat","profit");
fprintf("Ediff = %16.14f\n",Ediff(8,35))
Ediff = 7452.50000000000182
fprintf("profit = %16.14f",profit(8,35))
profit = 7452.50000000000364
John D'Errico
John D'Errico 2024-3-3
Ran in:
They only look the same, to 4 decimal places.
format short
x = 1.23456789;
y = 1.23456987;
[x,y]
ans = 1×2
1.2346 1.2346
They certainly look the same to me. But only if I don't look at their actual values, down to the last digits. If I subtract them of course, they are seen to be different.
x - y
ans = -1.9800e-06
Surely you would expect the difference to be non-zero, even though, when displayed as only 5 dignificant digits, they seem identical?
format long
[x,y]
ans = 1×2
1.234567890000000 1.234569870000000

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