While loop iterating further than expected?
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Hi. I am experiencing a strange behavior in this simple script:
% CASE 1 - COMPUTES CORRECTLY
m1=1;
while m1<4.80
m1=m1+0.1;
end
disp(m1);
% CASE 2 - DOES NOT COMPUTE CORRECTLY
m2=1;
while m2<4.90
m2=m2+0.1;
end
disp(m2);
I obtain m1 = 4.8, as expected. However, I obtain m2 = 5.0, which implies the loop in CASE 2 has gone on for one iteration too many. Any idea why this might be happening? Is it a precision-related issue? I'm afraid I must be missing something very silly. Thank you for your help.
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Stephen23
2018-8-29
"I'm afraid I must be missing something very silly."
The value that you are adding cannot be exactly represented using binary floating point numbers. What you see printed in the command window is the closest representation to 5 or 16 significant digits, depending on your current format setting. To see the "real" value download James Tursa's FEX submission:
disp is useless for this. Use James Tursa's num2strexact and you will see that none of those values really have the exact value 0.1, or their multiples. All you are looking at is a representation of those floating point numbers displayed in the command window, to the precision defined by your format setting. Just because you see 0.1 is displayed tells you nothing about the "real" floating point number's value.
You need to learn about the limits of floating point numbers. Start by reading these:
This is worth reading as well:
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