while loop ends too early
조회 수: 2 (최근 30일)
이전 댓글 표시
Hi everyone,
Why is this loop not executed for t_max = 5.0000e-04 anymore? if I output t, the maximum (last) value is 5.0000e-04 instead of 5.0100e-04
delta_t = 10e-7;
t_max = 5.0000e-04; %[s]
t = 0;
while t <= t_max
t = t+delta_t;
end
댓글 수: 0
채택된 답변
Ameer Hamza
2020년 3월 21일
편집: Ameer Hamza
2020년 3월 21일
Image Analyst and Sriram explained the reason as it is caused by the finite precision of floating-point representation. One way is to solve the issue with limited precision is to use variable precision arithmetic
delta_t = vpa(10e-7);
t_max = vpa(5.0000e-04); %[s]
t = vpa(0);
while t <= t_max
t = t+delta_t;
end
However, note that this will be slower as compared to the numeric version. The other solution is to add a tolerance value for comparison
delta_t = 10e-7;
t_max = 5.0000e-04; %[s]
t = 0;
while t <= t_max + 1e-10 % small tolerance in comparison
t = t+delta_t;
end
추가 답변 (2개)
Image Analyst
2020년 3월 21일
Take the semicolon off and format long and you'll see why
format long g;
format compact
delta_t = 10e-7;
t_max = 5.0000e-04; %[s]
t = 0;
while t <= t_max
t = t+delta_t
end
t =
0.000498000000000006
t =
0.000499000000000006
t =
0.000500000000000006
You see, it ends up being slightly larger than 0.0005 so it doesn't even enter the loop that last time.
So, basically you need to see the FAQ for an explanation of digitization error: FAQ Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero?
댓글 수: 0
Sriram Tadavarty
2020년 3월 21일
Hi Ann,
The loop runs as expected, since the floating values are represented in IEEE 754 format as described here.
To know what the value of t that end from the loop, use format long as below:
format long
>> t_max
t_max =
5.000000000000000e-04
>> t
t =
5.000000000000056e-04
Observe that t is lager than t_max and thus, the loop ended.
Hope this helps.
Regards,
Sriram
댓글 수: 0
참고 항목
카테고리
Help Center 및 File Exchange에서 Loops and Conditional Statements에 대해 자세히 알아보기
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!