'logical' discrepancy
显示 更早的评论
Dear MATLAB community,
I am running 'if' statement and encounter a problem as such:
The logical returns 'false', but the values themselves do equilibrate. Confused.
Thanks,
2 个评论
Greg
2017-1-4
It doesn't appear to be an order of operations problem, but I'd throw explicit ()s to force the proper comparison just to double-check.
Then, try looking more than 4 digits past the decimal. Either format long, or fprintf(1,'%.12f\n',ans)
Walter Roberson
2017-1-5
Having a | at that location is not syntactically valid. It is not possible to have a numeric operator directly before or after a logical operator.
采纳的回答
Star Strider
2017-1-4
1 个投票
I can’t run your code because I don’t have the data. however I believe you’re encountering a problem with Operator Precedence. The equality up to the or operator is being evaluated as a single expression, and then being compared to ‘cosd(theta)’.
Judicious use of parentheses could resolve that, but given the structure of the line you quote, I make no guarantees.
4 个评论
Yan Liu
2017-1-4
Star Strider
2017-1-5
My pleasure.
No, they’re not the same:
check = (abs(xn(1,29) - xn(1,5) )) - (0.5 * sqrt(2) * l * cosd(theta))
check =
-3.77475828372553e-15
This was my second hypothesis. For details, see Why is 0.3 - 0.2 - 0.1 (or similar) not equal to zero?
Yan Liu
2017-1-5
Star Strider
2017-1-5
As always, my pleasure!
Happy New Year to you too! I wish you the same!
更多回答（1 个）
John BG
2017-1-5
Yan
1. with command vpa taking below 15 precision digits seems to work ok:
vpa(0.5 * sqrt(2) * l * cosd(theta),10)== vpa(abs(xn(1,29) - xn(1,5) ),10)
ans =
0.35355339059327376208252680100941 == 0.35355339059327376208252680100941
>> n=0;if vpa(0.5 * sqrt(2) * l * cosd(theta),10)== vpa(abs(xn(1,29) - xn(1,5) ),10) n=1; end
n
=
1
2. when not reducing the amoung of digits used there is a small difference:
(0.5 * sqrt(2) * l * cosd(theta))-(abs(xn(1,29) - xn(1,5) ))
=
3.774758283725532e-15
the compared values:
(0.5 * sqrt(2) * l * cosd(theta))
=
0.353553390593274
>> (abs(xn(1,29) - xn(1,5) ))
=
0.353553390593270
cosd.m is not available with command type
type cosd.m
%COSD Cosine of argument in degrees.
% COSD(X) is the cosine of the elements of X, expressed in degrees.
% For odd integers n, cosd(n*90) is exactly zero, whereas cos(n*pi/2)
% reflects the accuracy of the floating point value for pi.
%
% Class support for input X:
% float: double, single
%
% See also ACOSD, COS.
% Copyright 1984-2010 The MathWorks, Inc.
% Built-in function.
.
but the comment hints that function cos is subject to precision of pi, therefore cosd may have the accurate value while perhaps the values of xn may be the ones that have too many decimals.
if you find these lines useful would you please mark my answer as Accepted Answer?
To any other reader, if you find this answer of any help please click on the thumbs-up vote link,
thanks in advance for time and attention
John BG
3 个评论
Yan Liu
2017-1-5
happy 2017 to you too.
but I provided a valid answer I kindly ask you to consider marking my answer as the Accepted Answer for your question.
Awaiting answer
John BG
Walter Roberson
2017-1-5
"the comment hints" and "the value of xn may be the ones that have too many decimal places". Hypotheses.
Star Strider provided a specific numeric analysis and a link to the faq on the topic, and did so hours before you posted your answer.
类别
在 帮助中心 和 File Exchange 中查找有关 Variables 的更多信息
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!
Translated by 
