Why find doesn't work for some number?

8 次查看（过去 30 天）
mariem farhat
mariem farhat 2015-10-23
评论： Steven Lord 2021-5-27
Hello,
I have the following code
if true
for i = 0:0.1:2
k = find (A==i);
disp(k);
end
This code works but for some number it doesn't work. For example, i=0.6 and 0.7. But when I wrote find(k==0.6) it works!!!
What is the problem? Any idea please

请先登录，再回答此问题。

回答（1 个）

Adam
Adam 2015-10-23
编辑：Adam 2015-10-23
Testing exact equality between double-precision data is un-safe in general. Due to the representation of these numbers a test for absolute equality can fail because of a difference of 0.000001 that you don't see in the variable editor or command window.
If your values such as 0.6 in the array A are the result of some calculation then this will often be the case that they are not precisely 0.6, but may instead be 0.600000000001
  6 个评论
显示 4更早的评论
Rik
Rik 2021-5-27
@Rotem Bachar
You can either use ismembertol, uniquetol, etc, or you can do something like this:
% instead of A==B
abs(A-B)<=tol
Steven Lord
Steven Lord 2021-5-27
Ran in:
Close only counts in horseshoes, hand grenades, and comparing floating-point numbers. The tolerance controls what is "close enough" to a match to count as a match.
x = 1;
y = 2;
If you compare x and y with a tolerance of 5 ("close enough" being within 5 units) then x and y match.
doTheyMatch = abs(x-y) <= 5
doTheyMatch = logical
1
If you compare x and y with a tolerance of 0.5 ("close enough" being within 0.5 units) then they do not.
doTheyMatch = abs(x-y) <= 0.5
doTheyMatch = logical
0
If you used a tolerance of 0 you're saying "close enough isn't good enough, it has to be an exact match down to the last bit" and in that case sometimes numbers that look like they match don't actually match.
tenth = 0.1; % Not exactly one tenth, but close
z1 = tenth + tenth + tenth
z1 = 0.3000
z2 = 0.3
z2 = 0.3000
doTheyMatch = abs(z1-z2) <= 0 % essentially equivalent to z1 == z2
doTheyMatch = logical
0
howFarApart = abs(z1-z2) % very small but not 0
howFarApart = 5.5511e-17

请先登录，再进行评论。

类别

在 Help Center 和 File Exchange 中查找有关 Resizing and Reshaping Matrices 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by