If statement in for loop does not work

1 次查看（过去 30 天）
Mohammed Radha Al Khazraji
评论： Jan 2021-2-9
I am trying to model a particle bouncing around in a box. The size of the box is 0<=x<=1 and 0<=y<=1. so given an initial position and velocity the particle will move until it hits a wall and then it bounces. I have an idea of how this should work but as I was working through the project I hit this problem. This is a simplified version of the code that is causing the problem. the value of x is 1 at some point in the iteration but the if statement doesnt recognmize it.
x = 0.5;
for i=1:6
x = x+0.1
if x == 1
disp('yes')
end
end

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回答（2 个）

KALYAN ACHARJYA
KALYAN ACHARJYA 2021-2-9
编辑：KALYAN ACHARJYA 2021-2-9
It's because floating point numbers representation (Exactly). Hence If you're trying to do logical comparision (==) with floating-point numbers, be careful.
Please refer the following
https://in.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero
More:
https://matlab.fandom.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F
But if you are insisted to way out, you may consider difference between the numbers as allowable tolerance (error) and use > or < logical comparision in the "if statement".
Hope it Helps!
randerss simil
randerss simil 2021-2-9
%if true
x = 0.5;
for i=1:6
x = (x)+0.1
if strcmp(num2str(x),'1')
disp('yes')
end
end
Try the above
  1 个评论
Jan
Jan 2021-2-9
Letting strcmp perform a rounding is a very indirect and instable way. The actual poblem of floating point number is not addressed here. Using a limit for the numeric comparison is better: if abs(x - 1) < 10 * eps

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