If statement in for loop does not work

Question

Mohammed Radha Al Khazraji 2021-2-9

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/740157-if-statement-in-for-loop-does-not-work

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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Answer 1

KALYAN ACHARJYA 2021-2-9

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https://ww2.mathworks.cn/matlabcentral/answers/740157-if-statement-in-for-loop-does-not-work#answer_618557

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!

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Answer 2

randerss simil 2021-2-9

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https://ww2.mathworks.cn/matlabcentral/answers/740157-if-statement-in-for-loop-does-not-work#answer_618667

在 MATLAB Online 中打开

%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

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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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