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Why do I get false results when using equal relational operation for the logical operation?

Asked by Prabhakar on 18 Jan 2011
I would like to get true when I use equal rational operation in a code including if condition. But I got false for the equal rational operation like as the below example. For example:
a = 0.2;
b = zeros(3,1);
if 12*a == 2.4
b(1) = 1;
end
if 14*a == 2.8
b(2) = 2;
end
if 17*a == 3.4
b(3) = 3;
end
>> b b = 0 0 0

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2 Answers

Answer by Anish
on 18 Jan 2011

If you look carefully at the definition of fundamental arithmetic operations like addition and multiplication, you soon encounter the mathematical abstraction known as the real numbers. But actual computation with real numbers is not very practical because it involves limits and infinities. So you need to avoid to using the result of multiplying floating points for the equal (==) operation because the result of multiplying floating points make the round off error.
Here is the link for the reference of floating points including IEEE Standard unifies arithmetic model:
As workarounds for this issue, use the integer for the variable 'a=2' instead of using floating points 'a=0.2' or rewrite your code like as below:
if (abs(a*12 - 2.4) < eps*10)

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Answer by Walter Roberson
on 20 Jan 2011

See also FAQ #6.1

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