MATLAB: Problem comparing two variables

Question

Can someone please explain why the comparison result comes out false for this code?

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Node.active_time=1;

temp=0.5;

%through some iterations temp increases. Lets hard code it here

temp=temp+0.1;

temp=temp+0.1;

temp=temp+0.1;

temp=temp+0.1;

temp=temp+0.1;

%by now value of temp is 1.0000

temp==Node.active_time

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Matlab answers 0 for this, which means they are not equal. Why?

However, if we manually assign values and then compare them, it agrees with comparision. For example,

temp=1.0000;

Node.active_time=1;

temp==Node.active_time

matlab's answer to this is 1, which means they are equal (rightly so).

Why is it that comparison in previous code shows false?

Answer 1

Floating point operation limitations. E.g., see here

http://www.mathworks.com/matlabcentral/answers/41536-why-is-2-24-10-22-4-not-equal-to-0

In your particular case, 0.1 cannot be represented exactly in IEEE double floating point arithmetic, leading to the differences you are seeing.

For floating point comparisons you generally need to use a tolerance, e.g.,

abs(temp-Node.active_time) < tol