Relational Operators Issue - 1 is not equal to 1.000

Hey there,

the following CODE (MATLAB 2011b) results in an unexpected problem, which I'd like to know the reason for:

x = 0;
y = 1;

for i = 1:10
   x = x + 0.1;
end

Result:

x = 1.0000
y = 1

The use of some relational operators to compare x and y

a = y == x;
b = y < x;
c = y > x;

Results:

a = 0
b = 0
c = 1

It seems as if the value of x after passing the for-loop is less than 1, even though the workspace displays x = 1.0000... and y = 1.

Actually I experienced this problem in some measurement automation loop and solved it by using

d =  y == round(x*100000)/100000;

Result:

d = 1

The question arises: Is this the only/best way to solve this problem or is there a better work-around - maybe a generalized solution - that avoids such problems in advance!

Additional information: Both variables x and y are DOUBLE after the creation.

Thanks for your help!

Answer 1

[Thomas]

This wiki might help

http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F

[Walter Roberson]

Yes, see that FAQ.

You are only displaying a few digits after the decimal place, and for *display* purposes the value you are displaying rounds to 1.000 but that does not mean that the value *is* 1