bug in power calcultion while in loop ?
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Aswin Prasanna Suriya Prakash
on 26 Sep 2020
Commented: Rik
on 26 Sep 2020
So, I've been doing a project lately and I come upon this interesting bug.
here's a variable W which uses the euler's formula i.e., cos(2*pi*1i/8) +
W = cos(2*pi/8) + sin(2*pi/8);
or even the exponent form
W = exp(2*pi*1i/8);
Now, if i use a power k = 2 on this variable, I get the result as intended
>> k = 2
k =
2
>> W^k
ans =
0.0000 + 1.0000i
Here's the one in a loop in the command window which is fine again
for k = 1:4
disp(W^(k-1));
end
1
0.7071 + 0.7071i
0.0000 + 1.0000i
-0.7071 + 0.7071i
but, when i put this in a loop inside my program this is the result
for k = 1:4
g(k) = W^(k-1);
disp("W^(k-1) " + W^(k-1));
end
W^(k-1) 1
W^(k-1) 0.70711+0.70711i
W^(k-1) 2.2204e-16+1i
W^(k-1) -0.70711+0.70711i
how did this bug occur such that it's working perfectly fine in commands but not in program ?
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Accepted Answer
Rik
on 26 Sep 2020
This is not a bug. Look at the exponent: 2e-16 is very small. It is close enough to eps that you can assume that is just 0.
The reason in the way computers store values. Just like you can't store the result of 1/3, computers can't store every number. If you can only store 4 decimals 3*(1/3) will be 0.9999, not 1.
2 Comments
Rik
on 26 Sep 2020
You probably don't have to do anything, but you could round to some maximum precision.
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