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How parentheses effects multiplication with pi ?

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Emre Doruk
Emre Doruk on 4 Aug 2020
Edited: Bruno Luong on 5 Aug 2020
Hi,
I am trying to generate sine wave. I am usning following two code lines. But they are slightly different (about e-15). Why is it happenning ? What is the differences of two lines;
f0=5e2;
fs=500e2;
len=3e3;
dt=1/fs;
t=0:dt:(len-1);
sing1= sin (2*pi*f0*t);
sing2= sin(2*pi*(f0*t));
isequal(sing1,sing2)
Thanks for your help,

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Accepted Answer

Bruno Luong
Bruno Luong on 5 Aug 2020
Edited: Bruno Luong on 5 Aug 2020
"While floating-point addition and multiplication are both commutative (a + b = b + a and a × b = b × a), they are not necessarily associative. That is, (a + b) + c is not necessarily equal to a + (b + c). ... "
  • "(2*pi*f0*t)" interpreted by MATLAB as ((2*pi)*f0)*t
  • "2*pi*(f0*t)" interpreted by MATLAB as (2*pi)*(f0*t)
As floating-point multiplication is NOT ASSOCIATIVE, both results might be different.

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Emre Doruk
Emre Doruk on 5 Aug 2020
Thanks for your answer, again. I can see your point.
Bruno Luong
Bruno Luong on 5 Aug 2020
Sometime this non-associativity is a real headeach, such as finding a limits of a sign- alternate series by partial sum, or integral of an oscillated signal (any wave propagation SW might encounters this). There is a real challenge to know what really the limits and the order of the sum can make the result change widely.
Emre Doruk
Emre Doruk on 5 Aug 2020
Actually I realized it recently, I thougth this problem is about pi. Because it is irrational and maybe MATLAB coverge it sth. I asked for this reason but I see that it is more common from this. I should check all of my old codes which needs exact values. Genereally it can be accept like precision mistake but sometimes I really need exact value.

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More Answers (1)

madhan ravi
madhan ravi on 4 Aug 2020
In the first the order of operation is from left to right.
In the second the order of operation is inside the parenthesis and then the outer.

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Stephen Cobeldick
Stephen Cobeldick on 5 Aug 2020
"Because it is the same oparetaion, left to right or the right to left. "
No, that is incorrect.
In general operations on binary floating point numbers are NOT associative:
A classic example of this is called catastrophic cancelation:
>> (1 + 1e100) - 1e100
ans =
0
>> 1 + (1e100 - 1e100)
ans =
1
"Why is it happenning?"
Because of the well-documented properties of binary floating point numbers.

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