1×0 empty double row vector using find
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Hi, I have problem with this code:
clc; clear; close all
x=[0 0.1 0.2 0.3 0.4 0.5];
y=[1 7 4 3 5 2];
h=0.1;
n=(max(x)-min(x))/h
suma=0;
for i=2:n
aux=h*(i-1)
[row,col] = find(x==aux)
suma=suma+y(col);
end
when I run the for cicle and aux is equal to 0.3, the result of find is "1×0 empty double row vector", but there is a 0.3 in x. I'm really confused about this, someone can help me, please?
Thanks in advance.
Answers (2)
James Tursa
on 1 Jun 2023
Edited: James Tursa
on 1 Jun 2023
Welcome to the world of floating point arithmetic. For your specific example, they are not equal. E.g.,
x=[0 0.1 0.2 0.3 0.4 0.5];
h=0.1;
i = 4;
aux=h*(i-1);
[row,col] = find(x==aux)
fprintf('%20.18f\n',x(4))
fprintf('%20.18f\n',aux)
isequal(0.3,3*0.1)
You can see that these numbers are close but not exactly equal. They differ by one least significant bit in the floating point bit pattern:
num2hex(0.3)
num2hex(3*0.1)
To understand why you get this difference between 0.3 and 3*0.1, see this link:
It is usually bad practice to test for exact equality when floating point arithmetic is involved. Your code needs to be written to account for these small differences.
0 Comments
VBBV
on 1 Jun 2023
clc; clear; close all
x=[0 0.1 0.2 0.3 0.4 0.5];
y=[1 7 4 3 5 2];
h=0.1;
n=(max(x)-min(x))/h
suma=0;
for i=2:n
aux=h*(i-1)
[row,col] = find((x==round(aux,1)))
suma=suma+y(col);
end
3 Comments
John D'Errico
on 1 Jun 2023
Be careful with rounding decimals. Even then, you do not get exactly what you think you get. For example,
x = 0.3;
sprintf('%0.55f',x)
y = round(x,1);
sprintf('%0.55f',y)
0.3 is not exactly representable in floating point arithmetic, and rounding will not change that fact.
VBBV
on 1 Jun 2023
Edited: VBBV
on 2 Jun 2023
round function will output the same what we expect based on the number of input decimal precision given to the function. However, sprintf is different thing, which again displays outputs based on the input precision specified in the function
x = 0.3;
sprintf('%0.55f',x)
y = round(x,1)
% round while displaying
sprintf('%0.9f',y)
0.3 is not exactly representable in floating point arithmetic, and rounding will not change that fact.
Then what is the purpose of round function ?
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