Why is my if counter not working

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Kaden Bauernfeind
Kaden Bauernfeind 2022-4-22
编辑： Manash Sahoo 2022-4-22
Im trying to get my if counter to add each point when s is negative but it just produces 0 all the time
my counter here is total
s= 0
x = linspace(-1,1, 50)
n = 1:2:10
L = 2
total = 0
for i = 1:n
s = (8/(pi^2))*(s + (-1^(i-1/2)*(sin(i*pi*x/L)/i^2)))
if s(i) < 0
total = total + 1
end
end
plot(x,s)

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回答（3 个）

VBBV
VBBV 2022-4-22
Ran in:
s= 0
s = 0
x = linspace(-1,1, 50)
x = 1×50
-1.0000 -0.9592 -0.9184 -0.8776 -0.8367 -0.7959 -0.7551 -0.7143 -0.6735 -0.6327 -0.5918 -0.5510 -0.5102 -0.4694 -0.4286 -0.3878 -0.3469 -0.3061 -0.2653 -0.2245 -0.1837 -0.1429 -0.1020 -0.0612 -0.0204 0.0204 0.0612 0.1020 0.1429 0.1837
n = linspace(1,10,50)
n = 1×50
1.0000 1.1837 1.3673 1.5510 1.7347 1.9184 2.1020 2.2857 2.4694 2.6531 2.8367 3.0204 3.2041 3.3878 3.5714 3.7551 3.9388 4.1224 4.3061 4.4898 4.6735 4.8571 5.0408 5.2245 5.4082 5.5918 5.7755 5.9592 6.1429 6.3265
L = 2
L = 2
total = 0
total = 0
s= zeros(size(n))
s = 1×50
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
for i = 1:length(n)
s(i) = (8/(pi^2))*(s(i) + (-1^(i-1/2)*(sin(i*pi*x(i)/L)/i^2)));
if s(i) < 0
total = total + 1;
end
end
total
total = 26
plot(x,s)
Bruno Luong
Bruno Luong 2022-4-22
编辑：Bruno Luong 2022-4-22
Your for loop index is probably wrong, n should be scalar
n = 1:2:10
...
for i = 1:n
...
end
or perhaps you want this
n = 1:2:10
...
for i = n
...
end
Manash Sahoo
Manash Sahoo 2022-4-22
编辑：Manash Sahoo 2022-4-22
First off, your for loop.
Doing
n = 1:2:10
for i = 1:n
disp(i)
end
shows that your for loop will only run one time, so your if statement is only checked once.
If you want to loop through each element of n and use it in equation s you need to do this:
x = linspace(-1,1, 50)
n = 1:2:10
L = 2
figure;hold on;
for k = 1:numel(n)
i = n(k)
s = (8/(pi^2))*(s + (-1^(i-1/2)*(sin(i*pi*x/L)/i^2)))
end
If you want a total of how many numbers in s that are < 0, you can just do this:
total = numel(find(s < 0 == 1))
MS

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