Why is my if counter not working

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)

Answer 1

[VBBV]

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)

Answer 2

[Bruno Luong]

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

Answer 3

[Manash Sahoo]

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