# Update a for loop with if statements

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gamer on 14 Jun 2021
Answered: Joseph Cheng on 14 Jun 2021
Hello community,
thanks for trying to help me! I would be so happy if this will work.
First of all I created n spheres in a 15 x 7.5 field. They should move with the speed of v, which is always reduced by 0.001
Now thr question: How is it possible to change the direction ( make the element of v negative) when the spheres hit the borders. I tried it like this but it just changes the direction for only "one moment"
a = 15;
b = 7.5;
p=[r + (a-2*r)*rand(n,1),r + (b-2*r)*rand(n,1)];
v = rand(n,2);
for f = 1:1000
temp = v - 0.001 ;
temp(temp < 0.001) = 0 ;
v = temp;
if ~any(v(:))
break
end
p = p+v;
for i = 1:n
% check if ball hit borders
if (p(i,1)<=r)
p(i,1)=r; v(i,1) = -v(i,1); % new direction of speed
end % unfortunately only for one moment
if (p(i,1)>=(a-r))
p(i,1)=(a-r); v(i,1) = -v(i,1);
end
if (p(i,2)<=r)
p(i,2)=r; v(i,2) = -v(i,2);
end
if (p(i,2)>=(b-r))
p(i,2)=(b-r); v(i,2) = -v(i,2);
end
end
end

Joseph Cheng on 14 Jun 2021
Without running your code i think you can create a sign variable of (-1)^(0 or 1) which switches between 0 or 1 depending on when it hits. then you keep using that value. This will also give you a variable that changes sign for either adding or subtracting the velocity slow down.