crazy loop 1

[Star1976]

Hi Guys
Have a look at the following simple matlab code:

x=[1;2;3;4;5;6;7;8;9;10];
y=[5;4;3;6;7;8;5;4;3;2];
z=[1;2;-7;-4;5;-1;4;-9;-6;5]

x1 = zeros(m, 1);% initialise output matrix
y1 = zeros(m, 1);
z1 = zeros(m, 1);

for i = 1:m;
x1(i) = x(i);
y1(i) = y(i);
z1(i) = z(i);

if z1(i)<0 % to choose the negative elements
if z1(i+1)>0 % if the next element is positiv
plot(x(i),y(i),'o')
hold on
end
end
end
------------------

I want to plot x versus y when z sign change from negative to positive ( z passes through zero). Matlab does not consider the second loop where I try to examine the following element. The answer is probably easy and silly but I spent few days without any success.

Any idea or suggestion??

Help is greatly appreciated.

Regards

Star

 

[Star1976]

Sorry I forgot to define m, the code should be:

x=[1;2;3;4;5;6;7;8;9;10];
y=[5;4;3;6;7;8;5;4;3;2];
z=[1;2;-7;-4;5;-1;4;-9;-6;5]

m = size(x, 1);

x1 = zeros(m, 1);% initialise output matrix
y1 = zeros(m, 1);
z1 = zeros(m, 1);

for i = 1:m;
x1(i) = x(i);
y1(i) = y(i);
z1(i) = z(i);

if z1(i)<0 % to choose the negative elements
if z1(i+1)>0 % if the next element is positiv
plot(x(i),y(i),'o')
hold on
end
end
end

 

[JockeK]

Your problem is

if z1(i+1)>0

Your looking one sample ahead of the computation and according to

z1 = zeros(m,1)

this sample is always zero.

Something like this should solve the problem.

for i = 2:m
   if (z1(i-1)<0 & z1(i)>0)
      plot(....)
   end
end

 

[Star1976]

Thanks Jockek.

That really works nicely.

Well done!!!!