graph of y = 1 / ( cos(x) -c)

i am trying to plot a graph of y = 1 / ( cos(x) -c) where c comes from initial conditions.

When I do a graph by hand for each of the initial conditions I get plots above and below the axis but all my matlab plots are below zero.

C = cos(c) - 1/y

When I hard code the values for C the graph works fine so I expect it is these lines of code.

if true
x0 = [0 pi/2 pi/2 pi/2 pi/2 pi/2 pi/2]
y0 = [1 -1/2 1/2 -2/3 2/3 -1 1]   
C = cos(x0) - (1./y0);
for C= 1:7
end

if true
  % code
hold on
grid on
x=[0: pi/20: 4*pi]
axis([0 4*pi -5 1])
x0 = [0 pi/2 pi/2 pi/2 pi/2 pi/2 pi/2]
y0 = [1 -1/2 1/2 -2/3 2/3 -1 1]   
C = cos(x0) - (1./y0);
for C= 1:7
y = 1./(cos(x) -C);
plot (x,y,'color',rand(1,3))
end
set(gca,'xTick',0:pi/2:4*pi) 
set(gca,'xTickLabel',{'0', ' ', 'pi', ' ', '2pi', ' ', '3pi', ' ', '4pi'})
% Define y-ticks and let them be auto-labelled... 
set(gca,'yTick',-5:.5:1)

title('1/ (cos(x) - c)') xlabel('x') ylabel('y') legend('y(0)= 1','y(pi/2)= -1/2', 'y(pi/2)= 1/2', 'y(pi/2)= -2/3', 'y(pi/2)= 2/3', 'y(pi/2)= -1', 'y(pi/2)= 1')

end
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Any help would be appreciated.

Answer 1

[Hollie Bell]

How good are you!

Thanks

[Jos (10584)]

Is this really the answer you wish to accept? ;-)

Answer 2

[Jos (10584)]

I think these lines do not behave as expected:

C = cos(x0) - (1./y0); % these values are never used
for C= 1:7
y = 1./(cos(x) -C); % C is 1, 2, 3 ...
...

where my guess is that you want to use a specific value, like this:

C = cos(x0) - (1./y0); % 7 values
  for k = 1:7
  % use a specific value of C in each iteration
  y = 1./(cos(x) - C(k)); 
  ...