Help with looping problem

Here I am defining x and y coordinates of rays vertically approaching a boundary (currently sin(x))

x = 0:0.1:pi

rx1 = [x(1) x(1)]            % x-coordinates, incoming ray 1
ry1 = [y0 sin(x(1))]         % y-coordinates, incoming ray 1

rx2 = [x(2) x(2)]            % x-coordinates, incoming ray 2
ry2 = [y0 sin(x(2))]         % y-coordinates, incoming ray 2

rx3 = [x(3) x(3)]            % x-coordinates, incoming ray 3
ry3 = [y0 sin(x(3))]         % y-coordinates, incoming ray 3

and so on for the length of vector (x) = 32

Clearly this isn't a good way to write the program. The alternative approach I have tried to use is the following:

i = 1

for i = 1:length(x)
rx(i) = [x(i) x(i)]
ry(i) = [y0 sin(x(i))]
end

Which returns the error:

In an assignment A(I) = B, the number of elements in B and I must be the same.

So clearly this isn't the way to achieve what I was trying to do. I would greatly appreciate if someone could help me resolve this, as I have been stuck on it for some time.

Answer 1

[Matt Fig]

Why would you want to create 64 variables like that? What a mess it would be to deal with in further steps. Perhaps two cell arrays could get the job done:

for ii = 1:length(x)
  rx{ii} = [x(ii) x(ii)];  % rx is a cell array.
  ry{ii} = [y0 sin(x(ii))];
end

Now the really important question is why do you need to do this at all? What is your next step that requires you to have these arrays as individuals, rather than just using x and sin(x)?

[Tom]

Haha! I wouldn't be surprised to hear I have gone the complete wrong way about this. Perhaps I could ask you then: How would you plot a number of vertical lines in increments along the x-axis, which each terminate when they reach the function sin(x)?

Really appreciate the help by the way.

p.s. when I ran your suggestion I received the error: Error using plot Not enough input arguments.

[Matt Fig]

Do this instead:

x = 0:0.1:pi;           
y = sin(x);
plot(x,y,'k-')          
hold on

for ii = 1:length(x)
  line([x(ii) x(ii)],[-1 y(ii)])
end

[Tom]

That is exactly what I needed. I cannot thank you enough!

[Matt Fig]

Or without the FOR loop at all:

x = 0:0.1:pi;           
y = sin(x);
plot(x,y,'k-')  
L = line([x;x],[-ones(1,length(x));y]);
set(L,'color','b')

[Tom]

I haven't encountered this 'line' parameter yet. How come it is plotted without even being asked?

[Matt Fig]

LINE is not a parameter, it is a function.

help line

Answer 2

[Matt Kindig]

I'm not entirely clear what you want, but I think this will do the trick:

x = (0:0.1:pi)';
rx = [x, x];
ry = [ repmat(y0, size(x)), sin(x)];

This implements the vector as specified in the for loop you have shown, which is the preferred way to specify matrix data in Matlab (not creating variables for each line, as your rx1, rx2, etc. example shows).

[Tom]

Thanks for the swift response!

That didn't seem to work, perhaps it would make my problem clearer if you run the following:

x = 0:0.1:pi;           
y = sin(x);
plot(x,y,'k-')          
hold on

rx1 = [x(1) x(1)]
ry1 = [-1 sin(x(1))]

rx2 = [x(2) x(2)]
ry2 = [-1 sin(x(2))]

rx3 = [x(3) x(3)]
ry3 = [-1 sin(x(3))]

plot(rx1,ry1,rx2,ry2,rx3,ry3)

This is what I am trying to achieve (although I have only shown the first 3 vertical lines).

My inexperienced attempt to achieve this with a For loop goes:

for i = 1:length(x)
    rx(i) = [x(i) x(i)]
    ry(i) = [y0 sin(x(i))]
end

Which returns the error shown above, making clear I'm going completely wrong. Any idea what I should be doing?

Answer 3

[Walter Roberson]

http://matlab.wikia.com/wiki/FAQ#How_can_I_create_variables_A1.2C_A2.2C....2CA10_in_a_loop.3F

Answer 4

[Azzi Abdelmalek]

y0=1
x = 0:0.1:pi
n=length(x);
eval(sprintf('rx%d=[x(%d) x(%d)],',repmat((1:n),3,1)))
eval(sprintf('ry%d=[y0 sin(x(%d))],',repmat((1:n),2,1)))