Help with Alogrithm????

im trying to do linear interpolation the parametric equations for the ellipse i have are

x=30*cos(t)
y=15*sin(t)

when i simply put it and plot it the graph is fine but when i try something like this

s=pi/180; %to make radians.
>> for i=1:360; %for every degree
t=s*i;          %degree in to radians
x=30*cos(t);
y=15*sin(t);
end
>> plot(x,y)

please help Im new with this coding

Answer 1

[James Tursa]

You could save the x and y result from each iteration in a vector, but rather that do that you don't really need the for loop at all. E.g.,

s=pi/180; %to make radians.
i=1:360; %for every degree
t=s*i;          %degree in to radians
x=30*cos(t);
y=15*sin(t);
plot(x,y)

[qasim zia]

That actually works but can u please tell me why didnt it print out the graph when i tried a for loop? (was it because it was unnecessary?)

[James Tursa]

It didn't print out what you expected because you were not saving the results of each iteration in a vector. The way you have it coded, each iteration the previous values of x and y get overwritten by new values of x and y. At the end of the loop, you only have the last values of x and y, not all of the values of x and y. Here is how you could fix that if you wanted to use the loop (making x and y vectors):

s=pi/180; %to make radians.
for i=1:360; %for every degree
    t=s*i;          %degree in to radians
    x(i)=30*cos(t); % save in the i'th element
    y(i)=15*sin(t); % save in the i'th element
end
plot(x,y)

But note that the simple changing of x and y to x(i) and y(i) only works well because the indexing i=1:360 started at 1 and counted by 1. Also, we didn't pre-allocate x or y (not really needed for this small problem). But if the indexing changed or the problem got bigger, this simple "fix" I have shown would need to be modified.