# beginner question array loops with values lower than 1

6 views (last 30 days)
Tony on 28 Oct 2013
Commented: Cedric Wannaz on 28 Oct 2013
hello, i am starting to use matlab on my own for research that i am getting into for image recognition and i don't have a formal class to learn matlab under so sorry if this is a silly question.
how would i do this: <= means less then and equal to, don't know how to insert the proper way
plot this curve for 0 <= t <= 10pie x= 6t+3sin(2t) y=3+3cos(2r)
so i wrote a for loop with something like this.
for t=0:(10*pi)
x(t)=6*t+3*sin(2*t)
y(t)=3+3*cos(2*t)
end
with this i get an error because arrays can't start with 0. how would i get 0 into this and if its not a similar process how would i be able to solve a problem similar to this but had values below 0.

Cedric Wannaz on 28 Oct 2013
Edited: Cedric Wannaz on 28 Oct 2013
We usually do this in a vector way, and not element by element. However, your first attempt is almost correct for a loop-based approach. The only modification to bring is not to use t as a loop index, as it is not (and should not be) an integer greater or equal to 1.
t = 0 : 0.1 : 10*pi ;
for k = 1 : length(t)
x(k) = 6*t(k) + 3*sin(2*t(k)) ;
y(k) = 3 + 3*cos(2*t(k)) ;
end
As you can see, elements of t can now have any value. If you do this though, the size of x and y increases as the loop progresses. This isn't efficient, because each time MATLAB must resize these variables, it has to "ask" for a bigger chunk of free memory, copy the old content, update the last element, and free the previous chunk of memory, which is slow. Therefore, we usually preallocate memory for variables like x and y in this context:
t = 0 : 0.1 : 10*pi ;
x = zeros(size(t)) ;
y = zeros(size(t)) ;
for k = 1 : length(t)
x(k) = 6*t(k) + 3*sin(2*t(k)) ;
y(k) = 3 + 3*cos(2*t(k)) ;
end
This way, memory for the full/final size of x and y is reserved before the loop starts (filled with 0's), and the code in the loop is just re-defining each element of x and y without changing their size.
But as I mentioned above, we usually do this kind of operations in a vector way:
t = 0 : 0.1 : 10*pi ;
x = 6*t + 3*sin(2*t) ;
y = 3 + 3*cos(2*t) ;
If you evaluate sin(t), you will see that most MATLAB functions and operators can take vectors as inputs, and output vectors. The only thing that you have to care for is to use dotted operators to enforce element-wise operations when it is relevant. E.g.
C = A * B .. is a matrix multiplication of A by B.
C(1,1) = A(1,1)*B(1,1) + A(1,2)*B(2,1) for example.
whereas
C = A .* B .. is an element-wise multiplication of elements of A
by elements of B. C(2,2) = A(2,2)*B(2,2) for example.
Additions and subtractions don't need to be dotted, but most other operators need to be dotted for performing element-wise operations.
##### 2 CommentsShowHide 1 older comment
Cedric Wannaz on 28 Oct 2013
PS: if you are just starting, I recommend the Primer, Mathematics, and Programming Fundamentals available here: