help with writing an equation with iterations

Hi I am trying created a code with aim of finding theta and then alpha(i). Then plotting alpha(i) against i. It is known that alpha(0)=30.

>alpha= 30; % This is alpha(0) i tried writing alpha(0) but kept getting error

>for i=1:10

>theta= 8*alpha*i+6*alpha %I want 8*alpha(i)*i+6*alpha(i)

>alpha(i+1)= theta +alpha(i) %Want to creating a list of alpha(i) values to plot against i

>end

>plot(i,alpha(i))

Also tried this but with no success produced almost all zero values

>alpha=[ ]

>alpha= [30];

>alpha(1) %this is alpha(0) as it gives value 30

>for i=1:10

>theta= 8*alpha(i)*i+6*alpha(i)

>alpha(i+1)= theta +alpha(i)

>end

>plot(i,alpha(i))

Many thanks in advance

Answer 1

[Mischa Kim]

Ken, you could do:

 alpha = 30;
 n     = 10;
 for i = 1:n
    theta(i)   = 8*alpha(i)*i + 6*alpha(i); 
    alpha(i+1) = theta(i) + alpha(i); 
 end
 plot(1:n+1,alpha)

however, you probably end up with some huge numbers. Note, that indexing in MATLAB starts at 1, not 0. Also, since you are generating n+1 values for alpha you need to have n+1 data points for i, for plotting.

[Ken]

Hi thanks for the reply

For some reason creates a graph like this when plotting i against alpha

When I test to see what alpha is produces this;

    0.0000    0.0000    0.0000    0.0000    0.0000

    0.0000    0.0000    0.0000    0.0014    0.1143

    9.9443

Though when i type in alpha(1) returns 30

[Mischa Kim]

That's because of the huge numbers you are computing. Have a look at the y-axis scale factor: 10^17. 30 = 0.0000...00030 * 10^17.

If you remove semi-colons after the two commands in the for-loop the values for theta and alpha are displayed in the MATLAB command window. The first values are:

 theta =
         420        9900      310500    12192300   575604900
 alpha =
          30         450       10350      320850    12513150   588118050

The results are correct. So I am wondering, if this is indeed the series you need to compute. What exactly is the problem statement?