my for loops do not work

hello, my for loops do not seem to work.I appreciate if anyone could help me with that.

clear all;
m=500;
n=50;
G=zeros(m,n);
G(250,1)=1;
S(1,1)=1;
for i=2:1:300
sol=roots([1 -S(i-1,1) -1]);
 S(i,1)=sol(sol>=0);
end
S=10*(S);
y=ceil(S);
y(1,1)=1;
G(250,1)=1;
G(250+y(1,1),1)=1;
for t=1:1:50
    for k=50:1:1
        if y(t+50-k,1)<=250
           for j=1:1:y(t+50-k,1)
              G(250+j,k)=1;
              G(250-j,k)=1;
           end
      end
  end
end

Answer 1

[Star Strider]

What does ‘do not seem to work’ mean? Your code runs without errors.

What do you want your for loops to do?

[shiva]

hello, thanks for your message. let me try to clarify my issue. actually I'm running this code to get some specific result and at the end I do not get that and that's why I think that my for loops do not work. in the beginning I have a zero matrix. then as time increases from 1 to 50, the elements of matrix G changes to one in this way:at t=1 the G(250+y(1,1),50) and G(250-y(1,1),50) and all the elements between them change to one, and G(250+y(2,1),49) and G(250-y(2,1),49)and all the elements between them change to one. and this keeps happening to the element of G(250-y(50,1),1) and (250-y(50,1),1)and all the elements between them. this must be operated by this loop: at t=1: for k=50:1:1 if y(t+50-k,1)<=250 for j=1:1:y(t+50-k,1) G(250+j,k)=1; G(250-j,k)=1; end end end and then for the second time step t=2 we have : the G(250+y(2,1),50) and G(250-y(2,1),50) and all the elements between them change to one + and G(250+y(3,1),49) and G(250-y(3,1),49)and all the elements between them . . . and G(250+y(51,1),1) and G(250-y(2,1),49)and all the elements between them and these changes end once t=50. eventually I'm expecting the matrix G with elements 1 and that's what we do not obtain, we have the same zero matrixG.

I hope I could convey my message clearly.

thanks in advance

[Stephen23]

I hope you took a breath during that explanation... it seems to be one sentence!

[Image Analyst]

Doesn't this help you solve it: http://blogs.mathworks.com/videos/2012/07/03/debugging-in-matlab/

[Stephen23]

Congratulations Star Strider, your two questions and one statement seem to have earned you the Accepted Answer!

OPs never cease to amaze.

[Star Strider]

I found one problem: MATLAB increments colon operators (like for loop counters) by default. If you want to decrement them, you have to tell it explicitly to do so. If you do not, since the end index is less than the start index, MATLAB considers the loop complete and will not iterate it.

Change this line:

for k=50:1:1

to this:

for k=50:-1:1

and your loop runs successfully (I hope).

If you want to see your ‘G’ matrix graphically, add these lines after the code you posted:

figure(1)
mesh(G)
grid on

It’s easier than trying to display such a large matrix in the Command Window.

[shiva]

thank you so much!!it finally worked:)

[shiva]

just one more question. do you know how I can make the movie of the evolution of G matrix in 2 dimensional plot?

[Star Strider]

@Stephen — I may have actually solved it at this point, after going through the code myself to see why the inner loop wasn’t iterating.

I figure that for every quickly Accepted Answer, I have at least two ‘Wow! Thank you very much! That was just what I needed!’ leaving me with nothing except what I learned in developing the Answer. Sometimes my subsequent plaintive plea to Accept my Answer results in it being Accepted, but most of the time, not. I am consoled in knowing that they’re experiences we all share here eventually.

[Star Strider]

@shiva —

My pleasure!

I haven’t made movies from my code in a while. The documentation on Animation will guide you to creating a movie of your evolving plot. You will have to put the code for mesh(G) just after the ‘G’ calculation, in the inner loop, as well as getframe and any other commands you need. You may have to experiment.

[shiva]

great I will take a look at that. thanks again Stephen for sharing your experiences.

[Image Analyst]

I also have a little movie making demo that you may be interested in - it's attached.

[Star Strider]

@ I A — Thank you!