Break the for loop

Hi! I have a for loop that fills my vector with fibonacci numbers if random number from 0 to 1 is greater than 0,1. In cases when random number is less than 0,1, instead of adding the leading digits, it subtracts. So normally result looks like this:

1

1

2

3

5

8

13

5

18

23

But in some cases random number from 0 to 1 gives value less than 0,1 twice in a row and i get a table with fibonacci number <= 0 that looks like this:

1

1

2

3

1

-2

-1

-3

-4

-7

So i have two questions that i have not managed to solve.

1. How to stop the loop when i get first negative value (in this case -2) and write a message about that?
2. How to make this for loop automaticly go over and over again (implement it in while loop?) until i get a negative value and than stop it and then write this message? Is it possible to count how many times (while?) loop had to start before negative value was found?

n = 10;

limitf = 0.1;

F = zeros(n,1);

F(1) = 1;

F(2) = 1;

for k = 3:n

r = rand(1);

if r > limitf

F(k) = F(k-1) + F(k-2);

else

F(k) = F(k-1) - F(k-2);

end

end

disp(F)

Answer 1

[Mathieu NOE]

hello

I modified a bit your code so the condition were the two successive identical random r :

n = 10;
limitf = 0.1;
F = zeros(n,1);
F(1) = 1;
F(2) = 1;
r_old = -1;

for k = 3:n
    r_old = r;
    r = rand(1);

    if r > limitf 
       F(k) = F(k-1) + F(k-2);
    else
        if abs(r-r_old)>eps
        F(k) = F(k-1) - F(k-2);
        end
    end

end
disp(F)

[Julenissen]

Hi! Thank you for the answer. I don’t quite get it. Could you explain what are you using abs and eps for?

[Mathieu NOE]

hello

so I modified the code so that in case r would be identical twice in a row , this case would be rejected

this portion of the code ensure that F(k) = F(k-1) - F(k-2); is executred only if r and r-old are differnt by an amount greater than eps

        if abs(r-r_old)>eps
        F(k) = F(k-1) - F(k-2);
        end

r_old is the previous r value and we compare if to the current r value

instead of making a direct comparison like

 if r == r_old

which can be sensitive to round off errors , it's much preferable to check that the difference (absolute value) is very low ; eps = 2.2204e-016

is a small number defined bu default in matlab , but of course you can define your own tolerance value

[Julenissen]

Oh, i see now :) I don´t really need r to be identical, but r to be bellow 0,1 twice in the row.

But still the best criteria is when we get first negative value written in the vector

1

1

2

3

1

-2

-1

-3

-4

-7

So when in this case -2 is written in the table, the loop should stop, write content of the vector, in this case it will be:

1

1

2

3

1

-2

0

0

0

0

and write a message

[Mathieu NOE]

hello

I modified the code so it implements the requirement : the best criteria is when we get first negative value written in the vector

n = 10;
limitf = 0.1;
F = zeros(n,1);
F(1) = 1;
F(2) = 1;
for k = 3:n
    r = rand(1);
    if r > limitf
    F(k) = F(k-1) + F(k-2);
    else
    F(k) = F(k-1) - F(k-2);
    end
    
    if F(k)<0
        disp(F)
        disp(['number of iterations ' num2str(k)]);
        break
    end
        
end

[Julenissen]

Thank you :)

[Mathieu NOE]

My pleasure !

Answer 2

[Alan Stevens]

Something like this perhaps:

fib = [1; 1];
its = 0;
keepgoing = true;
while keepgoing 
    its = its+1;
    r = rand;
    if r>0.1
       fib(its+2) = fib(its+1) + fib(its);
    else
        fib(its+2) = fib(its+1) - fib(its);
    end
    if fib(its+2)<0
       keepgoing = false; 
    end
end
disp(['number of iterations ' num2str(its)])

[Julenissen]

Hi, Alan! Thank you for your answer! What vould you do on question 1 (edit for loop so it stops when we get first negative value and write a message about that)?

[Julenissen]

The problem with your answer is that it expands matrice until it find this value, but original matrice was limited to 10 values in length. So the loop have to start again when desired negative value is not found.

Probably it will be easiest to find solution on question number 1 and stop at this point :) thank you for trying

[Alan Stevens]

The following restircts your vetor to 10 elements. Not clear to me if you want to start from 1 1 each time the 10 values are exceeded, or if you just want to carry on with the larger Fibonacci numbers. The following does the latter. It answers the first question automatically (the message I've printed just indicates the number of iterations. Replace with whatever message you want.)

fib = [1; 1];
its = 0;
keepgoing = true;
p1 = 1; p2 = 2; p3 = 3;
while keepgoing 
    its = its+1;
    
    if p1>10
       p1 = 1; 
    end
    if p2 > 10
       p2 = 1; 
    end
    if p3 > 10
         p3 = 1;
    end
    
    
    r = rand;
    if r>0.1
       fib(p3) = fib(p2) + fib(p1);
    else
        fib(p3) = fib(p2) - fib(p1);
    end
    if fib(p3)<0
       keepgoing = false; 
    end
    p1 = p1+1;
    p2 = p2+1;
    p3 = p3+1;
end
disp(['number of iterations ' num2str(its)])

[Julenissen]

The next loop have to start from 1 and 1 again :)