Question

dan, I love you

Answer 1

@dan815 the file I have so far...it's basic, but that's all there is according to the example I was given... Only the task is to set r=1 and K=20 fixed which I was trying to do but the code keeps going whacko

Answer 2

if I remove r =1 and K=20, it's fine, but that's not what my goal is.

Answer 3

function f=ram(ry,y,K)
f=ry(1-y/K);

Answer 4

well what do u wanna do to it

Answer 5

i dont see anything wrong with it

Answer 6

the task issss

Solve the logistic DE dy/dt= ry(1-y/K) with r=1, K=20
% initial conditions y(0)= 5, 10, 15, 20, 25, (use a for loop)
% plotting the solutions on the same figure for t in [0 10]

Answer 7

ok so rewrite the equatio now

Answer 8

u are missing * sign in there

Answer 9

which I noticed there are three variables, r,y, and K. If I do something like

function f=ram(r,y,K)
f=ry(1-y/K);
K=20; r=1;

it won't work.

if I do
function f=ram(ry,y,K)
f=ry(1-y/K);

it works but I need to have K=20 and r=1 fixed and that's not what is going on here

If I do

function f=ram(ry,y,K)
f=ry(1-y/K);
K=20; r=1;

Matlab whines

Answer 10

and ry is not a paremeter is it

Answer 11

isnt it r*y*(1-y)/K)

Answer 12

star sign? oh shxxx

Answer 13

one sec

Answer 14

is ry 1 variable?

Answer 15

matlab needs to make their fonts bigger I swear

Answer 16

well I saw r y and K. I thought it was 3?

Answer 17

hold on refine your function with a t input, u need that when u use ode45

Answer 18

so it's t y K?

Answer 19

anyway
>> ram(1,1, 20)

ans =

0.9500

>>

if I just solve XD

Answer 20

OH WAIT. hold on

Answer 21

you cant do that, this is just dy/dt, its only telling u the slope

Answer 22

btw ode45 is just a nurmerical method, our prof made us once write the code behind ode45 and the 90 or something its pretty straight forwad just different step sizes for the approximations

Answer 23

pft ... doing ram(1,y,20) on the code makes it scream

Answer 24

so if u are ever having trouble u can just write your own numerical method and it shud be fine

Answer 25

but I need a graph...

Answer 26

yeah okay

Answer 27

wait lets start over...

Answer 28

k

Answer 29

the task issss Solve the logistic DE
dy/dt= ry(1-y/K)
r=1, K=20 % initial conditions y(0)= 5, 10, 15, 20, 25,(use a for loop)
% plotting the solutions on the same figure for t in [0 10]

Answer 30

mhm but there's r y and K in there and we have to let it be fixed at r=1 and K=20

Answer 31

function f=ram(t,y)
r=1
K=20
f=r*y*(1-y/K);
end

Answer 32

function f=ram(t,y)
f=r*y*(1-y/K);
K=20;
r=1;
end
?

Answer 33

[tv f]=ode45('ram',[0 5],1);
Undefined function or variable "r".

Error in ram (line 2)
f=r*y*(1-y/K);

Error in odearguments (line 88)
f0 = feval(ode,t0,y0,args{:}); % ODE15I sets args{1} to yp0.

Error in ode45 (line 114)
[neq, tspan, ntspan, next, t0, tfinal, tdir, y0, f0, odeArgs,
odeFcn, ...


I will kill this thing

Answer 34

the example code for the graph is


[tv f]=ode45('fun',[0 5],1);
plot(tv,f,'-')
title('y''=-ty/sqrt(2-y^2), y(0)=1, t in [0, 5]')
grid
axis([0 5 0 1])


so I'm just doing the first two for now with

[tv f]=ode45('ram',[0 5],1);
plot(tv,f,'-')

and I'm getting messed up

Answer 35

yup proof I need glasses... I was aiming to type, dan I need you. x.x omg fml not my night

Answer 36

http://prntscr.com/60wmu3

Answer 37

does it gotta look something like that

Answer 38

getting therer my scripts were running wild

Answer 39

yeah that graph is right...

Answer 40

okay heres the code i used so far

Answer 41

function out=usukicode()
for i=[5,10,15,20,25]
timerange=[0 10];
initialcond=i
[t,y]=ode45(@ram,timerange, initialcond)
plot(t,y)

hold all;
end
end



function f=ram(t,y)

f=y.*(1-y./20);
end

Answer 42

all in 1 file and named it usukicode.m

Answer 43

u can make that forloop a little more classy by
i=5:5:25

Answer 44

ok, but what happens if r =2?

Answer 45

ok if u wanna pass that in too then

Answer 46

do u have to be able to pass it in? or u just wanna see with a new r

Answer 47

hold on let me try get r = 2 on this one ^_^ and if I crash I'll let you know.

Answer 48

r=2 : http://prntscr.com/60wp24
r=1 : http://prntscr.com/60wmu3

Answer 49

a faster convergence

Answer 50

wtf all I got is just one graph T___T!

Answer 51

oh u want it in differnt graph?

Answer 52

no all lines have to be in one graph... why do I only have 1?

Answer 53

lol what even is that

Answer 54

save and pass usukicode(r) for r=1

Answer 55

oh i see initial cond is not 1

Answer 56

its i

Answer 57

XD! told ya matlab needs to have bigger fonts

Answer 58

yeah

Answer 59

what the rabbit.. if I input 2 into the function, the code displays a red warning

Answer 60

its working for me

Answer 61

maybe u shud define the ram function in another file instead of under the code like i did

Answer 62

i have it defined in 2 files so im not sure which instance its refering to

Answer 63

oh like a secondary

Answer 64

gotcha

Answer 65

got em

function out = nike()
for i=[5,10,15,20,25]
timerange=[0,10]
initialcond=i
[t,y]=ode45(@card,timerange,initialcond)
plot(t,y)
hold all;
end
end
function f=card(t,y)
f=2*y*(1-y/20)
end

Answer 66

looks like the graphs are retracting and the 20 is the equilibrium

Answer 67

it has something to do with the population... eh I can type that later.

Answer 68

xlabel time
ylabel population is missing but that's ok. I can always fix it... I know that the original problem has 2 equlibriums 0 and K and the graphs have something to do with the population when it's going towards the equilibrium or something like that

Answer 69

yep it does

Answer 70

there is a saturation point for this ode it can be used to model some population growths

Answer 71

do you know what a bifurcation graph is?

Answer 72

i dunno what bifucation means

Answer 73

but the main things to observe on your graph is there is a point where its increasing exponentially then decreasing exponentially

Answer 74

just like population growths, when u are far from the lands supporting population u will increase like a normal interest rate growth function

Answer 75

http://www.sosmath.com/diffeq/first/bifurcation/bifurcation.html

Answer 76

but as u get closed to the saturation point the growth rate goes lower and lower and evnetually decays

Answer 77

closer*

Answer 78

oh ok ya i know this ode :)

Answer 79

and then the population is messed up

Answer 80

what about graphing stability?

Answer 81

0 is a unstable point

Answer 82

i dunno what u call it exactly but

Answer 83

its the one where things diverge away from

Answer 84

yeah but do you graph by hand x)

Answer 85

u wanna graph what happens at 0?

Answer 86

ya

Answer 87

add more inital conditions then

Answer 88

but a different function

Answer 89

like

dx/dt = x(x^2-a^2)

besides x =0,-a,a which are the equilibrium points... how do I graph it manually?

Answer 90

graph what exactly?