Can't get plot to work

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Matt Baron
Matt Baron 2021-3-25
评论： Matt Baron 2021-3-27
Hello Mathworks team!
I created this Euler's method code below but can't get the graph to show anything at all. Even if I make a separate code with just xn=1 yn=1, plot(xn,yn), the graph shows up but nothing on it. Any help would be much appreciated.
%Euler's Method
h=.25;%step size
xinitialcondition=0;%initial condition for x
yinitialcondition=1;%initial condition for y
n=0;%start at 0
xn=xinitialcondition;
yn=yinitialcondition;
f=@(xn,yn) (2*(cos(xn))*yn);%the function to be evaluated
while xn < 10%what value do you want it estimated to
xn = (xinitialcondition) + ((n) .* (h));
[n xn yn]
yn = (yn) + ((h) .* f(xn,yn));
if xn >= 10
break
end
plot(xn,yn,"r-")
hold on
n=n+1;
end
hold off

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采纳的回答

Walter Roberson
Walter Roberson 2021-3-25
Ran in:
%Euler's Method
h=.25;%step size
xinitialcondition=0;%initial condition for x
yinitialcondition=1;%initial condition for y
n=0;%start at 0
xn=xinitialcondition;
yn=yinitialcondition;
f=@(xn,yn) (2*(cos(xn))*yn);%the function to be evaluated
while xn < 10%what value do you want it estimated to
xn = (xinitialcondition) + ((n) .* (h));
[n xn yn]
yn = (yn) + ((h) .* f(xn,yn));
if xn >= 10
break
end
plot(xn,yn,"r-*") %CHANGED
hold on
n=n+1;
end
ans = 1×3
0 0 1
ans = 1×3
1.0000 0.2500 1.5000
ans = 1×3
2.0000 0.5000 2.2267
ans = 1×3
3.0000 0.7500 3.2037
ans = 1×3
4.0000 1.0000 4.3758
ans = 1×3
5.0000 1.2500 5.5579
ans = 1×3
6.0000 1.5000 6.4342
ans = 1×3
7.0000 1.7500 6.6618
ans = 1×3
8.0000 2.0000 6.0681
ans = 1×3
9.0000 2.2500 4.8055
ans = 1×3
10.0000 2.5000 3.2961
ans = 1×3
11.0000 2.7500 1.9758
ans = 1×3
12.0000 3.0000 1.0627
ans = 1×3
13.0000 3.2500 0.5367
ans = 1×3
14.0000 3.5000 0.2699
ans = 1×3
15.0000 3.7500 0.1435
ans = 1×3
16.0000 4.0000 0.0846
ans = 1×3
17.0000 4.2500 0.0570
ans = 1×3
18.0000 4.5000 0.0443
ans = 1×3
19.0000 4.7500 0.0396
ans = 1×3
20.0000 5.0000 0.0403
ans = 1×3
21.0000 5.2500 0.0461
ans = 1×3
22.0000 5.5000 0.0579
ans = 1×3
23.0000 5.7500 0.0784
ans = 1×3
24.0000 6.0000 0.1121
ans = 1×3
25.0000 6.2500 0.1659
ans = 1×3
26.0000 6.5000 0.2489
ans = 1×3
27.0000 6.7500 0.3704
ans = 1×3
28.0000 7.0000 0.5358
ans = 1×3
29.0000 7.2500 0.7377
ans = 1×3
30.0000 7.5000 0.9472
ans = 1×3
31.0000 7.7500 1.1114
ans = 1×3
32.0000 8.0000 1.1691
ans = 1×3
33.0000 8.2500 1.0840
ans = 1×3
34.0000 8.5000 0.8749
ans = 1×3
35.0000 8.7500 0.6116
ans = 1×3
36.0000 9.0000 0.3728
ans = 1×3
37.0000 9.2500 0.2030
ans = 1×3
38.0000 9.5000 0.1030
ans = 1×3
39.0000 9.7500 0.0517
ans = 1×3
40.0000 10.0000 0.0272
hold off
  2 个评论
Walter Roberson
Walter Roberson 2021-3-25
plot() only draws a line if there are two adjacent finite values in the same call. You are only plotting one point at a time, so it cannot draw lines.

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更多回答（1 个）

KSSV
KSSV 2021-3-25
Ran in:
Save into a Matrix and plot all at once:
%Euler's Method
h=.25;%step size
xinitialcondition=0;%initial condition for x
yinitialcondition=1;%initial condition for y
n=0;%start at 0
xn=xinitialcondition;
yn=yinitialcondition;
f=@(xn,yn) (2*(cos(xn))*yn);%the function to be evaluated
A = zeros([],2) ;
while xn < 10%what value do you want it estimated to
xn = (xinitialcondition) + ((n) .* (h));
yn = (yn) + ((h) .* f(xn,yn));
if xn >= 10
break
end
n=n+1;
A(n,:) = [xn yn] ;
end
plot(A(:,1),A(:,2))
  6 个评论
显示 4更早的评论
Matt Baron
Matt Baron 2021-3-27
Do I have the questions correct in bold below;
A = zeros([],2) ; %so this is like creating an empty matrix that will be x by 2 for future use?
while xn < 10%what value do you want it estimated to
xn = (xinitialcondition) + ((n) .* (h));
yn = (yn) + ((h) .* f(xn,yn));
if xn >= 10
break
end
n=n+1;
A(n,:) = [xn yn] ; %this is inputting xn yn into the nth row of matrix A?
end
plot(A(:,1),A(:,2)) %this plots matrix A all rows of column 2 vs matrix A all rows of column 1?
Matt Baron
Matt Baron 2021-3-27
It wasn't working with n=1 so I re-wrote the code using matrices according to your code above and it worked!!
This looks so much better.
h=.05;%step size
init=[1 1];%initial conditions for x and y
n=1;%start at 1
x=init(1,1);
y=init(1,2);
A=zeros([],2);%create an empty 0 by 2 matrix for later
f=@(x,y) (.2 * x * y);%the function to be evaluated
while x <= 1.5%what value do you want it estimated to
A(n,:)=[x y];%input x and y into row n
y = (y + ((h) * f(x,y)));
x = (init(1,1) + ((n) * (h)));
if x > 1.5
break
end
n=n+1;
end
plot (A(:,1),A(:,2))%plot y vs x

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