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it is not plotting

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Meva 2015-5-9

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关闭： MATLAB Answer Bot 2021-8-20

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Hi,

I try this;

beta1 = 0.4*pi;
alphabar = 1;
dx=0.01;
for i=1:101
  xx(i) = (i-1)*dx
end
dt=0.0001;
  for nt=1:10001
  t=(nt-1)*dt
  ...
  if t==0.1
    for i=1:101
      t
      if xx(i) < t
        dp2dx(i) = -beta1-3*beta1^2.*xx(i);
      else
        dp2dx(i) = -beta1+2.*xx(i)*(-beta1*alphabar-1.5*beta1^2) + ...
                         t*2*beta1*alphabar;
      end
    end
    figure(16) 
    plot(xx,dp2dx)
where xx(i) = (i-1)*0.01.  But it is not plotting. Any suggestions??

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Geoff Hayes 2015-5-9

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Meva - what are the dimensions of xx and dp2dx? What is the maximum value of xx? Note how you have a condition

if xx(i) < t

Is this ever satisfied?

Meva 2015-5-9

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I am editing my question

Meva 2015-5-9

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xx and dp2dx 1*101 arrays

Meva 2015-5-9

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yes the condition

if xx(i) < t

is satisfied since xx(i) can take 0<xx(i)<0.1 and time is 0.1 so it is satisfied.

Geoff Hayes 2015-5-9

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What are the minimum and maximum values of your xx and dp2dx arrays? Just do

 min(xx)
 max(xx)
 min(dp2dx)
 max(dp2dx)

Meva 2015-5-9

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编辑：Meva 2015-5-9

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min of xx is 0 max of xx is 1 and dp2dx is calculated from xx as can be seen. Also dp2dx both min and max values show

-1.2566

So I assume it does not compute dp2/dx

dpb 2015-5-9

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Doesn't answer the question raised...you're assuming something is happening that may not be. As Geoff, says, "show us".

You say "it is not plotting"; what is happening, specifically?

I note you have

figure(16)

in the script before plot---you sure you're looking at the proper figure or have you got the axes there fixed outside the bounds of the data from a previous iteration or somesuch? We can't know these things from here.

I'd suggest just

figure
plot(...

and see wht happens.

NB: Besides the < test Geoff asks about, you also have the conditional

if t==0.1

containing the whole rest of the code snippet. Relying on floating point exact comparison is risky altho in this case it should work as you have an integer multiple of a delta but in general you can't count on a computed floating point value being identical to the constant to the last significant bit; you really should double-check this is actually happening here as well and write the test as

if abs(t-CONDITION)<3*eps(t)

or somesuch other appropriate for the case relative error value.

Meva 2015-5-9

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编辑：Meva 2015-5-9

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I see that is why I changed the if condition for

if t==0.1

I changed this with

if nt==1001

which gives t=0.1 so they are same.

I now typed

figure

instead of

figure(16)

The result is, I am still not getting the plot. The error should be in dp2/dx lines

Meva 2015-5-9

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编辑：Meva 2015-5-9

I now see the reason of not plotting. It is because dp2dx is 1 dimensional just a value not 1*101 array. But I have used this in a for loop. It is supposed to be 1 *101 array.

Meva 2015-5-9