MATLAB: If condition does not store the variables

if statementvariables

Hello,
Please forgive me my post is long. But I debugged and I came up with no solution. I want to store my u, p and gap arrays at time t=2, t=3, t=4, and t=5, then plot all. To do this, I put if commands associated with the times above as can be seen from the code below. At first, Matlab gave me the plots. However,now matlab says undefined variable u2 which is the first stored variable.
clear all;close all;clc;
Tsim=4;
ii=101;
dt=10^(-5);
tstart = 1;    
dx=0.01;      
Nt=(Tsim/dt)+1;
% Nt = 1;
ii_vect = 1:ii;
x = (ii_vect-1)*dx;
%----------------------------------------------------------------

b = 0.5;                                                       %|






kappa = 1;                                                     %|
gbar = .1;                                                     %|
M = 1;                                                         %|
g = gbar/M;                                                    %|
Gamma = 0.2;                                                   %|
F = kappa.*x.*(1-x);                                           %|
h = kappa*b*(1-b);        % h0 = F(b);                         %|
theta = kappa*(1-2*b);    % htheta0 = F'(b);                   %|
%----------------------------------------------------------------
pi=4.*atan(1);
hD=0;
thetaD=0;
gap0 = 0.5;
cb=gap0;
ub=zeros(1,ii); u=zeros(1,ii); p=zeros(1,ii); q=zeros(1,ii); gap=zeros(1,ii);
gap=gap0+(-F+h+theta.*(x-b));
ub=(cb-hD.*(x-b)-thetaD/2.*(x-b).^2)./gap;
dc=-10^(-5);
%______________ TIME ITERATION____________%
for nt=1:Nt
    flag=0;
    mmm=1;
    m=1;
    c=cb;
    t=tstart+(nt-1)*dt;
    while (flag==0)
        u=(c-hD.*(x-b)-thetaD/2.*(x-b).^2)./gap;     
        p(1)=0.5*(1-u(1)^2);
        q(1)=0;
        for i=2:ii
            q(i)=q(i-1)-dx*(u(i-1)-ub(i-1))/dt;
            p(i)=0.5*(1-u(i)^2)+q(i);
        end
        st(m)=p(ii)-0;
        m=m+1;
        if (m==2)
            c=c+dc;
        end
        if (m==3)
            c=(c*st(1)-(c-dc)*st(2))/(st(1)-st(2));
        end
        if (m==4)
            mmm=mmm+1;
            if (mmm==2)
                m=1;
            else
                flag=1;    
            end         
        end               
      end
      sumint1=zeros(1,ii); sumint2=zeros(1,ii);
      sumint1=0.5*(p(1)); sumint2=0.5*(p(1))*(-b);    
      for k=2:ii-1
          xx=(k-1)*dx;
          sumint1=sumint1+(p(k));
          sumint2=sumint2+(p(k))*(xx-b);
      end
      hDDOT=(sumint1*dx - M*g)/M;
      hD=hD+dt*hDDOT;
      h=h+dt*hD;
      thetaDDOT=sumint2*dx/(Gamma*M);
      thetaD=thetaD+dt*thetaDDOT;
      theta=theta+dt*thetaD;
      hL=h-theta*b;
      hR=h+theta*(1-b);  
      gap=gap0+(-F+h+theta.*(x-b));
  %____THESE IF CONDITIONS ARE NOT RECOGNISED  !___%
      if  t==2   %%(nt==(2-tstart)/dt +1)  
          u2=u;
          p2=p;
          gap2=gap;
      end
      if  t==3   %%(nt==(3-tstart)/dt +1)  
          u3=u;
          p3=p;
          gap3=gap;
      end
      if  t==4   %%(nt==(4-tstart)/dt +1)  
          u4=u;
          p4=p;
          gap4=gap;
      end
      if  t==5   %%(nt==(5-tstart)/dt +1)     
          u5=u;
          p5=p;
          gap5=gap;
      end
  end
  ub(:)=u(:);
  cb=c;
  %_______PLOTTING_______%
figure(1)
subplot(2,2,1)
plot(x,u2,'k-')
hold on
plot(x,u3,'k:')
hold on
plot(x,u4,'k--')
hold on
plot(x,u5,'k-.')
hold on
legend('t=2', 't=3', 't=4','t=5')
subplot(2,2,2)
plot(x,p2,'k-')
hold on
plot(x,p3,'k:')
hold on
plot(x,p4,'k--')
hold on
legend('t=2', 't=3', 't=4','t=5')
subplot(2,2,[3,4])
plot(x,gap2,'k-')
hold on
plot(x,gap3,'k:')
hold on
plot(x,gap4,'k--')
hold on
plot(x,gap5,'k-.')
legend('t=2', 't=3', 't=4','t=5')

Best Answer

I have not gone through your code in detail, but I have an educated guess.
When you do your tests such as
if t==2
t is a floating-point variable which it looks to me might not be exactly equal to 2 (or whatever), because you are calculating it via a for loop that will not lead to exact integers. [It might be equal to 2.000000000001, for example.]
I expect you need to either calculate t in a way that ensure it is exactly equal to the integer you expect, or make your comparison in a way the includes a tolerance.
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