the command after "if" not work after awhile

the "counter"=1 at start, the command under if work until t==8 but after that it's looklike "if" not working beside that "t" continue untill 200.

thanks

if  t==(1*counter)
    filename = 'y.xlsx';
    filename1= 'V.xlsx';
    filename2= 'Q.xlsx';
    A=double(yp);
    C=double(vp);
    D=double(Qp);
    for i=1:16
        maty(counter,i)=A(1,i);
        matQ(counter,i)=D(1,i);
        matv(counter,i)=C(1,i);
        my{counter,i}=[A(1,i)];
        mv{counter,i}=[C(1,i)];
        mq{counter,i}=[D(1,i)];
    end    
    counter=counter+1;
end

[Geoff Hayes]

mahammad - how does t change over time? Do you ever reset the counter to zero? Please show more of your code.

[mohammad gudarzi]

the step of t=0.005.

as i know "counter" never reset

clc;clear all;close all;
%%initial value
B=xlsread('input','input','A2');aaaa=0;
s0=xlsread('input','input','B2');counter=1;
delt=xlsread('input','input','C2');
delx=xlsread('input','input','D2');
teta=xlsread('input','input','E2');
a=xlsread('input','input','H2');
b=xlsread('input','input','I2');
T=xlsread('input','input','J2');
g=xlsread('input','input','K2');
X0=xlsread('input','input','B4:Q4');
Q0=xlsread('input','input','B5:Q5');
y=xlsread('input','input','B6:Q6');
%% main program
maty=zeros(200,16);
matQ=zeros(200,16);
matv=zeros(200,16);
for t=0.005:delt:T
    for i = 1:16
    v(i)=Q0(i)/(B*y(i));
    end
    for i = 1:15
    syms x 
    [ys(i)]=vpasolve(0==(x-y(i))+(teta*(y(i+1)-y(i))/(1+teta*(v(i+1)-v(i)))*(v(i)-(g*x)^0.5)));
    vs(i)=(v(i)-teta*((g*y(i+1))^0.5*v(i)-(g*y(i))^0.5*v(i+1)))/(1-teta*(v(i)-v(i+1)-(g*y(i))^0.5+(g*y(i+1))^0.5));
    Qs(i)=B*ys(i)*vs(i);
    Sfs(i)= a* vs(i)+ b * vs(i)^2;
    [yr(i)]=vpasolve(0==(y(i+1)-x)-teta*(y(i+1)-y(i))/(1+teta*(v(i+1)-v(i)))*(v(i+1)+(g*x)^0.5));
    vr(i)=(v(i+1)-teta*((g*y(i))^0.5*v(i+1)-(g*y(i+1))^0.5*v(i)))/(1+teta*(v(i+1)-v(i)+(g*y(i+1))^0.5-(g*y(i))^0.5));
    Qr(i)=B*yr(i)*vr(i);
    Sfr(i)=a* vr(i)+ b * vr(i)^2;
    end
    %upstream condition
    Qp(1)= 0.0003+0.0075*exp(-0.5*abs((t-50.7261)/24.495)^2.9063);
    syms X
    [ypp]=vpasolve(0==((vs(1)-2*(g*ys(1))^0.5)+g*(s0-Sfs(1))*delt)+(2*(g*X)^0.5)-(Qp(1)/(B*X)));
    for i=1:14
    yppp(i) = 0.5*(yr(i)+ys(i+1))+ teta*((Qr(i)-Qs(i+1))/(2*B));
    end
    %downstream condition
    ypppp=0.1201*exp(-0.5*((t-81.7891)/23.2503)^2);
    %start velocity
    vpp=Qp(1)/(B*ypp);
    %end velocity
    vpppp= (vr(15)+2*(g*yr(15))^0.5)+(g*(s0-Sfr(15))*delt)-2*(g*ypppp)^0.5;
    
    for i = 1:14
    vppp(i)=0.5*(vr(i)+vs(i+1))+0.5*teta*((vr(i)^2-vs(i+1)^2)+g*(yr(i)-ys(i+1)))+g*delt*(s0-0.5*(Sfr(i)+Sfs(i+1)));
    end
    %combine yp & vp
    yp=horzcat(ypp,yppp,ypppp);
    vp=horzcat(vpp,vppp,vpppp);
    
    if t==0:1:200
    plot(X0,yp)
    end
    for i=1:15
    Qp(i+1)=B*vp(i+1)*yp(i+1);
    end
    
    %output
        if  t==(1*counter)
        filename = 'y.xlsx';
        filename1= 'V.xlsx';
        filename2= 'Q.xlsx';
        A=double(yp);
        C=double(vp);
        D=double(Qp);
        for i=1:16
            maty(counter,i)=A(1,i);
            matQ(counter,i)=D(1,i);
            matv(counter,i)=C(1,i);
        my{counter,i}=[A(1,i)];
        mv{counter,i}=[C(1,i)];
        mq{counter,i}=[D(1,i)];
        end    
        counter=counter+1;
        end
    Q0=Qp;
    y=yp;
    aaaa=aaaa+1
end
xlswrite(filename,my)
xlswrite(filename1,mv)
xlswrite(filename2,mq)

[Geoff Hayes]

What are you expecting to happen with this code?

if t==0:1:200
    plot(X0,yp)
end

Do you really mean to compare t to an array of values from 0-200?

[mohammad gudarzi]

that's not important, i can delete that but the problem is with next if

Answer 1

[the cyclist]

My best guess here is that you are running up against the limitations of floating point computation. For example, try putting this expression into MATLAB:

6*0.01 == (0.01+0.01+0.01+0.01+0.01+0.01)

You will find that the result is false (logical 0). This is probably unexpected by you.

Then try

6*0.01 - (0.01+0.01+0.01+0.01+0.01+0.01)

and you will see that the result is a number of order 1.e-18 (very tiny but not zero).

I suggest that you read this documentation page about floating point representation. There are also many, many answers here that talk about this issue. Be aware that this is not some kind of MATLAB error.

I also happened to notice that you have this line in your code:

if t==0:1:200
   ... 
end

I'm not sure what you are expecting to happen there, but you will only enter that loop is t is equal to the vector [0 1 2 ... 199 200].

[mohammad gudarzi]

thank you about answer, i test that and it's true, because of that tiny number the "if" not runnig.

thanks for help