the command after "if" not work after awhile

Question

mohammad gudarzi 2019-5-23

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/463658-the-command-after-if-not-work-after-awhile

评论： mohammad gudarzi 2019-5-23

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

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mohammad gudarzi 2019-5-23

在 MATLAB Online 中打开

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 2019-5-23

在 MATLAB Online 中打开

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 2019-5-23

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

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Answer 1

the cyclist 2019-5-23

2
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/463658-the-command-after-if-not-work-after-awhile#answer_376398

在 MATLAB Online 中打开

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: