MATLAB is giving me 0's instead of really small numbers

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Nicholas Morris 2021-8-17

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评论： Walter Roberson 2023-4-17

I don't have code to share, but MATLAB is outputing 0's instead carrying through small numbers. When I change the command window to format long, the calculation show non zero. However, when my code runs it is carrying these zeros through my calculations.

>> Calculation
ans = 
        0.0
        
>> Format long
>> Calculation
ans = 
        
        8.425293911978570e-07

How do I make matlab keep the small numbers?

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Steven Lord 2021-8-18

Can you show us a small segment of the code that appears to return 0's instead of the small numbers you expected?

How small is "really small" in this context? Smaller than realmin?

Nicholas Morris 2021-8-18

在 MATLAB Online 中打开

See the comments in the answer below.

The numbers are definitely not smaller than realmin. and I'm not really worried about the fact that my command window is displaying "0" for all values in my matrix, but more the fact that if I use the following:

>>find(array>0)
ans =
  0×1 empty double column vector

so these numbers are actually zero.

I have a similar array in my code with small numbers that you can see display properly

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Nicholas Morris 2021-8-18

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Problem solved...Indexing errors.

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Walter Roberson 2021-8-17

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MATLAB is keeping the small numbers throughout your calculation.

I recommend that you put

format long g

at the top of your code and run again.

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Walter Roberson 2021-8-18

Your code might be zeroing the entire array at each step.

But as well, "format long g" does not affect the variable browser. There is a drop-down above the variable name that can select the format for the variable browser; personally though I recommend setting Preferences -> Variables -> Default Array Format to "long g", which controls the default format for the variable browser.

Nicholas Morris 2021-8-18

I've already tried the "Preferences -> Variables -> Default Array Format to "long g"... : (

No luck.

What do you mean by my code "might be zeroing the entire array at each step"?? Why would it do this?

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vikas singh 2023-3-12

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编辑：Walter Roberson 2023-3-12

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I have written a code and it is giving me H3 and z2 zero. i dont know what is the problem.

clc;
clear all;
close all;
l=10000;
x=0:100:l;
t=[0;1;5;10;15;20;25];
H=30;
alpha=0.00384
Mu=2.8e-1;
D=31;
k=0.01132;
k1=l/H;
for n=1:100
    a=2*H;
    b=(1-(-1)^n*exp(-k*l))/((n*pi+(k^2*l^2)/(n*pi)));
    c=1/(n*pi);
    Cn(n)=a*(b-c);
end
s=0;
for n=1:100
    d= sin(n*pi*x/l);
    q= exp((-n^2*pi^2*t)/(Mu^2*l));
    aa=Cn(n).*d.*q;
    s=s+aa;
    bb=(2*alpha*Mu^2*l^2)/(n*pi)^3;
    s2=bb.*(1-q).*d;
    
end
J=(H+alpha*t).*x/l;
J1=alpha*t;
z1=s-s2-J+J1;
for n=1:100
    syms X T  
    disp(n)
    H0=(Cn(n)/(D-H)).*sin(n*pi*X).*exp(-n^2*pi^2*T/Mu)+(bb/(D-H)).*(1-exp(-n^2*pi^2*T/Mu)).*sin(n*pi*X) +(H/(D-H)+alpha*T*Mu*l/(D-H))*X+(H-alpha*T*Mu*l)/(D-H);
    y=diff(H0*diff(H0,X),X);
    Q=int(y*sin(n*pi*X),X,0,1);
    P=int(Q*exp(n^2*pi^2*T/Mu),T,0,T);
    Hn=-2*P/(exp(n^2*pi^2*T/Mu));
    H1=Hn.*sin(n*pi*X);
    for i=1:size(t,1)
        H2=subs(H1,T,t/(Mu*l));
    end
    format long g
    H3=subs(H2,X,x/l)
    
end
z2=double(((D-H)^2).*H3/l)

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Walter Roberson 2023-3-12

在 MATLAB Online 中打开

Well, I can get you non-zeros, but it is questionable as to whether the result is what you want.

format long g
l=10000;
x=0:100:l;
t=[0;1;5;10;15;20;25];
H=30;
alpha=0.00384
Mu=2.8e-1;
D=31;
k=0.01132;
k1=l/H;
for n=1:100
    a=2*H;
    b=(1-(-1)^n*exp(-k*l))/((n*pi+(k^2*l^2)/(n*pi)));
    c=1/(n*pi);
    Cn(n)=a*(b-c);
end
s=0;
for n=1:100
    d= sin(n*pi*x/l);
    q= exp((-n^2*pi^2*t)/(Mu^2*l));
    aa=Cn(n).*d.*q;
    s=s+aa;
    bb=(2*alpha*Mu^2*l^2)/(n*pi)^3;
    s2=bb.*(1-q).*d;
    
end
J=(H+alpha*t).*x/l;
J1=alpha*t;
z1=s-s2-J+J1;
syms X T 
for n=1:100
    disp(n)
    H0=(Cn(n)/(D-H)).*sin(n*pi*X).*exp(-n^2*pi^2*T/Mu)+(bb/(D-H)).*(1-exp(-n^2*pi^2*T/Mu)).*sin(n*pi*X) +(H/(D-H)+alpha*T*Mu*l/(D-H))*X+(H-alpha*T*Mu*l)/(D-H);
    y=diff(H0*diff(H0,X),X);
    Q=int(y*sin(n*pi*X),X,0,1);
    P=int(Q*exp(n^2*pi^2*T/Mu),T,0,T);
    Hn=-2*P/(exp(n^2*pi^2*T/Mu));
    H1=Hn.*sin(n*pi*X);
    for i=1:size(t,1)
        H2(i,1)=subs(H1,T,t(i)/(Mu*l));
    end
    H3(:,:,n)=subs(H2,X,x/l);
end
z2=double(((D-H)^2).*H3/l);