why do I receive two numbers when I use Find command?

could you please figure out the problem why do i get twi indecies for tg

which the first index t( 45633 ) is not giving the correct index but i still receive it.

clear
close all, clc


ca = 1014 ;             
ci = 2050 ;             
cw = 4218 ;             
eps = 27.03 ;            
LWC = 0.001  ;           
Haw = 500 ;             
His = 100 ;             
ki = 2.18 ;             
kw = 0.571 ;            
Le = 13.4 ;             
LF =  3.344 * 1e5 ;     
LE = 2.26 * 1e6 ;       
p0 = 6.3*1e4 ;          
v = 90  ;                
beta = 0.55 ;           
rho_g = 917 ;           
rho_r = 880 ;           
rho_w = 1000 ;          
X = 11 ;                

r = 0.9 ;               
R = 287.05 ;            
Tf = 273.15 ;           
cp = 1012 ;             

Ta= 263.15;
Ts=Ta;
Qa=r*Haw*((v)^2/(2*cp));
Qk=LWC*beta*((v^3)/2);
sigma = 5.67*1e-8;
t=0:0.001:320;
for i=1:length(t)
   
    Bg =  (ki*(Tf-Ts)) / (LWC*beta*v*LF + (Qa+Qk)-((Haw+LWC*beta*v*cw+X*eps)*(Tf-Ta)));
    tg = (rho_r /(LWC*beta*v))*Bg ;             % time when first glaze appears

%     B(i)=((Haw+X*eps+LWC*beta*v*cw-Qa-Qk)/(rho_g*LF))*t(i);
end

tg_index=find(abs(t-tg)<1e-3)

tg_index = 1×2

45633 45634

[Geoff Hayes]

@mehmet salihi - but both of those indices satisfy the condition that you are using in your call to find. For example,

>> abs(t(45633)-tg)

ans =

   5.9622e-04

and

>> abs(t(45634)-tg)

ans =

   4.0378e-04

where the ans to each is less than 1e-3.

Answer 1

[Adam Danz]

> why do i get two indices for tg?

Let's look at your data. Your code:

ca = 1014 ;             
ci = 2050 ;             
cw = 4218 ;             
eps = 27.03 ;            
LWC = 0.001  ;           
Haw = 500 ;             
His = 100 ;             
ki = 2.18 ;             
kw = 0.571 ;            
Le = 13.4 ;             
LF =  3.344 * 1e5 ;     
LE = 2.26 * 1e6 ;       
p0 = 6.3*1e4 ;          
v = 90  ;                
beta = 0.55 ;           
rho_g = 917 ;           
rho_r = 880 ;           
rho_w = 1000 ;          
X = 11 ;                
r = 0.9 ;               
R = 287.05 ;            
Tf = 273.15 ;           
cp = 1012 ;             
Ta= 263.15;
Ts=Ta;
Qa=r*Haw*((v)^2/(2*cp));
Qk=LWC*beta*((v^3)/2);
sigma = 5.67*1e-8;
t=0:0.001:320;
for i=1:length(t)
   
    Bg =  (ki*(Tf-Ts)) / (LWC*beta*v*LF + (Qa+Qk)-((Haw+LWC*beta*v*cw+X*eps)*(Tf-Ta)));
    tg = (rho_r /(LWC*beta*v))*Bg ;             % time when first glaze appears
%     B(i)=((Haw+X*eps+LWC*beta*v*cw-Qa-Qk)/(rho_g*LF))*t(i);
end

tg_index=find(abs(t-tg)<1e-3)

tg_index = 1×2

45633 45634

tg_index contains the indices of all values abs(t-tg) that are less than 0.001. Let's plot those values,

plot(abs(t-tg), '-o')
yline(1e-3) % draw a line at your specified threshold
ylim([0,0.002]) % zoom into the bottom to see the threshold
% add red lines at the tg_index values for confirmation
xline(tg_index, 'r--')

Here we can see that there are two values less than your threshold and they are at the expected indices returned by find().

> the first index t( 45633 ) is not giving the correct index but i still receive it.

The investigation above shows that both indices correctly find values less than 0.001. Were you trying to find the minimum value? If so, [~, tg_index] = min(abs(t-tg)).

[mehmet salihi]

I am really in love with you explanation.

thank you very much for your help

Answer 2

[Voss]

Evidently those two elements of t are within 0.001 of tg. If you want just the first element of t that is within 0.001 of tg you can say

tg_index=find(abs(t-tg)<1e-3,1);

but it seems more likely that you want the element of t that is closest to tg, in which case you can say

[~,tg_index]=min(abs(t-tg));

[mehmet salihi]

thank you very much for your help