Array indices must be positive integers or logical values.

I just simply want to use previous Re, f and V values in the next one and trying to stop iteration when it reaches the criteria i made. But for the many times i defined my array differently i couldnt solve this error message. It gives the error every time at line 31. Here is my code, any help would be much appreciated.

clear all
clc
close all

%Variables
H=8; %m, =H1
D=0.3;  %m
ks=0.0001;  %m
nu=0.001;   %Ns/m^2
rho=1000;   %kg/m^3
g=9.81; %m/s^2
gamma=rho*g;
deltat=0.02;    %s
T=60;   %s
E=0.01; %criteria for steady state
N=10^10;    %time to iterate until finding stady-state velocity

%1-) Steady State Velocity, Vs
%writing Bernoulli Eq. between (1) and (3);
Vsteady=sqrt(H*2*g);  %m/s,steady-state velocity
Resteady=Vsteady*D*rho/nu;  %Reynolds Number
fsteady=0.25/(log((ks/3.7*D)+(5.74/Resteady^0.9)))^2;
Qsteady=Vsteady*(pi()*D^2)/4;

for j=1
V(j)=Vsteady;
Re(j)=Resteady;
f(j)=fsteady;
for i=1:deltat:N
    for L=50
Re(i)=V(i)*D*rho/nu;  %Reynolds Number
f(i)=0.25/(log((ks/3.7*D)+(5.74/Re(i)^0.9)))^2;
V(i+1)=V(i)+deltat.*(H-(1+(f(i)*L*V(i)^2)./(D*2*g))).*(g./L);
Q=(V(i+1))*(pi()*D^2)/4;  %m^3/s
if abs((V(i+1)-V(i)) ./ (V(i)))==E
    break
end
%writing Bernoulli Eq. between (1) and (2);
P2=gamma*H-(gamma*Vsteady^2)/(2*g)-(fsteady*L*Vsteady^2*gamma)/(D*2*g);  %H1=H2
    end
end
end

Answer 1

[Sindar]

You are trying to access Re(i),V(i),etc. with i=1.02. There doesn't seem to be anywhere that you use i as a number rather than index, so try replacing

for i=1:deltat:N

with:

for i=1:N

Answer 2

[James Heselden]

So the error comes from the line:

Re(i)=V(i)*D*rho/nu;  %Reynolds Number

Where your i value on the second iteration is 1.02

i =
    1.020

You cannot take the 1.02 index of an array, this must be a integer. the issue isnt with your array but with your for loop on line 29:

    for i=1:deltat:N
        ...
    end

I do not know much on the specifics in what your code is for, but you have 2 options to solve the issue:

1. Take the floor() / ceil() / round() of your index.

Re(i)=V(i)*D*rho/nu;  %Reynolds Number
%becomes
Re(floor(i))=V(floor(i))*D*rho/nu;  %Reynolds Number

2. Add a secondary index to identify the location in your array. and rpelace every instance of indexing with (i) to (idx).

    idx = 0; %Initialise index to 0
    for i=1:deltat:N
        idx=idx+1; %Increment index
        ...
            Re(idx)=V(idx)*D*rho/nu;  %Reynolds Number
            ...

Given the context the latter would be the most appropriate.

Hope that helps :)