Please help with fsolve, i keep getting this error

%% Question 2
% lengths given in the tables
L1 = 20;
L2 = 10;
L3 = 30;
% diameter given in the tables
D1 = .167;
D2 = .2083;
D3 = .125;
% Given values of ro, mu, and e from table
ro = .0631;
mu = 0.0000205;
e = .00085;
% converting the diameter to area
A1 = pi*(D1/2)^2;
A2 = pi*(D2/2)^2;
A3 = pi*(D3/2)^2;
% equation to solve for the velocites
V1 = Q1/A1;
V2 = Q2/A2;
V3 = Q3/A3;
% if statement for reynolds number
Re1 = 2000;
if Re1 <= 2300
    f = 64/Re1;
elseif Re1>2300
    f=0.25*(log(((e/D1)/3.7)+5.74/Re1^0.9))^-2;
end
% function for fsolve
fun = @(Q) [Q(1)+Q(2)+Q(3)-750; f*(L1/D1)*((Q(1)/A1)^2/2)-deltaP/ro; f*(L2/D2)*((Q(2)/A2)^2/2)-deltaP/ro; f*(L3/D3)*((Q(3)/A3)^2/2)-deltaP/ro];
Q0 = [120;21;24;23];
Q_2 = fsolve(fun,Q0);
fprintf('The value of Q1 is %d\n',Q1)
fprintf('The value of Q2 is %d\n',Q2)
fprintf('The value of Q3 is %d\n',Q3)
fprintf('The value of Q is %d\n',Q)
fprintf('The value of deltaP is %d\n',deltaP)

this is the error i keep getting

Array indices must be positive integers or logical values.

Error in Project_2_updated (line 100)

Q(0) = [120;21;24;23];

PLEASE HELP PLEASE

[John D'Errico]

When you get an error, give the COMPLETE error. Not just part of it. EVERYTHING IN RED.

Next, we don't actually have enough information to even show you what is wrong, since you don't tell us the value of deltaP. As well, Q1 is undefined.

We cannot see inside your computer. Sorry.

Answer 1

[Walter Roberson]

Your posted code has

Q0 = [120;21;24;23];

which is fine.

But your error message is from running some other code -- code that has

Q(0) = [120;21;24;23];

That other code is attempting to index element #0 of variable Q. MATLAB indices start at 1 -- and if you had used Q(1) = [120;21;24;23]; then you would have received an error from trying to store 4 elements into a single location.

MATLAB does not have any notation for defining initial conditions of a variable in terms of index 0.

The posted code that uses Q0 instead of Q(0) is fine, at least with regards to that part of the code.

Q_2 = fsolve(fun,Q0);
fprintf('The value of Q1 is %d\n',Q1)
fprintf('The value of Q2 is %d\n',Q2)
fprintf('The value of Q3 is %d\n',Q3)
fprintf('The value of Q is %d\n',Q)

Variables Q1, Q2, Q3, and Q are not defined in that code.

Reminder: giving a variable name followed by a number does not mean that you want to index the variable. Q1 does not mean that you want to index variable Q at the first location. And besides, you assigned to Q_2 not to Q. Use () for indexing, such as

fprintf('The value of Q1 is %d\n',Q_2(1))