Why do I get the "Array indices must be positive integers or logical values" error?

I keep getting the "Array indices must be positive integers or logical values" error for the line of my code "fs_2(i) = 1 - exp((log((CS_scheil(i)/(k*C0)))/(k-1)));" that comes after the "else". How do I fix this? My entire entry of code is below.

C0 = 8;
k = 0.13;
FS = linspace(0,0.4079,100);
CS_scheil = k*C0*(1 - FS).^(k - 1);

for i = 0:0.01:1
    if i == 1
        fs_2(i) = 1 - exp((log((CS_scheil(i)/(k*C0)))/(k-1)));
        a(i) = 1;
        b(i) = (k*C0*((1 - fs_2(i))^(k - 1)));
    else
        fs_2(i) = 1 - exp((log((CS_scheil(i)/(k*C0)))/(k-1)));
        a(i) = (fs_2(i) - fs_2(i-1));
        b(i) = (1/2)*[(k*C0*((1 - fs_2(i))^(k - 1))) + (k*C0*((1 - fs_2(i-1))^(k - 1)))];
    end
SUM(i) = a(i)*b(i);
end

for n = 1:1:100
        if n == 1
            SUM(n) = a(n)*b(n);
        else
            SUM(n) = SUM(n-1) + (a(n)*b(n));
        end
    end
C_avg = SUM;

Answer 1

[FScherff]

Hello,

Your for-loop is running over 0:0.01:1, resulting in an index-vector [0 0.1 0.2 0.3 ... 0.99 1].That means that already in the first iteration of your foor loop, you are using the value 0 as an array index. But as the error message states, "Array indices must be positive integers or logical values" in Matlab (as you are corrctly using in the second code snippet with n=1:100). That means you should e.g. use 1:101 as index-vecotr of your foor loop, resulting in the same number of iterations, but with valid integer indices. If you really need the values of 0 to 1 in your code (although I don't see that place in the code you posted), use something like

valuesBetweenZeroAndOne = 0:0.01:1;
for i = 1:101
    if i == 1
        ...
    else
        ...
    end
    %Line where you need the value, e.g. for scaling or something like that
    SUM(i) = a(i)*b(i)*valuesBetweenZeroAndOne(i);
end

I have an additional remark:

In your first for-loop you run through 101 iterations (0 to 1 in 0.01-steps), in your second for loop you only have 100 iterations (1 to 100 in 1-steps). As you are using the same vector SUM in booth loops, so I'm relativly sure this is not on purpose.

Greetings!

[Thomas Mittelbrun]

Thank you for the help! I was able to fix my code.

Answer 2

[Mario Malic]

% i = 0:0.01:1 % 101 element
i = 1 : 1 : 101 % 101 element

You should index arrays/variables with integers.

[Stephen23]

i = 0 : 1 : 100 % 101 element <- zero is not a valid MATLAB index.

[Mario Malic]

Woopsie!

Answer 3

[Walter Roberson]

for i = 0:0.01:1

i is first set to 0, then to 0.01, then about 0.02, then about 0.03, and so on.

fs_2(i) = 1 - exp((log((CS_scheil(i)/(k*C0)))/(k-1)));

and there you use i as a subscript for CS_scheil and for the output variable fs_2

First you have to ask what is meant by CS_scheil(i) . It would seem to imply the value of CS_scheil that is associated with the floating point value stored in i . But look at how you created CS_scheil :

FS = linspace(0,0.4079,100);
CS_scheil = k*C0*(1 - FS).^(k - 1);

There might be some ground for saying that you want CS_scheil(i) to mean that value of CS_scheil that is associated with FS having the same value as i, but FS runs 0 to 0.4079 while i runs 0 to 1, so when i > 0.4079 then there would be no obvious meaning to CS_scheil. Furthermore, if you ask for ismembertol(0:0.01:1, FS) you will find that the only intersection between the two, to within reasonable tolerances, is 0 .

Beyond that, you need to solve the problem of indexing with i for the output variable. For that, please read https://www.mathworks.com/matlabcentral/answers/?term=tag%3A%22generalized+for+loop%22