My function won't accept the vector

x = 0:1:10;
f(x) = x.^2-x-1;

I keep getting an error message saying Array indices must be positive integers or logical values. I am trying to find values of f(x) within certain parameters using the find function.

Answer 1

[Arif Hoq]

You can not specify 0 as an index

x = 0:1:10;
f = x.^2-x-1

f = 1×11

-1 -1 1 5 11 19 29 41 55 71 89

[Natalie]

This Fixed my problem thank you very much for the help

Answer 2

[Walter Roberson]

x = 0:1:10;

okay, x is a list of values [0 1 2 3 4 5 6 7 8 9 10]

f(x) = x.^2-x-1;

you have to substitute in that list of values, so your statement is effectively

f(0:10) = (0:10).^2-(0:10)-1;

which tries to assign to index 0, 1, 2, ... 10. But MATLAB does not permit index 0, so you have a problem.

You could write

f(x+1) = x.^2-x-1;

which would then be equivalent to

f((0:10)+1) = (0:10).^2-(0:10)-1;

which would assign to index locations 1, 2, 3... 11.

But if you are going to define x that way, you might easily have wanted to do something like

x = 0:0.1:10;

and if you add 1 to that you would just end up with indices such as 1, 1.1, 1.2, and so on. Non-integer indices are not permitted either.

[Walter Roberson]

When you have a statement such as

f(x) = x.^2-x-1;

you need to decide whether you are working with arrays or if you are working with formulas .

If you are working with arrays, then the (x) part of f(x) has to be non-negative integers.

If you are working with formulas then the (x) on the left has to be a symbolic variable (Symbolic Toolbox)

syms x
f(x) = x.^2-x-1;
X = 0:1:10;
plot(X, f(X))

[Walter Roberson]

You should learn this programming pattern: you will use it a lot.

%can be negative, non-integer, can include duplicates,\
% does not need to be sorted
xvals = -1:.1:1; 
num_x = numel(xvals);   %how many are there?

f = zeros(size(xvals)); %pre-allocate output same size as input

for xidx = 1 : num_x   %loop over INDICES
    x = xvals(xidx);   %pull out one SPECIFIC value into your x
    y = x.^2-x-1;      %calculate based on that ONE x
    f(xidx) = y;       %store into a location according to the INDEX
end

plot(xvals, f)         %use the entire vector of values, NOT plot(x, f)

Answer 3

[Adam Danz]

Perhaps you're looking for

x = 0:1:10;
f = x.^2-x-1;

Answer 4

[Image Analyst]

See the FAQ:

https://matlab.fandom.com/wiki/FAQ#%22Subscript_indices_must_either_be_real_positive_integers_or_logicals.%22

You probably want

x = 0:1:10;
f = x.^2-x-1

f = 1×11

-1 -1 1 5 11 19 29 41 55 71 89