Array indices must be positive integers or logical values.

clc;

x = -6; %input value for following expression

if x < -5

f(x) = 0;

elseif -5 <= x && x < 5;

f(x) = 2 * x^1/2 + exp(x/4);

elseif 5 <= x && x <= 20;

f(x) = log((x^2));

else

f(x) = 0;

end

f(x)

how can i get my code to work for values -infinity to 0 for this expression? i keep getting this error code

Array indices must be positive integers or logical values.

Error in Homework_two (line 11)

f(x) = 0;

Answer 1

[Torsten]

f is interpreted here as an array, and arrays can only be indexed with positive integers or logical values.

So f(-6) is forbidden.

You can use

x = -6:0.1:10 ; %input value for following expression
f = zeros(size(x));
f(-5 <= x & x < 5) = 2 * sqrt(x(-5 <= x & x < 5)) + exp(x(-5 <= x & x < 5)/4);
f(5 <= x & x <= 20) = log(x(5 <= x & x <= 20).^2);
plot(x,f)

Warning: Imaginary parts of complex X and/or Y arguments ignored.

But note that x^(1/2) gives imaginary results for x < 0.

[Alonso]

Yah that makes sense, Thank you for clarifying that for me.

Answer 2

[Steven Lord]

x = -6; %input value for following expression
if x < -5
    f(x) = 0;

There's no such thing as element -6 of an array in MATLAB. If you want f to be one element long and you want its value to depend on x, assign to f not f(x).