Array indices must be positive integers or logical values.

Hey, I m trying to do a bissecion method code.

But when trying to give a var fa the value of f(a) it gives me a :'Array indices must be positive integers or logical values.'

Could anyone explain me why please ?

clear;

tol = 10^(-6);

x1= -2:0.01:2;

fx = x1.^2-1-sin(x1);

a=input('Intervalo inferior: ');

b=input('Intervalo superior: ');

fa=fx(a);

Array indices must be positive integers or logical values.

fb=fx(b);

if sign(fa)==sign(fb)

disp('no tiene solucion')

return

end

while (b-a)/2 > tol

c = (a+b)/2;

fc= fx(c);

if sign(a)==sign(c)

a=c;

fa=fc;

else

b=c;

fb=fc;

end

end

c

[DGM]

a and b are not constrained to being positive nonzero integers, so you can't use them as array indices.

[Grégoire]

yeah but they can be negative too, the roots are in between [-1;0] and [1;2]

Answer 1

[Matt J]

fx = @(X) X.^2-1-sin(X);

[Grégoire]

It works thanks a lot !

So the @(X) allows you to be able to give to X every variable you want right ?

[Matt J]

Yes. In other words, it makes fx a function (or more precisely a function handle).

[Grégoire]

ok thanks :)

Answer 2

[VBBV]

Matlab arrays are indexed using whole integers, when you give a fractional or decimal values as input it throws such errors

clear;
tol = 10^(-6);
x1= -2:0.01:2;
fx = x1.^2-1-sin(x1)

fx = 1×401

3.9093 3.8735 3.8378 3.8023 3.7668 3.7315 3.6962 3.6611 3.6260 3.5911 3.5563 3.5216 3.4870 3.4525 3.4181 3.3838 3.3496 3.3155 3.2815 3.2476 3.2138 3.1802 3.1466 3.1131 3.0798 3.0465 3.0133 2.9803 2.9473 2.9144

a=2.7 %input('Intervalo inferior: ');

a = 2.7000

b=3.2 %input('Intervalo superior: ');

b = 3.2000

fa=fx(round(a))  % round the input values 

fa = 3.8378

clear;
tol = 10^(-6);
x1= -2:0.01:2;
fx = x1.^2-1-sin(x1)

fx = 1×401

3.9093 3.8735 3.8378 3.8023 3.7668 3.7315 3.6962 3.6611 3.6260 3.5911 3.5563 3.5216 3.4870 3.4525 3.4181 3.3838 3.3496 3.3155 3.2815 3.2476 3.2138 3.1802 3.1466 3.1131 3.0798 3.0465 3.0133 2.9803 2.9473 2.9144

a=2.5; %input('Intervalo inferior: ');
b=3.2; %input('Intervalo superior: ');
fa=fx(a)

Array indices must be positive integers or logical values.

[Grégoire]

ok thanks :)