# Array indices must be positive integers or logical values.

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Grégoire on 15 Feb 2024
Edited: Matt J on 15 Feb 2024
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 on 15 Feb 2024
a and b are not constrained to being positive nonzero integers, so you can't use them as array indices.
Grégoire on 15 Feb 2024
yeah but they can be negative too, the roots are in between [-1;0] and [1;2]

Matt J on 15 Feb 2024
fx = @(X) X.^2-1-sin(X);
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Matt J on 15 Feb 2024
Edited: Matt J on 15 Feb 2024
Yes. In other words, it makes fx a function (or more precisely a function handle).
Grégoire on 15 Feb 2024
ok thanks :)

VBBV on 15 Feb 2024
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 on 15 Feb 2024
ok thanks :)

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