Array indices must be positive integers or logical values.

Question

Grégoire 2024-2-15

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2082403-array-indices-must-be-positive-integers-or-logical-values

编辑： Matt J 2024-2-15

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

c

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DGM 2024-2-15

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

Grégoire 2024-2-15

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

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Answer 1

Matt J 2024-2-15

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https://ww2.mathworks.cn/matlabcentral/answers/2082403-array-indices-must-be-positive-integers-or-logical-values#answer_1409313

在 MATLAB Online 中打开

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

3 个评论
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Matt J 2024-2-15

编辑：Matt J 2024-2-15

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

Grégoire 2024-2-15

ok thanks :)

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Answer 2

VBBV 2024-2-15

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2082403-array-indices-must-be-positive-integers-or-logical-values#answer_1409333

在 MATLAB Online 中打开

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.