Why am I getting Error "Array indices must be positive integers or logical values"?

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Seungryul Lee
Seungryul Lee 2022-4-8
评论： KSSV 2022-4-8
N=[5 6 7 8 9 10 11 12 13 14 15];
fprintf('%5s\t%15s\t%15s\t%15s\n','n','T(h)','En','En/E(n-1)')
trueValue = exp(-2.5).*(-sin((2.5).^3)-sin((2.5).^2)+2.*(2.5).*cos((2.5).^3)+3.*(2.5).^2.*cos((2.5).^3)+3.*(2.5)-3);
err1=-1;
for i=1:length(N)
n=N(i);
x=2.5;
h=2.^(-n);
f=exp(-x).*(sin(x.^3)+sin(x.^2)-3.*x);
fprime = MyCenteredDifference(f, x, h);
err2=abs(fprime-trueValue);
ratio = err2/err1;
err1=err2;
if i==1
ratio=1;
end
fprintf('%5d\t%15.10f\t%15.10f\t%15.10f\n',n,I,err1,ratio)
end
_I'm getting an error for the "fprime = MyCenteredDifference(f, x, h);" line, and I don't know why it says "Array indices must be positive integers or logical values".
Please help me.

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回答（1 个）

KSSV
KSSV 2022-4-8
Ran in:
You have not given/ shown us the function MyCenteredDifference, so we cannot check the function. But the error is simple and staright. In matlab array indices must be positive integers and logicals. In your case, you have voilated that crieteria and ended up with error.
EXample:
A = rand(1,10) ;
A(1) % no error
ans = 0.0417
A(10) % no error
ans = 0.8617
A(1.5) % error, index cannot be fraction
Array indices must be positive integers or logical values.
Use of logicals:
idx = A > 0.5 ;
A(idx) % no error
A(0) % error index cannot be double 0, 0 is not logical here
In the function, check where index is voilating the rule.
  2 个评论
Seungryul Lee
Seungryul Lee 2022-4-8
function fprime = MyCenteredDifference(f, x, h)
fprime = (f(x+h) - f(x-h)) ./ (2*h);
end
Here is my function.
Can you plz check?
KSSV
KSSV 2022-4-8
Ran in:
Your f is not a function handle, it is a number as you have already substitued x in it. You may follow like shown below.
N=[5 6 7 8 9 10 11 12 13 14 15];
fprintf('%5s\t%15s\t%15s\t%15s\n','n','T(h)','En','En/E(n-1)')
n T(h) En En/E(n-1)
trueValue = exp(-2.5).*(-sin((2.5).^3)-sin((2.5).^2)+2.*(2.5).*cos((2.5).^3)+3.*(2.5).^2.*cos((2.5).^3)+3.*(2.5)-3);
err1=-1;
f=@(x) exp(-x).*(sin(x.^3)+sin(x.^2)-3.*x); %<--- function handle
for i=1:length(N)
n=N(i);
x=2.5;
h=2.^(-n);
fprime = MyCenteredDifference(f, x, h);
err2=abs(fprime-trueValue);
ratio = err2/err1;
err1=err2;
if i==1
ratio=1;
end
fprintf('%5d\t%15.10f\t%15.10f\t%15.10f\n',n,i,err1,ratio)
end
5 1.0000000000 0.9039129196 1.0000000000 6 2.0000000000 0.8406641934 0.9300278547 7 3.0000000000 0.8245927387 0.9808824322 8 4.0000000000 0.8205585495 0.9951076585 9 5.0000000000 0.8195489799 0.9987696557 10 6.0000000000 0.8192965236 0.9996919570 11 7.0000000000 0.8192334055 0.9999229606 12 8.0000000000 0.8192176257 0.9999807384 13 9.0000000000 0.8192136808 0.9999951845 14 10.0000000000 0.8192126946 0.9999987961 15 11.0000000000 0.8192124480 0.9999996990
function fprime = MyCenteredDifference(f, x, h)
fprime = (f(x+h) - f(x-h)) ./ (2*h);
end

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