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

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.

Answer 1

[KSSV]

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.

[Seungryul Lee]

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]

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