# Array indices must be positive integers or logical values.

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Mark Prentice on 9 Jul 2021
Edited: DGM on 10 Jul 2021
h = 0.2;
T = 4:h:8
X_s=[5.87 -4.23 -2.55 -0.89 0.67 2.09 3.31 4.31 5.06 5.55 5.78 5.77 5.52 5.08 4.46 3.72 2.88 2.00 1.10 .23 -0.59]
for X=4.2:0.2:7.8
k=[X]
x=(((k/(0.2))-(4/(0.2))))+1
V = ((X_s((x)+1))-(X_s((x)-1)))/(2*h)
end
Trying to get the for loop to spit out an array of answers for a central diffrencing problem but cannot get the loop to run without an array error. when I do the math I'm seeing whole positive integers being imputed. Not sure whats going on and would appreciate help.

Matt J on 9 Jul 2021
Edited: Matt J on 9 Jul 2021
Since x is not an integer, you cannot use it to look up an entry in a vector X_s(x). Perhaps the following is what you really want?
h = 0.2;
X_s=[5.87 -4.23 -2.55 -0.89 0.67 2.09 3.31 4.31 5.06 5.55 5.78 5.77 5.52 5.08 4.46 3.72 2.88 2.00 1.10 .23 -0.59];
V=(X_s(3:end)-X_s(1:end-2))/(2*h)
V = 1×19
-21.0500 8.3500 8.0500 7.4500 6.6000 5.5500 4.3750 3.1000 1.8000 0.5500 -0.6500 -1.7250 -2.6500 -3.4000 -3.9500 -4.3000 -4.4500 -4.4250 -4.2250
##### 2 CommentsShowHide 1 older comment
DGM on 10 Jul 2021
First note that in your loop, you're not actually calculating a vector V. You're calculating V as a scalar and discarding all the results except the last one. It's assumed that's not what you intend.
Nothing in the loop depends on prior results, so the loop isn't necessary. We can look at the vectors that are being generated and look for a pattern:
h = 0.2;
T = 4:h:8;
X_s=[5.87 -4.23 -2.55 -0.89 0.67 2.09 3.31 4.31 5.06 5.55 5.78 ...
5.77 5.52 5.08 4.46 3.72 2.88 2.00 1.10 .23 -0.59];
k=4.2:0.2:7.8
k = 1×19
4.2000 4.4000 4.6000 4.8000 5.0000 5.2000 5.4000 5.6000 5.8000 6.0000 6.2000 6.4000 6.6000 6.8000 7.0000 7.2000 7.4000 7.6000 7.8000
x=k/0.2 - 4/0.2 + 1
x = 1×19
2.0000 3.0000 4.0000 5.0000 6.0000 7.0000 8.0000 9.0000 10.0000 11.0000 12.0000 13.0000 14.0000 15.0000 16.0000 17.0000 18.0000 19.0000 20.0000
So x is ostensibly integer-valued, varying from 2 to 20. Since x is used as the basis for indexing in X_s:
V = (X_s(x+1) - X_s(x-1))/(2*h)
... and the length of X_s is 21, then the above indices are equivalent to
V = (X_s((2+1):(20+1)) - X_s((2-1):(20-1)))/(2*h)
% or
V = (X_s(3:end) - X_s(1:end-2))/(2*h)

G A on 9 Jul 2021
Edited: G A on 9 Jul 2021
floating point problem;
try to use x = round(k/0.2 - 4/0.2 + 1)
k =
4.200000000000000
x =
2
k =
4.400000000000000
x =
3
k =
4.600000000000001
x =
4
k =
4.800000000000001
x =
5.000000000000004
k =
5
x =
6
k =
5.200000000000000
x =
7
k =
5.400000000000000
x =
8
k =
5.600000000000001
x =
9
k =
5.800000000000001
x =
10.000000000000004

R2021a

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