Array indices must be positive integers or logical values.
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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.
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Risposta accettata
Matt J
il 9 Lug 2021
Modificato: Matt J
il 9 Lug 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)
2 Commenti
DGM
il 10 Lug 2021
Modificato: DGM
il 10 Lug 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
x=k/0.2 - 4/0.2 + 1
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)
Più risposte (1)
G A
il 9 Lug 2021
Modificato: G A
il 9 Lug 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
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