Derivative of unequally spaced data points without decreasing the length of a vector?

Hello Matlab,

I have data ponts that are not equally spaced

x=[0.097
0.477
0.934
4.835
15.590
46.645
69.371
100.709
165.249
277.846
382.978
1000.64
1519.83
1991.60
2544.96
3023.40
3589.94
4159.93
4473.43
5186.36
6316.71
7134.15
9125.71
10571.90
12565.82
14916.64
16859.92
];

y=[20.96
21.33
20.77
20.96
21.14
21.33
20.24
19.71
17.90
15.98
14.51
10.05
8.37
7.40
6.49
6.05
5.54
5.17
4.82
4.57
4.26
3.87
3.52
3.25
3.08
2.80
2.61
];

 dydx = diff(y(:))./diff(x(:))
 z= y.*dydx;

First is dydx the right way to differentiate the data points? Second dydx will give me will give me one element shorter than y and have trouble calculating z. What is the best way to calculate z without shortening. I'm currently using shorten the y axis:

    z= y(2:end).*dydx;

However, I dont like shorting my y axis.

Any help is appreciated. Thanks GH

Answer 1

[Walter Roberson]

gradient(y, x)