Interpolation examples¶

Let’s define a small helper function:

def print_me(msg, val):

    print("{}: {}".format(msg, val))

Declare an Interpolation object:

i = Interpolation([5, 3, 6, 1, 2, 4, 9], [10, 6, 12, 2, 4, 8])

print(i)

# X: [1, 2, 3, 4, 5, 6]

# Y: [2, 4, 6, 8, 10, 12]

Note

They are ordered in ‘x’
The extra value in ‘x’ was dropped

Use the copy constructor. We can easily make a copy of an Interpolation object:

j = Interpolation(i)

print(j)

# X: [1, 2, 3, 4, 5, 6]

# Y: [2, 4, 6, 8, 10, 12]

j = Interpolation([0.0, 1.0, 3.0], [-1.0, -2.0, 2.0])

print(j)

# X: [0.0, 1.0, 3.0]

# Y: [-1.0, -2.0, 2.0]

print_me("j(2)", j(2))

# j(2): -1.0

print_me("j(0.5)", j(0.5))

# j(0.5): -1.75

Test with a value already in the data table:
```
print_me("j(1)", j(1))

# j(1): -2.0
```

Get the number of interpolation points internally stored:

print_me("Number or interpolation points in 'j'", len(j))

# Number or interpolation points in 'j': 3

We can interpolate Angles too:

k = Interpolation([27.0, 27.5, 28.0, 28.5, 29.0],

                  [Angle(0, 54, 36.125), Angle(0, 54, 24.606),

                   Angle(0, 54, 15.486), Angle(0, 54, 8.694),

                   Angle(0, 54, 4.133)])

print_me("k(28.278)", Angle(k(28.278)).dms_str())

# k(28.278): 54' 11.4279073579''

Let’s work with a new Interpolation object:

m = Interpolation([-1.0, 0.0, 1.0], [-2.0, 3.0, 2.0])

print(m)

# X: [-1.0, 0.0, 1.0]

# Y: [-2.0, 3.0, 2.0]

Get some interpolated values:

print_me("m(-0.5)", m(-0.5))

# m(-0.5): 1.25

print_me("m(0.5)", m(0.5))

# m(0.5): 3.25

Get derivatives:

print_me("m'(-1.0)", m.derivative(-1.0))

# m'(-1.0): 8.0

print_me("m'(-0.5)", m.derivative(-0.5))

# m'(-0.5): 5.0

print_me("m'(0.0)", m.derivative(0.0))

# m'(0.0): 2.0

print_me("m'(0.5)", m.derivative(0.5))

# m'(0.5): -1.0

print_me("m'(1.0)", m.derivative(1.0))

# m'(1.0): -4.0

Get the root within the interval:

print_me("m.root()", m.root())

# m.root(): -0.720759220056

Get the extremum within the interval:

print_me("m.minmax()", m.minmax())

# m.minmax(): 0.333333333333

Let’s work now with the interpolation of sine function:

m = Interpolation([29.43, 30.97, 27.69, 28.11, 31.58, 33.05],

                  [0.4913598528, 0.5145891926, 0.4646875083,

                   0.4711658342, 0.5236885653, 0.5453707057])

print_me("sin(29.5)\t", m(29.5))

# sin(29.5) : 0.492423560118

print_me("sin(30.0)\t", m(30.0))

# sin(30.0) : 0.500000000018

print_me("sin(30.5)\t", m(30.5))

# sin(30.5) : 0.507538362978

Derivatives must be adjusted because degrees were used instead of radians:

print_me("sin'(29.5)\t", degrees(m.derivative(29.5)))

# sin'(29.5)        : 0.870355696916

print_me("sin'(30.0)\t", degrees(m.derivative(30.0)))

# sin'(30.0)        : 0.866025403791

print_me("sqrt(3.0)/2.0\t", sqrt(3.0)/2.0)

# sqrt(3.0)/2.0     : 0.866025403784

print_me("sin'(30.5)\t", degrees(m.derivative(30.5)))

# sin'(30.5)        : 0.861629160353