interp1

Syntax

yi = interp1(x,y,xi)

yi = interp1(x,y,xi,method)

yi = interp1(x,y,xi,extrap)

yi = interp1(x,y,xi,method,extrap)

pp = interp1(x,y,...,'pp')

Inputs

x,y

The known points with which to interpolate.

Type: double

Dimension: vector

xi

The domain points at which to interpolate.

Type: double

Dimension: vector

method

The available options are:

'linear' (default)

Returns the linear interpolation.

'pchip'

Returns shape preserving piece-wise cubic Hermite interpolation.

'spline'

Returns the cubic spline interpolation.

Type: string

extrap

The available options are:

extrapVal (default: NaN)

Out of bounds values are set to this scalar value.

'extrap'

Allows extrapolation beyond the interval using the current method.

'noextrap'

Does not allow extrapolation.

Outputs

yi

The interpolated y values corresponding to xi.

pp

A piecewise polynomial struct.

For this output, the yi input is omitted.

See mkpp for details.

Examples

With 'linear' method and no extrapolation needed:

x = [1,2,3,4,5];
y = [1,4,9,16,25];
xi = [1.5,2.5,3.5,4.5];
yi = interp1(x,y,xi,'linear')

yi = [Matrix] 1 x 4
2.50000  6.50000  12.50000  20.50000

With 'linear' method and extrapolation:

x = [1,2,3,4,5];
y = [1,4,9,16,25];
xi = [0.5,1.5,2.5,3.5,4.5,5.5];
yi = interp1(x,y,xi,'linear','extrap')

yi = [Matrix] 1 x 6
-0.50000  2.50000  6.50000  12.50000  20.50000  29.50000

With 'spline' method and no extrapolation needed:

x = [1,2,3,4,5];
y = [1,4,9,16,25];
xi = [1.5,2.5,3.5,4.5];
yi = interp1(x,y,xi,'spline')

yi = [Matrix] 1 x 4
2.25000  6.25000  12.25000  20.25000

Comments

When y is a matrix, the function operates along the first dimension, which is the column dimension in the 2D case. This convention is the opposite of spline.

For the 'linear' method, duplicate x values are allowed if the vector is sorted, indicating a discontinuous function. The function is treated as right-continuous if x is in ascending order, and left-continuous if descending. More than two identical x values are not allowed.

For methods other than 'linear', x values must be unique.

The 'spline' method uses the not-a-knot cubic spline.