# Documentation

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# interpft

1-D interpolation using FFT method

## Syntax

`y = interpft(x,n)y = interpft(x,n,dim)`

## Description

`y = interpft(x,n)` returns the vector `y` that contains the value of the periodic function `x` resampled to `n` equally spaced points.

If `length(x) = m`, and `x` has sample interval `dx`, then the new sample interval for `y` is `dy = dx*m/n`. Note that `n` cannot be smaller than `m`.

If `X` is a matrix, `interpft` operates on the columns of `X`, returning a matrix `Y` with the same number of columns as `X`, but with `n` rows.

`y = interpft(x,n,dim)` operates along the specified dimension.

## Examples

Interpolate a triangle-like signal using an interpolation factor of 5. First, set up signal to be interpolated:

```y = [0 .5 1 1.5 2 1.5 1 .5 0 -.5 -1 -1.5 -2 -1.5 -1 -.5 0]; N = length(y);```

Perform the interpolation:

```L = 5; M = N*L; x = 0:L:L*N-1; xi = 0:M-1; yi = interpft(y,M); plot(x,y,'o',xi,yi,'*') legend('Original data','Interpolated data')```

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### Algorithms

The `interpft` command uses the FFT method. The original vector `x` is transformed to the Fourier domain using `fft` and then transformed back with more points.