Masi - 4 months ago 23

R Question

I am trying to interpolate linearly in R.

Pseudocode is

`u = interp1(u, linspace(1, numel(u), numel(u)-1));`

- is equivalent to MATLAB pseudocode
`approx rule=1`

I am not sure about the second

`interp1`

`y <- x`

`interp1(x, y, xi, method = "linear")`

Minimal code example (real one has > 500 k points so linear will work) and its output at the top

`List of 2`

$ : num [1:3] 1 2 3

$ : num [1:2] 1 2

num [1:2] 0 1

Error in interp1(x, y, xi, method = "linear") :

Points 'xi' outside of range of argument 'x'.

Execution halted

library("pracma") # http://finzi.psych.upenn.edu/library/pracma/html/interp1.html

files <- vector("list", 2)

files[[1]] <- c(1,2,3)

files[[2]] <- c(1,2)

str(files)

# Wanted, MATLAB: u = interp1(u, linspace(1, numel(u), numel(u)-1));

xi <- seq(0,1, len = length(files[[1]]) - 1)

x <- files[[1]]

y <- files[[1]]

str(xi)

files[[1]] <- interp1(x, y, xi, method = "linear")

str(files)

I know the thread using interp1 in R for matrix but I do not have a matrix.

Input:

`c(1,2,3)`

Expected output: [1:2] datastructure

R: 3.3.1

OS: Debian 8.5

Answer

If you're willing to get `NA`

values on extrapolation, as is the default for linear interpolation/extrapolation in interp1, then `approx()`

works fine:

```
files <- list(1:3,1:2)
xi <- seq(0,1, len = length(files[[1]]) - 1)
x <- files[[1]]
y <- files[[1]]
a <- approx(x,y,xi)
```

You said you wanted just a two-element vector so presumably you just want the output y-values:

```
a$y
## [1] NA 1
```

This may seem wrong, but is the correct answer to the question you actually posed. You've used `files[[1]]`

for both `x`

and `y`

, so `approx()`

should return y=x when the input is in the range from 1 to 3, and `NA`

otherwise. In this case `xi`

is `[0 1]`

, so the first element is out of the range of the x/y data provided ...

PS I can appreciate wanting to use `pracma`

for similarity to MATLAB's syntax, but - although `pracma`

is high-quality and widely used - base R functions are even more widely used/thoroughly tested ...