R raster package problem with extract() function

Question

I'm trying to extract pixel value using the extract() function from raster package in R and get the following error, bellow in my code bellow a well, where: df$PixelNumber is the "cell number value". The error :

Error in (function (classes, fdef, mtable) : unable to find an inherited method for function ‘extract’ for signature ‘"character", "numeric"’

the code I'm using:

vec=as.vector(as.numeric(as.matrix(df$PixelNumber)));
rast = file.choose();
VlaueSlope=extract(x= rast, vec)

What could I be doing wrong in the extract() code that cannot get the pixel value in the variable VlaueSlope.

Answer 1

The error:

    Error in (function (classes, fdef, mtable) : 
     unable to find an inherited method for
     function ‘extract’ for signature ‘"character", "numeric"’

is saying you are calling extract with a "character" argument and a "numeric" argument. How are you calling it? Let's see...

VlaueSlope=extract(x= rast, vec)

so what is rast? Let's see:

rast = file.choose();

rast is the file name it is NOT the raster data. You need to read in the raster using something like:

rasterpath = file.choose()
rast = raster(rasterpath)

and then you can extract the pixel values if everything else is correct.

In summary: try and understand error messages, read the documentation for functions, and check your arguments match the ones documented.