How to remove some values from a 4-dimensional matrix?-CodePudding

I'm working with a 4-dimensional matrix (Year, Simulation, Flow, Time instant: 10x5x20x10) in R. I need to remove some values from the matrix. For example, for year 1 I need to remove simulations number 1 and 2; for year 2 I need to remove simulation number 5.

Can anyone suggest me how I can make such changes?

CodePudding user response：

Arrays (which is how R documentation usually refers to higher-dimensional 'matrices') can be indexed with negative values in the same way as matrices or vectors: a negative value removes the corresponding row/column/slice. So if you wanted to remove year 1 completely (for example), you could use a[-1,,,]; to remove simulation 5 completely, a[,-5,,].

However, arrays can't be "ragged", there has to be something in every row/column/slice combination. You could replace the values you want to remove with NAs (and then make sure to account for the NAs appropriately when computing, e.g. using na.rm = TRUE in sum()/min()/max()/median()/etc.): a[1,1:2,,] <- NA or a[2,5,,] <- NA in your examples.

If you knew that all values of Flow and Time would always be present, you could store your data as a list of lists of matrices: e.g.

results <- list(Year1 = list(Simulation1 = matrix(...),
                             Simulation2 = matrix(...),
                             ...),
                Year2 = list(Simulation1 = matrix(...),
                             Simulation2 = matrix(...),
                             ...))

Then you could easily remove years or simulations within years (by setting them to NULL, but it would make indexing a little bit harder (e.g. "retrieve Simulation1 values for all years" would require an lapply or a loop across years).