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Course 2: R Programming (Subsetting (Matrices (By default, if a single…

- - - - x <- list(foo = 1:4, bar = 0.6)
        
        x[1]
        
        this returns a list, foo with the sequence 1through 4 in it
        
        x[[1]]
        
        this returns a object with just a sequence of numbers. it's no longer a list
        
        x$bar
        
        this returns just the number 0.6
        
        x[["bar"]]
        
        can also feed a string into the double bracket subset. this also just returns the value that "bar" is associated with
        
        x["bar"]
        
        returns a list with the 0.6 in it
        
        x[c(1,3)]
        
        extracts multiple elements of a list
  - - - x[2,1]
        
        this command would subset the value in the 2nd row and 1st column of the index.
    - - this subsets only the values in the first row of the matrix
    - - this returns the values only in the 2nd column
    - - you can change this default by setting drop = FALSE
      - another exception, is that by default if you subset a single row then this also gets returned as a vector. not a 1 by ncol matrix. this behavior can also be turned off by setting drop = FALSE
  - - - x <- c(1, 2, NA, 4, NA, 5)
        
        bad <- is.na(x)
        
        this command stores a logical vector where it is true if the element is missing and false if the element is not missing
        
        x <- x[!bad]
        
        this command will store in 'x' all the values that are NOT in bad. so this will eliminate the missing values