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R Software 2 (Exploring Data (head() (default shows first 6 rows, but can…

- - - - it takes 3 arguments: a list, a function that will do something you want, and the ...
        
        Note: the function you use can be a R defined function, or you can write your own on the fly and this "anonymous" function will only exist with the lapply framework
      - note that lapply always returns a list no matter what the input was. so the lapply function will coerce the object to a list first and then do the computation. if the object can't be coerced then it returns an error
    - - a variant of "lapply", it tries to simply the result that would come out of lapply
  - - - 1st argument: an array
      - What's an array in R?
        
        it's an object with rows and columns
        
        a matrix a 2-D array
      - 2nd argument: Margin which is an integer vector which specifies which margin ( or dimension) is to be retained
        
        you have a matrix assigned to object "x" with 20 rows and 10 columns. you want to calculate the mean of each column.
        
        so using the apply function on the matrix: apply(x, 2, mean).
        
        what you get back is a vector with 10 numbers in it, since there are 10 columns. the '2' of the 2nd argument says to keep the 2nd dimension of the matrix, or keep the columns and collapse the 1st dimension, or the rows. this command is taking the mean of each column and just returning the value of the mean for each column.
        
        apply(x, 1, mean)
        
        having a '1' as the 2nd argument tells R to retain the rows, so take the mean across a whole row and collapse the columns. so the output here would be a vector of 20 numbers which are the means of each row.
        
        Special Note if you are wanting to calculate the sum of rows or mean of rows in a matrix. There are shortcut functions for this that do the apply function for you.
        
        rowSums()
        
        rowMeans()
        
        colSums()
        
        colMeans()
      - 3rd argument: the function you want applied
- - - - 1) click Rstudio in upper tool bar 2) click preferences 3)code editing 4) enter # for tab width. min 4, 8 ideal 5) can also set max number of columns for width
        
        this improves readability for the code