1. Ch 9-10 Subsetting Ordering Array operations Iteration
© Fall 2004 Don Edwards and the University of South Carolina

2. 9 Subsetting Extraction with “[]” for vectors and matrices
: and c()
Rows, columns, and sub-matrices
Indices are not extracted
Extraction with logical vectors
Statements look self-referential
We can recover index information with match() or which() for unique entries.

3. 9 Sorting Order function Sort function lacks functionality
Creates vector of indices that can then be used to “extract” ordered data files
Like a logical vector, the focus is on the index object, not the output object
Sort function lacks functionality
Ordering of factor levels may not be alphabetical

4. 10 Matrix Operations Splus was very inefficient at looping
R is improved
Both Splus and R have built-in iterative capabilities
Functions that operate on vectors and arrays
The apply() function
tapply() works on groups (defined by factors)
tapply() is quite a versatile feature

5. 10 Iteration
If we need to iterate, we use for() and while() statements
for() uses a counter
while() uses a logical comparison
If for() is slow, R can call both C++ and Fortran subroutines