This functions below aim to give a bit of background on data and data manipulation in R.
 which.max(x) returns the index of the greatest element of x
 which.min(x) returns the index of the smallest element of x
 rev(x) reverses the elements of x
 sort(x) sorts the elements of x in increasing order; to sort in decreasing order: rev(sort(x))
 cut(x,breaks) divides x into intervals (factors); breaks is the number of cut intervals or a vector of cut points
 match(x, y) returns a vector of the same length than x with the elements of x which are in y (NA otherwise)
 which(x == a) returns a vector of the indices of x if the comparison operation is true (TRUE), in this example the values of i for which x[i] == a (the argument of this function must be a variable of mode logical)
 choose(n, k) computes the combinations of k events among n repetitions = n!/[(n−k)!k!]
 na.omit(x) suppresses the observations with missing data (NA) (suppresses the corresponding line if x is a matrix or a data frame)
 na.fail(x) returns an error message if x contains at least one NA
 unique(x) if x is a vector or a data frame, returns a similar object but with the duplicate elements suppressed
 table(x) returns a table with the numbers of the differents values of x (typically for integers or factors)
 subset(x, …) returns a selection of x with respect to criteria (…,
 typically comparisons: x$V1 < 10); if x is a data frame, the option
 select gives the variables to be kept or dropped using a minus sign
 sample(x, size) resample randomly and without replacement size elements in the vector x, the option replace = TRUE allows to resample with replacement
 prop.table(x,margin=) table entries as fraction of marginal table
Functions for Manipulating Character Variables


nchar(x)  a vector fo the lengths of each value in x 
paste(a,b,sep=”_”)  concatenates character values, using sep between them 
substr(x,start,stop)  extract characters from positions start to stop from x 
strsplit(x,split)  split each value of x into a list of strings using split as the delimiter 
grep(pattern,x)  return a vector of the elements of x that included pattern 
grepl(pattern,x)  returns a logical vector indicating whether each element of x contained pattern 
regexpr(pattern,x)  returns the integer positions of the first occurrence of pattern in each element of x 
gsub(pattern,replacement,x)  replaces each occurrence of pattern with occurrence 
tolower(x)  converts x to all lower case 
toupper(x)  converts x to all upper case 
Logical Operators  
==  is equal to 
!=  is not equal to 
>  greater than 
>=  greater than or equal to 
<  less than 
<=  less than or equal to 
%in%  is in the list 
!  not (reverses T & F 
&  and 
  or 