22015-03-15 09:49:00mandy.vogel@googlemail.com12015-03-15 09:48:13mandy.vogel@googlemail.com

there are circumstances where we want to select only some of the elements of a vector/array/dataframe/list this selection is done using subscripts (also known as indices) subscripts have square brackets [2] while functions have round brackets (2) Subscripts on vectors, matrices, arrays and dataframes have one set of square brackets [6], [3,4] or [2,3,2,1] when a subscript appears as a blank it is understood to mean all of thus [,4] means all rows in column 4 of an object [2,] means all columns in row 2 of an object. subscripts on lists have (usually) double square brackets [ [2] ] or [ [i,j] ] A vector of positive integers as index:The index vector can be of any length and the result is of the same length as the index vector. For example, ]]> ]]> A logical vector as index: Values corresponding to T values in the index vector are selected and those corresponding to F or NA are omitted. For example, ]]> ]]> creates a vector without missing values. Also ]]> ]]> replaces the missing value by zeros. A common operation is to select rows or columns of data frame that meet some criteria. For example, to select those rows of painters data frame with Colour >= 17: ]]> ]]>= 17,]]> We may want to select on more than one criterion. We can combine logical indices by the 'and', 'or' and 'not' operators. For example, ]]>= 17 & ]]>

Operation Description! logical NOT ¦ | logical OR < less than <= less than or equal to > greater than >= greater than or equal to == logical equals (double =) != not equal ¦¦ OR with IF xor(x,y) exclusive OR isTRUE(x) an abbreviation of identical(TRUE,x)If we want to select a subgroup, for example those with schools A, B, and D. We can generate a logical vector using the <latex> \mathtt{\%in\%}</latex> operator as follows: ]]> Sometimes we are interested in the indices of rows satisfying a certain condition. To extract these indices we use the which() command. ]]>

A vector character strings with variable names can be used to extract those variables relevant for analysis. This is very useful when we have a large number of variables and we need to work with a few ones. For example, ]]> ]]> a vector of character strings could a index on a vector when the vector has names: ]]> ]]> ]]> ]]>

an extremely useful facility is to use negative indices to drop terms from a vector suppose we wanted a new vector, z, to contain everything but the first element of x ]]> (]]>