4.3 Indexing and Subsetting
Subsetting works by using indices of interest with square brackets.
In matrix, using a single index treats it as a vector and selects the cells based on their order
matrix_data2## Thea Pravin Troy Albin Clementine
## Thea 1 1 1 1 0
## Pravin 1 0 1 1 1
## Troy 0 0 1 0 1
## Albin 1 1 0 1 0
## Clementine 1 0 0 1 0matrix_data2[c(1,7,20,25)]## [1] 1 0 1 0Using two indices subsets the matrix based on [row,column] pairs
matrix_data2[1,4]## [1] 1matrix_data2[c(1,2),c(4,5)]## Albin Clementine
## Thea 1 0
## Pravin 1 1We can also subset based on the column/row names
matrix_data2["Thea", "Troy"]## [1] 1We can use subsetting to change specific cells in the matrix
matrix_data2[c(1,7,20,25)] <- 7
matrix_data2## Thea Pravin Troy Albin Clementine
## Thea 7 1 1 1 0
## Pravin 1 7 1 1 1
## Troy 0 0 1 0 1
## Albin 1 1 0 1 0
## Clementine 1 0 0 7 7matrix_data2[c(1,2),c(4,5)] <- 9
matrix_data2## Thea Pravin Troy Albin Clementine
## Thea 7 1 1 9 9
## Pravin 1 7 1 9 9
## Troy 0 0 1 0 1
## Albin 1 1 0 1 0
## Clementine 1 0 0 7 7Subsetting in dataframe using a single index works differently.
#select columns 1, 2, and 3
df_data2[c(1,2,3)]## Thea Pravin Troy
## Thea 1 1 1
## Pravin 1 0 1
## Troy 0 0 1
## Albin 1 1 0
## Clementine 1 0 0Finally, we may wish to remove columns in a data.frame, matrix or vector. We can use the subset function to do this.
#
(subset( vector_123, vector_123 > 1))## [1] 2 3(vector_123[vector_123>1])## [1] 2 3