Extract Values from Matrix by Column and Row Names in R

23 May, 2021
In this article, we will discuss how to extract values from the matrix by column and row names in R Programming Language.
A row subset matrix can be extracted from the original matrix using a filter for the selected row names. Since a matrix’s elements are accessed in a dual index format, particular row selection can be carried out.
Syntax:
matrix[ vec , ] 
Where, vec contains the row names to be fetched
All the columns are retrieved from the data frame. The order of the rows and columns remains unmodified. The rows and column names remain unchanged after extraction. The result returned is a subset of the original matrix. The row names should be a proper subset of the original row names pertaining to matrix. 
Example:
R
# declaring matrix mat <- matrix(letters[1:12], ncol = 3)  # naming columnscolnames(mat) <- c("C1","C2","C3")  # naming rowsrownames(mat) <- c("R1","R2","R3","R4")print ("Original Matrix")print (mat)  # extracting rows row_vec <- c("R2","R4")row_mat <- mat[row_vec ,]  print ("Modified Matrix")print (row_mat)
Output
[1] "Original Matrix"
  C1  C2  C3
R1 "a" "e" "i"
R2 "b" "f" "j"
R3 "c" "g" "k"
R4 "d" "h" "l"
[1] "Modified Matrix"
  C1  C2  C3
R2 "b" "f" "j"
R4 "d" "h" "l"
A column subset matrix can be extracted from the original matrix using a filter for the selected column names. Since a matrix’s elements are accessed in a dual index format, particular row selection can be carried out.
Syntax:
matrix[  , vec ] 
Where, vec contains the column names to be fetched
All the rows for the selected columns are retrieved from the data frame. The order of the rows and columns remains unmodified. The rows and column names remain unchanged after extraction. The result returned is a subset of the original matrix. The column names to be chosen should be a proper subset of the original row names pertaining to the matrix. 
Example:
R
# declaring matrix mat <- matrix(letters[1:12], ncol = 3)  # naming columnscolnames(mat) <- c("C1","C2","C3")  # naming rowsrownames(mat) <- c("R1","R2","R3","R4")print ("Original Matrix")print (mat)  # extracting rows col_vec <- c("C1","C3")col_mat <- mat[,col_vec]print ("Modified Matrix")print (col_mat)
Output
[1] "Original Matrix"
  C1  C2  C3
R1 "a" "e" "i"
R2 "b" "f" "j"
R3 "c" "g" "k"
R4 "d" "h" "l"
[1] "Modified Matrix"
  C1  C3
R1 "a" "i"
R2 "b" "j"
R3 "c" "k"
R4 "d" "l"
Similar to the above approaches, the rows, and columns can also be extracted, by specifying the chosen vectors for both indexes. 
Syntax:
matrix[ rowvec , colvec ]  
Where, rowvec contains the row names to be fetched and colvec the column names
Example:
R
# declaring matrix mat <- matrix(letters[1:12], ncol = 3)  # naming columnscolnames(mat) <- c("C1","C2","C3")  # naming rowsrownames(mat) <- c("R1","R2","R3","R4")print ("Original Matrix")print (mat)  # extracting rows row_vec <- c("R1","R3")  # extracting columns col_vec <- c("C1","C3")col_mat <- mat[row_vec ,col_vec]print ("Modified Matrix")print (col_mat)
Output
[1] "Original Matrix"
  C1  C2  C3
R1 "a" "e" "i"
R2 "b" "f" "j"
R3 "c" "g" "k"
R4 "d" "h" "l"
[1] "Modified Matrix"
  C1  C3
R1 "a" "i"
R3 "c" "k"
Any single element or cell value can also be fetched from the matrix, by simply specifying the row name and column name as the indexes for retrieval. 
Example:
R
# declaring matrix mat <- matrix(letters[1:12], ncol = 3)  # naming columnscolnames(mat) <- c("C1","C2","C3")  # naming rowsrownames(mat) <- c("R1","R2","R3","R4")print ("Original Matrix")print (mat)  # extracting single elementcol_mat <- mat["R3" , "C2"]print ("Modified Matrix")print (col_mat)
Output
[1] "Original Matrix"
  C1  C2  C3
R1 "a" "e" "i"
R2 "b" "f" "j"
R3 "c" "g" "k"
R4 "d" "h" "l"
[1] "Modified Matrix"
[1] "k"
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