Matrix Indexation
Good. Now you know how to create a matrix in several ways. Let's work with already created objects. First, we need to learn how to access certain matrix elements.
Back to the Vectors section, you should remember that indexation in R starts with 1. Since the matrix is a two-dimensional structure, we need to refer to two positions: among rows and columns. Like in vectors, you need to use square brackets and put two integers (the first is the row number, the second is the column number) divided by a comma. For example, let's extract element 5 (row 2, column 2) and 3 (row 1, column 3) from the matrix below.
[,1] [,2] [,3]
[1,] 1 2 3
[2,] 4 5 6
[3,] 7 8 9
123456789# Vector of integers num <- 1:9 # Matrix m <- matrix(num, nrow = 3, ncol = 3, byrow = T) # Element `5` m[2, 2] # Element `3` m[1, 3]
Also you can extract multiple rows and/or columns at once. Pass vector of indices instead of a single index. For example, we can extract the 2nd and 3rd columns for the 3rd row.
1234num <- 1:9 m <- matrix(num, nrow = 3, ncol = 3, byrow = T) # 2nd and 3rd columns for 3rd row m[3, c(2,3)]
If you want to extract an entire row/column, do not set an index for the remaining part. For example, let's pull the first row and the third column.
123456num <- 1:9 m <- matrix(num, nrow = 3, ncol = 3, byrow = T) # The first row m[1,] # The third column m[,3]
Swipe to start coding
Given matrix named m.
2 4 6 8
10 12 14 16
Your tasks are:
- Extract the element
12. - Extract the elements
4 6. - Extract the third column.
Lösung
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Can you explain how to extract a submatrix from a matrix in R?
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Good. Now you know how to create a matrix in several ways. Let's work with already created objects. First, we need to learn how to access certain matrix elements.
Back to the Vectors section, you should remember that indexation in R starts with 1. Since the matrix is a two-dimensional structure, we need to refer to two positions: among rows and columns. Like in vectors, you need to use square brackets and put two integers (the first is the row number, the second is the column number) divided by a comma. For example, let's extract element 5 (row 2, column 2) and 3 (row 1, column 3) from the matrix below.
[,1] [,2] [,3]
[1,] 1 2 3
[2,] 4 5 6
[3,] 7 8 9
123456789# Vector of integers num <- 1:9 # Matrix m <- matrix(num, nrow = 3, ncol = 3, byrow = T) # Element `5` m[2, 2] # Element `3` m[1, 3]
Also you can extract multiple rows and/or columns at once. Pass vector of indices instead of a single index. For example, we can extract the 2nd and 3rd columns for the 3rd row.
1234num <- 1:9 m <- matrix(num, nrow = 3, ncol = 3, byrow = T) # 2nd and 3rd columns for 3rd row m[3, c(2,3)]
If you want to extract an entire row/column, do not set an index for the remaining part. For example, let's pull the first row and the third column.
123456num <- 1:9 m <- matrix(num, nrow = 3, ncol = 3, byrow = T) # The first row m[1,] # The third column m[,3]
Swipe to start coding
Given matrix named m.
2 4 6 8
10 12 14 16
Your tasks are:
- Extract the element
12. - Extract the elements
4 6. - Extract the third column.
Lösung
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Completion rate improved to 2.27single
