问题
How can I extract values from a matrix given a data frame containing the indexes of Rows and Columns?
So, I have a matrix and I have a data frame with two columns the first contains the indexes of the rows that I want to extract from the matrix and the second the indexes of the columns.
How can I get all the values from the matrix corresponding to the couples of indexes in the data frame?
回答1:
Without having more detail, my guess is you have a data.frame like so:
index_df = data.frame(rows = c(1:5),
cols = c(6:10))
> index_df
rows cols
1 1 6
2 2 7
3 3 8
4 4 9
5 5 10
I will use this matrix as an example:
dummy_matrix = matrix(1:100, ncol = 10, byrow = T)
> dummy_matrix
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,] 1 2 3 4 5 6 7 8 9 10
[2,] 11 12 13 14 15 16 17 18 19 20
[3,] 21 22 23 24 25 26 27 28 29 30
[4,] 31 32 33 34 35 36 37 38 39 40
[5,] 41 42 43 44 45 46 47 48 49 50
[6,] 51 52 53 54 55 56 57 58 59 60
[7,] 61 62 63 64 65 66 67 68 69 70
[8,] 71 72 73 74 75 76 77 78 79 80
[9,] 81 82 83 84 85 86 87 88 89 90
[10,] 91 92 93 94 95 96 97 98 99 100
Extract required entries:
dummy_matrix[as.matrix(index_df)]
[1] 6 17 28 39 50
thanks to those who spotted my silly error
来源：https://stackoverflow.com/questions/39949735/r-extract-values-from-matrix-given-dataframe-of-x-and-y