CODEWITHSUNDEEP
r
Saptarshi Roy
Saptarshi Roy

Reputation: 21

Adding a new column in dataframe and looking up values from matrix in R

Created a matrix named GroMatrix.

GroMatrix=matrix(c(1,2,3,4,5,2,3,4,5,6,3,4,5,6,7,4,5,6,7,8),byrow=TRUE,nrow=4)

I want to use this matrix as reference for entering data in a new column of my existing data frame by looking up data from two different columns of the same row to be treated as row number and column number of matrix. I was trying with following method but in vain.

Df$NewCol=GroMatrix[Df$Col1,Df$Col2]

Col1 and Col2 are two variables of Dataframe Df and both these columns are numeric (integers). On running the above command, I am getting a number of new observations added to my dataframe namely NewCol[,1], NewCol[,2], and so on. Need help .

Upvotes: 2

Views: 49

Answers (1)

AnilGoyal
AnilGoyal

Reputation: 26238

You just need to subset matrix through a matrix.

So, to subset matrix through matrix you have to convert your dataset into a matrix. Use as.matrix for that. Moreover, subsetting matrix through a matrix gives us a vector. For more reading, please see Advanced R by Hadley Wickham

Let's suppose your reference cols in data frame are named row and col. Just follow these steps

Gromatrix <- matrix(c(1,2,3,4,5,2,3,4,5,6,3,4,5,6,7,4,5,6,7,8),byrow=TRUE,nrow=4)
set.seed(1)
df_samp <- data.frame(
  row = sample(1:4, 10, replace = TRUE),
  col = sample(1:5, 10, replace = TRUE)
)
df_samp$new_col <- Gromatrix[as.matrix(df_samp[, c('row', 'col')])]
df_samp
#>    row col new_col
#> 1    1   3       3
#> 2    4   3       6
#> 3    3   1       3
#> 4    1   5       5
#> 5    2   5       6
#> 6    1   2       2
#> 7    3   2       4
#> 8    3   1       3
#> 9    2   5       6
#> 10   2   5       6

Created on 2022-07-07 by the reprex package (v2.0.1)

Upvotes: 2

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