Dataframe column to matrix by two other columns

Question

I have a dataframe

df<-data.frame(i=rep(1:3,3),j=sort(rep(1:3,3)),v=sample(1:9,9))
df
  i j v
1 1 1 3
2 2 1 1
3 3 1 9
4 1 2 8
5 2 2 5
6 3 2 4
7 1 3 7
8 2 3 2
9 3 3 6

that I want to transform to matrix M such that

M[i,j]<-df$v[which(df$i==i & df$j==j)] 

is there an easy way to do that?

Answer 1

Based on your description, you can just do,

matrix(df$v, ncol = max(df$j))
#      [,1] [,2] [,3]
#[1,]    2    4    7
#[2,]    3    1    5
#[3,]    8    6    9

Data Used:

dput(df)
structure(list(i = c(1L, 2L, 3L, 1L, 2L, 3L, 1L, 2L, 3L), j = c(1L, 
1L, 1L, 2L, 2L, 2L, 3L, 3L, 3L), v = c(2L, 3L, 8L, 4L, 1L, 6L, 
7L, 5L, 9L)), class = "data.frame", row.names = c(NA, -9L))