How to create a for in R with the indexes of a data frame

Question

If I have a date.:

> dato<-as.data.frame(matrix(1:64,8,8,T))[-3,]
> dato
  V1 V2 V3 V4 V5 V6 V7 V8
1  1  2  3  4  5  6  7  8
2  9 10 11 12 13 14 15 16
4 25 26 27 28 29 30 31 32
5 33 34 35 36 37 38 39 40
6 41 42 43 44 45 46 47 48
7 49 50 51 52 53 54 55 56
8 57 58 59 60 61 62 63 64

How can I "call" the index vector (1, 2, 4, 5, 6, 7, 8)?

Because I want to use it as an index in a for.

Answer 1

The key point here is to realize that for in the R is executed from an index vector. See example below:

for (j in 1:5){
  print(j^2)
}
[1] 1
[1] 4
[1] 9
[1] 16
[1] 25

To make this loop, I implicitly created an index vector with the command 1:5:

1:5
[1] 1 2 3 4 5

In particular, this vector starts at 1 and ends at 5, with increment of 1. If I had made it explicitly before to create my for, I would get the same result:

indices <- c(1, 2, 3, 4, 5)
for (j in indices){
  print(j^2)
}
[1] 1
[1] 4
[1] 9
[1] 16
[1] 25

Therefore, the for in the R is set on top of the positions of a vector. This vector can be whatever I want. Let’s assume that I wish to square not the first five positive integers, but the first five prime numbers. So I would have

indices <- c(2, 3, 5, 7, 11)
for (j in indices){
  print(j^2)
}
[1] 4
[1] 9
[1] 25
[1] 49
[1] 121

Notice how the logic is the same? I just need to correctly define my index vector and make my counter vary in it. In your case, it would be something like

indices <- c(1, 2, 4, 5, 6, 7, 8)
for (j in indices){
  # comandos a serem executados
}

Of course the vector indices could have been defined in a less explicit way. For example, the command

indices <- (1:8)[-3]
[1] 1 2 4 5 6 7 8

creates a vector with the first eight positive integers and removes the observation from the third position.

Anyway, there are several ways to solve the problem. The only feature that doesn’t change is that the R will always make the loop counter vary in the positions of a vector, from the first to the last.

Answer 2

Complementing @Marcus-Nunes' excellent response to get the indices automatically from your data.frame, you can use the function row.names():

dato <- as.data.frame(matrix(1:64, 8, 8, T))[-3, ]

# selectionar o nome das linhas e converter caracteres em numérico
rw <- as.numeric(row.names(dato))
rw
# [1] 1 2 4 5 6 7 8

Using rw in a loop for():

for(i in rw) {
  print(i)
}
# [1] 1
# [1] 2
# [1] 4
# [1] 5
# [1] 6
# [1] 7
# [1] 8