Extract every nth element of a vector

Question

I would like to create a vector in which each element is the i 6th element of another vector   For example  in a vector of length 120 I want to create another vector of length 20 in which each element is value i  i 6  i 12  i 18    of the initial vector  i e  I want to extract every 6th element of the original

User · Answer

I think you are asking two things which are not necessarily the same

I want to extract every 6th element of the original

You can do this by indexing a sequence:

foo <- 1:120
foo[1:20*6]

I would like to create a vector in which each element is the i+6th element of another vector.

An easy way to do this is to supplement a logical factor with FALSEs until i+6:

foo <- 1:120
i <- 1
foo[1:(i+6)==(i+6)]
[1]   7  14  21  28  35  42  49  56  63  70  77  84  91  98 105 112 119

i <- 10
foo[1:(i+6)==(i+6)]
[1]  16  32  48  64  80  96 112

User · Answer

To select every nth element from any starting position in the vector  nth element  lt - function vector  starting position  n       vector seq starting position  length vector   n           E g  vec  lt - 1 12  nth element vec  1  3     1   1  4  7 10  nth element vec  2  3     1   2  5  8 11

User · Answer

Another trick for getting sequential pieces  beyond the seq solution already mentioned  is to use a short logical vector and use vector recycling   foo  c  rep FALSE  5   TRUE

User · Answer

a  lt - 1 120 b  lt - a seq 1  length a   6

[r] Extract every nth element of a vector

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