How to select rows based on pre-determined gaps (R)?

Question

I recently post another question asking how I could create a new data.frame based on a colunm variable. I thought it would fix my problem but I realize now that I was asking the wrong thing. What I mean with my question is, how I can select rows in a constant gap and create a new data.frame with them? Like, if I have:

1   A   B   C
2   D   E   F
3   G   H   I
4   J   K   L
5   M   N   O
6   P   Q   R

I will want to select the rows that grow in two to two like:

2  D   E   F
4  J   K   L
6  P   Q   R

But actually in my case, I need to select the rows that are groing in 40 to 40 and create a new data.frame with them.

Sorry for another post, but I will be really glad if you guys could help me. I'm a new user of R.

what do you mean by growing in 40 to 40? – GordonShumway May 03 '18 at 23:40 — GordonShumway, May 03 '18 at 23:40
Do you mean you want to select rows 40,80,120,160.... ? – thelatemail May 03 '18 at 23:47 — thelatemail, May 03 '18 at 23:47

Kim · Accepted Answer · 2018-05-03T23:49:55.340

0

It's very easy with the dplyr package.

library(dplyr)
test %>% dplyr::filter(row_number() %% 2 == 0)

Basically, you are calling the row number and selecting only the even ones. If you wanted to go for every 40 rows, you would be doing row_number() %% 40 == 0. If you wanted to start from another row and get every 40 rows, you just need to change the 0 to another number, as the %% operator performs modular divsion.

edited May 03 '18 at 23:49

answered May 03 '18 at 23:42

Kim

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Thank you! It's very easy indeed... I was almost given up and selecting the rows by hand haha =) – Gabriel Lucas May 03 '18 at 23:56
I think it is entirely possible that you just missed the right search keywords---if you had searched for "every nth row r dataframe", you would have gotten [this post](https://stackoverflow.com/questions/24440258/selecting-multiple-odd-or-even-columns-rows-for-dataframe) and [this post](https://stackoverflow.com/questions/7942519/deleting-every-n-th-row-in-a-dataframe). Cheers! – Kim May 04 '18 at 00:17

score 0 · Answer 2 · answered May 04 '18 at 00:47

0

You can use R base functions if you don't want to load any extra packages:

Option 1

df[seq_len(nrow(df))%%2==0,]

Option 2

subset(df, seq_len(nrow(df))%%2==0)

answered May 04 '18 at 00:47

Jilber Urbina

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How to select rows based on pre-determined gaps (R)?

2 Answers2