Selecting multiple odd or even columns/rows for dataframe
Solution 1
You can always generate sequences with seq:
even_indexes<-seq(2,42,2)
odd_indexes<-seq(1,41,2)
Then,
x.loadings <- data.frame(x=data.pc$loadings[odd_indexes,1])
Solution 2
When logical vectors are used for indexing, they are recycled so this gets you odd columns or odd rows
calld[ c(TRUE,FALSE), ] # rows
calld[ , c(TRUE,FALSE) ] #columns
Even rows or columns:
calld[ !c(TRUE,FALSE), ] # rows
calld[ , !c(TRUE,FALSE) ] #columns
Every third column:
calld[ , c(TRUE,FALSE, FALSE) ] #columns 1,4,7 , ....
A recent commenter claims this no longer works. I'm not finding that in R 4.0.4 running in Ubuntu:
> d <- data.frame(as.list(1:10)) # simple example construction
> d
X1L X2L X3L X4L X5L X6L X7L X8L X9L X10L
1 1 2 3 4 5 6 7 8 9 10
> d[, c(TRUE,FALSE)]
X1L X3L X5L X7L X9L
1 1 3 5 7 9
> d[, c(TRUE,FALSE,FALSE)] # example: # of columns not exact multiple of length of logical vector
X1L X4L X7L X10L
1 1 4 7 10
Solution 3
I wish to add the tidyverse
style approach to this problem, using the %%
operator.
library(dplyr)
df <- data.frame(V1 = seq(26), V2 = letters)
df %>% filter(row_number() %% 2 == 0) ## Select even rows
df %>% filter(row_number() %% 2 == 1) ## Select odd rows
df %>% filter(row_number() %% 3 == 1) ## Select every 3rd row starting from first row
You can use the same idea to delete every n-th row, of course. See here.
Solution 4
Use %%
in combination with seq_len
to create vector for indexing your data frame to find even and odds columns/rows
Try something like this:
even <- seq_len(ncol(data.pc)) %% 2 # index
x.loadings <- data.frame(x=data.pc$loadings[even, ])
y.loadings <- data.frame(x=data.pc$loadings[!even, ] )
Solution 5
Here is data.table approach
- Make a simple data.table with columns (10) and rows (100)
df = data.table(sapply(1:10, rnorm, n=100))
- Get the even/odd rows:
Even
df[df[, .I%%2==0]]
or df[seq(2,.N,2)]
Odd
df[df[, .I%%2==1]]
or df[seq(1,.N,2)]
- Get the even/odd columns
Even
df[,.SD, .SDcols=seq(2,ncol(df),2)]
Odd
df[,.SD, .SDcols=seq(1,ncol(df),2)]
dmt
Updated on July 09, 2022Comments
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dmt almost 2 years
Is there a way in R to select many non-consecutive i.e. odd or even rows/columns?
I'm plotting the loadings for my Principal Components Analysis. I have 84 rows of data ordered like this:
x_1
y_1
x_2
.....x_42
y_42
And at the moment I am creating the dataframes for the x and y loadings figures like this:
data.pc = princomp(as.matrix(data)) x.loadings <- data.frame(x=data.pc$loadings[c(1, 3, 5, 7, 9, 11, 13 ,15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41), 1]) yloadings <- data.frame(y=data.pc$loadings[c(2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42), 1])
Surely there's an easier way to do this?
-
alko989 almost 10 yearsTake a look at
?seq
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Jilber Urbina almost 10 yearsNote that you're talking about selecting columns, but your code selects rows from column 1.
-
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Victor Klos about 9 yearsBeste answer. If N becomes big in "every Nth column/row" then
c(TRUE, rep(FALSE,7)
(with 7 being N-1) may be used as readable and concise alternative. -
Standin.Wolf over 6 yearsCould you explain the above code? calld[ !c(TRUE,FALSE), ]
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IRTFM over 6 years@Standin.Wolf : Not sure what part is confusing you. The "!" basically inverts the logical vector so that it really means:
calld[ c(FALSE,TRUE) , ]
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Agile Bean over 3 yearsin
R 4.0.0
very unfortunately, the syntax ` calld[ , !c(TRUE,FALSE) ]` doesn't work any more. You must specify exactly the correct number of columns or rows to repeat. -
W Barker about 3 yearsI don't understand why the accepted answer to a question about SELECTING odd/even rows or columns is about GENERATING odd or even numbers.
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IRTFM almost 3 years@AgileBean: I just tested it again on a simple 10 column dataframe and it seems to work for me. You need to post a counter example if you are still seeing evidence of failure to adhere to R's admittedly unusual feature of extending vectors by implicit repetition(AKA "recycling").
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Mali Remorker about 2 yearsCan you actually use
.SD
inside the brackets? -
langtang about 2 yearshmm, not sure.. Can you show me what you mean?
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langtang about 2 years@MaliRemorker I added the use of .SD for selecting odd/even columns - thanks!