问题
How do I generate a vector with a specified increment step (e.g. 2)? For example, how do I produce the following
0 2 4 6 8 10
回答1:
Executing seq(1, 10, 1) does what 1:10 does. You can change the last parameter of seq, i.e. by, to be the step of whatever size you like.
> #a vector of even numbers
> seq(0, 10, by=2) # Explicitly specifying "by" only to increase readability
> [1] 0 2 4 6 8 10
回答2:
You can use scalar multiplication to modify each element in your vector.
> r <- 0:10
> r <- r * 2
> r
[1] 0 2 4 6 8 10 12 14 16 18 20
or
> r <- 0:10 * 2
> r
[1] 0 2 4 6 8 10 12 14 16 18 20
回答3:
The following example shows benchmarks for a few alternatives.
library(rbenchmark) # Note spelling: "rbenchmark", not "benchmark"
benchmark(seq(0,1e6,by=2),(0:5e5)*2,seq.int(0L,1e6L,by=2L))
## test replications elapsed relative user.self sys.self
## 2 (0:5e+05) * 2 100 0.587 3.536145 0.344 0.244
## 1 seq(0, 1e6, by = 2) 100 2.760 16.626506 1.832 0.900
## 3 seq.int(0, 1e6, by = 2) 100 0.166 1.000000 0.056 0.096
In this case, seq.int is the fastest method and seq the slowest. If performance of this step isn't that important (it still takes < 3 seconds to generate a sequence of 500,000 values), I might still use seq as the most readable solution.
来源：https://stackoverflow.com/questions/7340472/how-do-i-generate-a-list-with-a-specified-increment-step