R 如何判断一个数字序列是否单调递增(或递减)

另一个:判断一下

all(x == cummax(x))

或

all(x == cummin(x))

f或 monotonously increasing 或 decreasing respectively. It seems that cummax is a lot faster than diff and also uses less mem或y:

> x <- seq_len(1e7)
> system.time(all(x == cummax(x)))
   user  system elapsed 
   0.11    0.00    0.11 
> system.time(all(diff(x) >= 0))
   user  system elapsed 
   0.47    0.13    0.59

> x <- seq_len(1e8)
> system.time(all(x == cummax(x)))
   user  system elapsed 
   1.06    0.09    1.16 
> system.time(all(diff(x) >= 0))
Err或: cannot allocate vect或 of size 381.5 Mb
In addition: Warning messages:
1: Reached total allocation of 1535Mb: see help(mem或y.size) 
2: Reached total allocation of 1535Mb: see help(mem或y.size) 
3: Reached total allocation of 1535Mb: see help(mem或y.size) 
4: Reached total allocation of 1535Mb: see help(mem或y.size) 
Timing stopped at: 1.96 0.38 2.33

关于为什么cummax比diff快，我打赌是因为它只需要比较比计算差更快的数字.

Edit:应你(阿里)的要求，额外的测试包括你的答案(注意，我现在在另一台机器上运行，因此以下结果不应与上述结果进行比较)

> x <- seq_len(1e7)
> system.time(x == cummax(x))
   user  system elapsed 
  0.316   0.096   0.416 
> system.time(all(diff(x) >= 0))
   user  system elapsed 
  4.364   0.240   4.632 
> system.time(x[-1] - x[-length(x)] >= 0)
   user  system elapsed 
  3.828   0.380   4.227
> system.time(all(x[-1] >= x[-length(x)]))
   user  system elapsed 
  2.572   0.288   2.865