r-有多少偶数索引值大于前一个值

Question

我想知道A[even number]的计数大于A[even number-1]

>A
[1] 1 2 3 1 4 5 5 6 7

我下面的代码不起作用：

for (i in 1:length(A)){
coun<-0
if (length(A)%%2!=0){
len <- length(A)-1
for (i in 1: len){
if (A[i+1]>A[len]){
coun<-coun+1
}
}
else if (A[i+1]>A[i]){
coun<-coun+1  
}
}

在A中，计数应为3，因为(A[2]>A[1]，A[4]<A[3]，A[6]>A[5]，A[8]>A[7]，A[9]将不被视为后面没有值(

Answer 1

两种方法：

## find the even indices and
## calculate the differences you want
even_ind = seq_along(A)[c(F, T)]
sum(A[even_ind] > A[even_ind - 1])
# [1] 3

## or, calculate all differences, 
## and take every other one
sum((diff(A) > 0)[c(T, F)])
# [1] 3