提取 R 中向量的每个第 N 个元素
在本文中，我们将看到如何在 R 编程语言中从向量中提取每个第N个元素。
方法一：使用迭代
在声明一个变量的同时对向量执行迭代，计数器初始化为 0。每次迭代，计数器递增，一旦访问第 n 个元素，它就会被打印出来，计数器被重新初始化为 0。结果，获得每个第 n 个索引处的元素序列。这种方法所需的时间复杂度为 O(n)，其中 n 是向量的长度。
例子
R
# declaring a vector
vec <- c(1,2,3,4,5,4,3,7,8,9)
# accessing every third element
# of the vector maintaining counter
count = 0
# looping over the vector elements
for (i in vec){
# incrementing count
count= count + 1
# checking if count is equal to
# third element
if(count == 3){
# printing the specific element
print (i)
# reinitialising count to 0
count = 0
}
}R
# declaring a vector
vec <- c("geeks","for","geeks","is","a","unique",
"portal","to","learn","and","grow")
# accessing every second element of
# the vector
n = 2
# maintaining counter
count = 0
# looping over the vector elements
for (i in vec){
# incrementing count
count= count + 1
# checking if count is equal to third
# element
if(count %% n == 0){
# printing the specific element
print (i)
}
}R
# declaring a vector
vec <- 5:20
print("Original vector:")
print(vec)
print("Extracting every 4th element")
# iterating over 1 to length of the
# vector to extract elements
extracted_vec <- vec[seq(1, length(vec), 4)]
print ("Resultant elements")
print(extracted_vec)输出
[1] 3
[1] 4
[1] 8
方法 2：使用迭代和取模
在声明变量的同时对向量执行迭代，计数器初始化为 0。每次迭代时，计数器都会递增，并使用指定的第 n 个元素检查其模数。如果模块操作的结果为 0，则打印该元素。这种方法所需的时间复杂度为 O(n)，其中 n 是向量的长度。
例子
电阻
# declaring a vector
vec <- c("geeks","for","geeks","is","a","unique",
"portal","to","learn","and","grow")
# accessing every second element of
# the vector
n = 2
# maintaining counter
count = 0
# looping over the vector elements
for (i in vec){
# incrementing count
count= count + 1
# checking if count is equal to third
# element
if(count %% n == 0){
# printing the specific element
print (i)
}
}
输出
[1] “for”
[1] “is”
[1] “unique”
[1] “to”
[1] “and”
方法 3：使用seq()函数
R 中的 seq() 方法用于从它们引用的对象中生成序列。 seq() 方法根据约束提取原始向量的子集，即开始和结束索引，以及每次迭代期间要增加的步数。它根据这些约束访问给定向量索引处的元素，然后将其附加到序列对象。
Syntax: seq (from, to , increment_step)
Parameters :
- from – the starting index of the sequence.
- to – the ending index of the sequence, till where we wish to iterate.
- increment_step – how many elements to skip
Returns :
A sequence based on the constraints.
例子
电阻
# declaring a vector
vec <- 5:20
print("Original vector:")
print(vec)
print("Extracting every 4th element")
# iterating over 1 to length of the
# vector to extract elements
extracted_vec <- vec[seq(1, length(vec), 4)]
print ("Resultant elements")
print(extracted_vec)
输出
[1] “Original vector:”
[1] 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
[1] “Extracting every 4th element”
[1] “Resultant elements”
[1] 5 9 13 17