解释R中的for循环 [英] Explaining a for loop in R

问题描述

我对R非常陌生，对R编程也很陌生.我有以下问题及其答案(这不是我的).我试图理解为什么某些值，从哪里获得的，为什么使用它们，等等.

I'm very new to R, and much more new to programming in R. I have the following question and its answer (which is not mine). I've trying to understand why some values, from where they are obtained, why they are used, etc.

问题:使向量3 5 7 9 11 13 15 17具有for循环.开始 用x = numeric()并用for循环填充此向量

Question: Make the vector 3 5 7 9 11 13 15 17 with a for loop. Start with x=numeric() and fill this vector with the for loop

我知道我必须创建x = numeric()，以便可以用从循环中获得的结果填充它.

I know I have to create x=numeric() so I can fill it with the result obtained from the loop.

同学的答案是:

> x <- numeric()
> for(i in 1:8){   
      if(i==1){                ## Why ==1 and not 0, or any other value   
          x[i] <- 3              
      }else{   
          x[i] <- x[i-1]+2  ### And why i-1   
     }

我在类似的问题中遇到类似的问题

I'm having similar problems in questions like:

进行for循环，将向量的第二个元素添加到第一个元素， 从结果中减去第三个元素，再次添加第四个元素，然后 这样整个向量的长度

Make a for loop that adds the second element of a vector to the first, subtracts the third element from the result, adds the fourth again and so on for the entire length of the vector

到目前为止，我已经创建了向量和空向量

So far, I created the vector and the empty vector

> y = c(5, 10, 15, 20, 25, 30)
> answer <- 0 

然后，当我尝试执行for循环时，我被卡在这里:

And then, when I try to do the for loop, I get stuck here:

for(i in 1:length(y)){
if(i...){ ### ==1? ==0?
    answer = y[i]   ###and here I really don't know how to continue. 
}else if()
}

当我告诉您时，请相信我，我已经在这里阅读了一些对问题的答复，例如

Believe me when I tell you I've read several replies to questions here, like in How to make a vector using a for loop, plus pages and pages about for loop, but cannot really figure how to solve these (and other) problems.
I repeat, I'm very new, so I'm struggling trying to understand it. Any help would be much appreciated.

推荐答案

首先，我将对循环进行注释，以回答循环的作用.

First, I will annotate the loop to answer what the loop is doing.

# Initialize the vector
x <- numeric()
for(i in 1:8){   
  # Initialize the first element of the vector, x[1].  Remember, R indexes start at 1, not 0.
  if(i==1){                
    x[i] <- 3              
  } else {   
    # Define each additional element in terms of the previous one (x[i - 1]
    # is the element of x before the current one.
    x[i] <- x[i-1]+2  ### And why i-1   
  }
} 

使用循环并扩大循环(如指令状态)的更好解决方案是这样的:

A better solution that uses a loop and grows it (like the instructions state) is something like this:

x <- numeric()
for(i in 1:8){
  x[i] <- 2 * i + 1
}

这仍然不是处理问题的好方法，因为在循环内增长向量非常缓慢.要解决此问题，您可以通过告诉numeric您想要的向量的长度来预分配向量:

This is still not a good way to do things because growing a vector inside a loop is very slow. To fix this, you can preallocate the vector by telling numeric the length of the vector you want:

x <- numeric(8)

解决此问题的最佳方法是:

The best way to solve this would be:

2 * 1:8 + 1

使用矢量化操作.

为帮助您解决其他问题，建议将循环的每个步骤写成表格.例如，对于我的解决方案，该表将是

To help you solve your other problem, I suggest writing out each step of the loop as a table. For example, for my solution, the table would be

i | x[i]
------------------
1 | 2 * 1 + 1 = 3
2 | 2 * 2 + 1 = 5

，依此类推.这将使您了解每次迭代中for循环的作用.

and so on. This will give you an idea of what the for loop is doing at each iteration.

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