Explaining a for loop in R

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.

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

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

The answer from a classmate was:

> 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
}

I'm having similar problems in questions like:

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

So far, I created the vector and the empty vector

> y = c(5, 10, 15, 20, 25, 30)

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()
}

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.

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.  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
}
}

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
}

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)

The best way to solve this would be:

2 * 1:8 + 1

using vectorized operations.

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

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

This is intentionally not an answer because there are better ways to solve the alternating sign summation problem than a for-loop. I suppose there could be value in getting comfortable with for-loops but the vectorized approaches in R should be learned as well. R has "argument recycling" for many of its operations, including the "*" (multiplication) operation: Look at:

(1:10)*c(1,-1)

Then take an arbitrary vector, say vec and try:

sum( vec*c(1,-1) )

The more correct answer after looking at that result would be:

vvec + sum( vec[-1]*c(1,-1) )

Which has the educational advantage of illustrating R's negative indexing. Look up "argument recycling" in your documentation. The shorter objects are automagically duplicatied/triplicated/however-many-needed-cated to exactly match the length of the longest vector in the mathematical or logical expression.

The answers/resolutions are collected from stackoverflow, are licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0 .

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