Loops and filling empty vectors in R - r

Question

I'm looking to fill in this empty vector:
empty_vec <- rep("", times=length(number_vec))
with sequential numbers from this loop:
for (numbers in number_vec) {
sqrt <- sqrt(numbers)
empty_vec[numbers] <- sqrt
}
where numbers_vec is c(16:49).
However, when I do this, the first positions (1-15) in my empty_vec are not filled?

Answer 1

You can address this in two ways:
First, you can create a counter, that will register which step of the loop you are, and use this as index to empty_vect, like this:
empty_vec <- rep("", times=length(number_vec))
counter=0
for (numbers in number_vec) {
counter=counter+1
sqrt<-sqrt(numbers)
empty_vec[counter]<-sqrt
}
Or you can just create an empty vector and concatenate each new value, like this:
empty_vec <- c()
for (numbers in number_vec) {
sqrt<-sqrt(numbers)
empty_vec <- c(empty_vec,sqrt)
}
The way you were doing, is like you started to fill your vector in 16th position, that's way you had error.

Answer 2

First you need to understand how for loop works
General express for for loop is
for(var in seq) expr
var = A syntactical name for a variable
seq = An expression evaluating to a vector (including a list and an expression) or to a pairlist or NULL. A factor value will be coerced to a character vector.
so note it , "seq" will be the value of the "var".
In your example , you wrote
for (numbers in number_vec)
where,
numbers = Name of the variable
number_vec = c(16:49)
So here , the initial value of "numbers" will be first value of "number_vec" which is 16.
in later step in loop, the expression
empty_vec[numbers]<-sqrt
where ,
empty_vec[numbers] indicate the 16th position of the empty_vec as initially numbers started with value 16
As you start with 16th position , the previous 15 position remain empty.
Possible solution of your problem :
number_vec = c(16:49)
empty_vec <- rep("", times=length(number_vec))
for (numbers in seq_along(number_vec)) {
sqrt<-sqrt(number_vec[numbers])
empty_vec[numbers]<-sqrt
}