Results differ between printed output and stored vector in nested for loop. What gives?

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I believe this may be straightforward, but I have been trying to figure this out for hours. The printed output is how I would like my vector. It is doing exactly what I want.

x1<-c(1,2,3,4)
x2<-c(1,0)
len<- length(filename)
output <- vector("numeric", 8)

for (i in x1){
    for(j in x2){
       print(sum(i+j))
       output[i]<- i
    }
}
output

But, I am confused as to why I get these two different results:

[1] 2
[1] 1
[1] 3
[1] 2
[1] 4
[1] 3
[1] 5
[1] 4
> output
[1] 1 2 3 4 0 0 0 0

How do I store the print results as a vector? Thanks.

nested-loops r for-loop
1 Answers

According to ?print

print prints its argument and returns it invisibly (via invisible(x)).

We may need to initialize an object (sum1) and start concatenating the output from sum(i + j) to the object while updating the output from each iteration back to 'sum1'

sum1 <- NULL
 output <- vector("numeric", 8)

 for (i in x1){
     for(j in x2){
      sum1 <- c(sum1, i + j)
  output[i]<- i
  }
 }

-output

> sum1
[1] 2 1 3 2 4 3 5 4

Regarding the difference between the print and 'output'. It is obvious - printing is done on the sum(i + j) (sum is redundant here as i + j does the sum as well) where as output is stored with the i value alone. Also the initialization of vector is a numeric vector with value 0 of length 8

>  output <- vector("numeric", 8)
> output
[1] 0 0 0 0 0 0 0 0

Initialization can also be done as

> output <- numeric(8)

where as the x1 values are of length 4. Thus, after the first 4 iteration, there is nothing to fill the 0 value remains as such.

In R, this can also be done with outer

> c(t(outer(x1, x2, FUN = `+`)))
[1] 2 1 3 2 4 3 5 4

Or with sapply

> c(sapply(x1, `+`, x2))
[1] 2 1 3 2 4 3 5 4
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