Table gets empty when looping through a column

Hi everyone,

I have a question concerning the use of R to transform data in Power BI.

I am working on this table and trying to replace the first column data.

I have trouble understanding how does R code can really interact with data, and I am a beginner in R. This might be why I get this error :

I wrote this script (trying, as a test, to replace all values of the first column by 1) and when I run it, the data seems to be empty :

ctr <- 0

for (day in dataset[,1]) {
ctr <- ctr + 1
dataset[,1][ctr] <- 1
}

I thought someone may have a solution.

Thanks,

Baptiste