Save loop results as csv table - r

Question

I have simple loop that generate a value at each step, and I want to save all results as a single table. Problem is that each step overwrites the previous.
for(i in 1:5){
x = 3*i
print(c(i,x))
}
This gives
[1] 1 3
[1] 2 6
[1] 3 9
[1] 4 12
[1] 5 15
Now I create a matrix that I will then save as a csv file, but it only shows the final step of the loop.
results = matrix(c(i,x), ncol = 2)
[,1] [,2]
[1,] 5 15
write.table(results, file = "Results.csv", col.names=NA, append = T)
How to show the entire list of results? Thanks in advance!
(ps.- I know that a similar question has been posted previously, e.g. Write output of R loop to file, but the problem was quite specific and I did not manage to adapt the answers to my case).

Answer 1

Your loop only prints, to the console, the results. The matrix you're creating only relies on the single (and last) value of i. There are many ways to do it but if you really want to write a matrix, then you need to store them somewhere to export all iteration intermediate results. You can try something like:
results <- matrix(NA, nrow=5, ncol=2)
for(i in 1:5){
results[i, ] <- c(i, 3*i)
}
write.table(results, file = "Results.csv", col.names=NA, append = T)
And by the way you don't really need a loop here:
i <- 1:5
m <- matrix(c(i, 3*i), nrow=5)
would do the job.

Answer 2

You can usually use sapply instead of for-loops:
results <- t(sapply(1:5, function(x) c(x, 3*x)))
write.table(results, file="Results.csv", col.names=NA, append=T)

Answer 3

Assuming you really want/need a for-loop
1) You store all the result into a matrix and then you write the whole matrix to file
n = 5;
results = matrix(NA, ncol=2, nrow=n);
for(i in 1:n) {
results[i, ] = c(i, x);
}
write.table(results, file = "Results.csv", col.names=NA, append = T);
This is a "good" solution if you don't have many results and you want to access the file just once.
2) You store current result only into a matrix and you write to file at each iteration
n = 5;
for(i in 1:n) {
results = matrix(c(i,x), ncol = 2)
write.table(results, file = "Results.csv", col.names=NA, append = T);
}
This is a "good" solution if you have many data and memory limits. Maybe slower than the previous one because you will open the file many times...

Answer 4

To append using a matrix you could use:
exampleMatrix <- matrix(ncol = 2)
for(i in 1:5){
x = 3*i
if(i ==1){
exampleMatrix<- rbind(c(x,i))
}else{
exampleMatrix<- rbind(exampleMatrix,c(x,i))
}}
To append to a dataframe using a loop you could use the following:
exampleDF <- data.frame()
for(i in 1:5){
x = 3*i
exampleDF <- rbind(exampleDF,c(x,i))
}
write.csv(exampleDF, "C:\\path")

Answer 5

So when you want to store you values while using a loop, it's important to index. Below, I created some code where a(the iteration) and x(the value x * 3) are each stored inside a vector.
After the loop has finished, I combine the two vectors into one data frame with the cbind() function
a <- vector()
x <- vector()
for(i in 1:5){
a[i] = i
x[i] = 3*i
}
df <- as.data.frame(cbind(a, x))
There are other ways to do this without loops. Once you start raising the number of iterations, or doing nested loops, the processing time starts to get really high. Other options are in the apply package.
Hope this helped!