write results sequentially in a loop in r - r

Question

I have a bunt of single files which need to apply a test. I need to find the way to write automatically results of each file into a file. Here is what I do:
library(ape)
stud_files <- list.files("path/dir/data",full.names = T)
for (f in stud_files) {
df <- read.table(f, header=TRUE, sep=";")
df_xts <- as.xts(df$cola, order.by = as.Date(df$colb,"%m/%d/%Y"))
pet <- testa(df_xts)
res <- data.frame(estimate = pet$estimate,
p.value=pet$p.value,
logi = pet$alternative)
write.dna(res,file = "res_testa.xls",format = "sequential")
}
This loop works well, except the last command which aim to write the results of each file consecutively, it saved only the last performance. And the results save as string, not a table as I define above (data.frame). Any idea in this case? Thanks in advance

Answer 1

Check help(write.dna).
write.dna(x, file, format = "interleaved", append = FALSE,
nbcol = 6, colsep = " ", colw = 10, indent = NULL,
blocksep = 1)
append a logical, if TRUE the data are appended to the file without
erasing the data possibly existing in the file, otherwise the file (if
it exists) is overwritten (FALSE the default).
Set append = TRUE and you should be all set.
As some of the comments point out, however, you are probably better off generating your table, and then writing it all at once to a file. Unless you have billions of files, you likely won't run out of memory.
Here is how I would approach this.
library(ape)
library(data.table)
stud_files <- list.files("path/dir/data",full.names = T)
sumfunc <- function(f) {
df <- read.table(f, header=TRUE, sep=";")
df_xts <- as.xts(df$cola, order.by = as.Date(df$colb,"%m/%d/%Y"))
pet <- testa(df_xts)
res <- data.table(estimate = pet$estimate,
p.value=pet$p.value,
logi = pet$alternative)
return(res)
}
lres <- lapply(stud_files, sumfunc)
dat <- rbindlist(lres)
write.table(dat,
file = "res_testa.csv",
sep = ",",
quote = FALSE,
row.names = FALSE)