Append data frames together in a for loop

Don't do it inside the loop. Make a list, then combine them outside the loop.

n = 5
datalist = list()
# or pre-allocate for slightly more efficiency
datalist = vector("list", length = n)

for (i in 1:n) {
    # ... make some data
    dat <- data.frame(x = rnorm(10), y = runif(10))
    dat$i <- i  # maybe you want to keep track of which iteration produced it?
    datalist[[i]] <- dat # add it to your list
}

big_data = do.call(rbind, datalist)
# or big_data <- dplyr::bind_rows(datalist)
# or big_data <- data.table::rbindlist(datalist)

This is a much more R-like way to do things. It can also be substantially faster, especially if you use dplyr::bind_rows or data.table::rbindlist for the final combining of data frames.

Again maRtin is correct but for this to work you have start with a dataframe that already has at least one column

model <- #some processing
df <- data.frame(col1=model)

for (i in 2:17)
{
     model <- # some processing
     nextcol <-  data.frame(model)
     colnames(nextcol) <- c(paste("col", i, sep="")) # rename the comlum
     df <- cbind(df, nextcol)
}

You should try this:

df_total = data.frame()
for (i in 1:7){
    # vector output
    model <- #some processing

    # add vector to a dataframe
    df <- data.frame(model)
    df_total <- rbind(df_total,df)
}