Vectorising for loops to save time in R

main.R

needs(tidyverse, magrittr, jsonlite, scales)

loopTimer <- function(rows){
  
  col1 <- runif (rows, 0, 2)
  col2 <- rnorm (rows, 0, 2)
  col3 <- rpois (rows, 3)
  col4 <- rchisq (rows, 2)
  df <- data.frame (col1, col2, col3, col4)
  
  r <- system.time({
    for (i in 1:nrow(df)) { # for every row
      if ((df[i, 'col1'] + df[i, 'col2'] + df[i, 'col3'] + df[i, 'col4']) > 4) { # check if > 4
        df[i, 5] <- "greater_than_4" # assign 5th column
      } else {
        df[i, 5] <- "lesser_than_4" # assign 5th column
      }
    }
  })
  
  output <- character (nrow(df)) # initialize output vector
  v <- system.time({
    for (i in 1:nrow(df)) {
      if ((df[i, 'col1'] + df[i, 'col2'] + df[i, 'col3'] + df[i, 'col4']) > 4) {
        output[i] <- "greater_than_4"
      } else {
        output[i] <- "lesser_than_4"
      }
    }
    df$output})
  
  data.frame(type=names(r),Appending=as.numeric(r),Vectorised=as.numeric(v))
}

rowNums <- c(1:10)^5
timerList <- vector("list", length(rowNums))
for(i in 1:length(rowNums)){
  n <- rowNums[i]
  timerList[[i]] <- loopTimer(n) %>% mutate(nrow = n)
}
timerList %>% bind_rows %>%
  filter(type == "elapsed") %>%
  gather(method, seconds, 2:3) %>%
  ggplot(aes(nrow, seconds, col=method, label = ifelse(nrow == max(nrow), paste0(method,' '), ''))) +
  theme_minimal(base_family = "Avenir") +
  theme(
    legend.position = "none",
    axis.line = element_line(color='black'),
    panel.grid.minor = element_blank()
    ) +
  geom_line() +
  geom_text(hjust=1, vjust=0) +
  scale_x_continuous(label = comma) +
  scale_color_manual(values = c("Appending"="#0083eb", "Vectorised"="#ff2b4f")) +
  labs(
    title = "How to save time when processing data in for loops",
    x = "Rows in data frame",
    y = "Seconds elapsed"
  )