Replace For Loop to fill column depending on other column value - r

Question

I have a two-column dataframe (HOME & AWAY) called 'gamelist' with sports games. The HOME column also includes some dates with the corresponding games listed below.
HOME AWAY
15 Oct 2019 Pre-season
Phoenix Suns Denver Nuggets
Utah Jazz Sacramento Kings
Dallas Mavericks Oklahoma City Thunder
Memphis Grizzlies Charlotte Hornets
14 Oct 2019 Pre-season
Miami Heat Atlanta Hawks
13 Oct 2019 Pre-season
Orlando Magic Philadelphia 76ers
Toronto Raptors Chicago Bulls
Washington Wizards Milwaukee Bucks
I want to create a new column with the dates for each game. Coming from a excel vba approach, I've used a for loop which is giving the result intented but I was wondering if there was a more efficient approach in R, and I'm sure there is.
This is the code I've used:
gamelist<-add_column(gamelist,SDATE="",.before = 1)
for(i in 1:nrow(gamelist)){
if(str_count(gamelist[[i,3]],"\\d")==6){
gamelist[i,2]<-gamelist[i,3]
}else{
gamelist[i,2]<-gamelist[i-1,2]
}
}
Which gives me this as intended
SDATE HOME AWAY
15 Oct 2019 15 Oct 2019 Pre-season
15 Oct 2019 Phoenix Suns Denver Nuggets
15 Oct 2019 Utah Jazz Sacramento Kings
15 Oct 2019 Dallas Mavericks Oklahoma City Thunder
15 Oct 2019 Memphis Grizzlies Charlotte Hornets
14 Oct 2019 14 Oct 2019 Pre-season
14 Oct 2019 Miami Heat Atlanta Hawks
13 Oct 2019 13 Oct 2019 Pre-season
13 Oct 2019 Orlando Magic Philadelphia 76ers
13 Oct 2019 Toronto Raptors Chicago Bulls
13 Oct 2019 Washington Wizards Milwaukee Bucks
My apologies for the dataframe formatting, couldn't figure out how to reproduce one properly here.
Thanks for your help

Answer 1

We could use str_extract to get only the 'dates' so that if there is no match it returns NA, then we use fill to fill the NA elements with the previous non-NA values
library(dplyr)
library(tidyr)
library(stringr)
gamelist %>%
mutate(SDATE = str_extract(HOME, "^\\d+ [A-Za-z]+ \\d{4}")) %>%
fill(SDATE)
# HOME AWAY SDATE
#1 15 Oct 2019 Pre-season 15 Oct 2019
#2 Phoenix Suns Denver Nuggets 15 Oct 2019
#3 Utah Jazz Sacramento Kings 15 Oct 2019
#4 Dallas Mavericks Oklahoma City Thunder 15 Oct 2019
#5 Memphis Grizzlies Charlotte Hornets 15 Oct 2019
#6 14 Oct 2019 Pre-season 14 Oct 2019
#7 Miami Heat Atlanta Hawks 14 Oct 2019
#8 13 Oct 2019 Pre-season 13 Oct 2019
#9 Orlando Magic Philadelphia 76ers 13 Oct 2019
#10 Toronto Raptors Chicago Bulls 13 Oct 2019
#11 Washington Wizards Milwaukee Bucks 13 Oct 2019
If we need the SDATE column first, we can use select
gamelist %>%
mutate(SDATE = str_extract(HOME, "^\\d+ [A-Za-z]+ \\d{4}")) %>%
fill(SDATE) %>%
select(SDATE, everything())
Or use add_column from tibble with either .after or .before
library(tibble)
gamelist %>%
add_column(SDATE = str_extract(.$HOME, "^\\d+ [A-Za-z]+ \\d{4}"),
.before = 1 ) %>%
fill(SDATE)
data
gamelist <- structure(list(HOME = c("15 Oct 2019", "Phoenix Suns", "Utah Jazz",
"Dallas Mavericks", "Memphis Grizzlies", "14 Oct 2019", "Miami Heat",
"13 Oct 2019", "Orlando Magic", "Toronto Raptors", "Washington Wizards"
), AWAY = c("Pre-season", "Denver Nuggets", "Sacramento Kings",
"Oklahoma City Thunder", "Charlotte Hornets", "Pre-season", "Atlanta Hawks",
"Pre-season", "Philadelphia 76ers", "Chicago Bulls", "Milwaukee Bucks"
)), class = "data.frame", row.names = c(NA, -11L))

Answer 2

If the date is always in the HOME column when the AWAY column is "Pre-season" (or some other predictable condition), then you could do something like:
# data
gamelist <- data.frame(
stringsAsFactors = FALSE,
HOME = c("15-Oct-19","Phoenix Suns",
"Utah Jazz","Dallas Mavericks","Memphis Grizzlies",
"14-Oct-19","Miami Heat","13-Oct-19","Orlando Magic",
"Toronto Raptors","Washington Wizards"),
AWAY = c("Pre-season","Denver Nuggets",
"Sacramento Kings","Oklahoma City Thunder",
"Charlotte Hornets","Pre-season","Atlanta Hawks","Pre-season",
"Philadelphia 76ers","Chicago Bulls","Milwaukee Bucks")
)
# create blank column to fill in
gamelist$date <- NA
# fill cases where there's a date
gamelist$date[gamelist$AWAY=="Pre-season"] <- gamelist$HOME[gamelist$AWAY=="Pre-season"]
# user zoo::na.locf() to fill in missing values
gamelist$date <- zoo::na.locf(gamelist$date)