Changing date format (NA error) - r

Question

So I have this data file which includes dates and other values. I've imported my data using the following code:
df <- read.csv(file.choose(), header=T, stringsAsFactors=F)
This is so that all the values in the data frame are in character. This makes the next step easier for me.
The data.frame (df) includes:
date x
20020102 1
20020102 2
The date changes every few thousand rows.
I want to change the date format so that it would be yyyy-mm-dd.
I've tried using the code:
df$date <- as.Date(df$date, format="%Y-%m-%d")
and have also used
df$date <- strptime(df$date, format="%Y-%m-%d")
but have always gotten NA values in the date column.
I'm a beginner at R so it would be very helpful if the solution could be simple or can be explained clearly.
Thanks so much!

Answer 1

You can use the correct format
df$date <- as.Date(df$date, format='%Y%m%d')
It is not clear whether you have numeric or non-numeric 'date' column. If it is 'numeric', convert to 'character' first
df$date <- as.Date(as.character(df$date), format='%Y%m%d')
But, strptime would work even if the column is numeric.
Or using library(lubridate)
library(lubridate)
ymd(df$date)

Answer 2

The problem is that your colunm "date" is not of class 'Date', it is a 'numeric' vector, thus the command as.Date returns NA`s.
You can check if the class of the colunm date is correct with this command:
class(df$date)
Following the advise from #akrun you should transform the date colunm into a 'character' vector, then you can format the style the way you want:
### your data example:
df <- data.frame(date = c(20020102, 20020102),
x = c(1,2))
class(df$date)
#> [1] "numeric"
#convert the colunm date to character
df$date <- as.character(df$date)
# Then, convert to the desired date format:
df$date <- as.Date(df$date, format='%Y%m%d')
# check the results:
df
#> date x
#> 1 2002-01-02 1
#> 2 2002-01-02 2
class(df$date)
#> [1] "Date"