[R] create an empty data frame and then fill in it (and then evaluate the mean of semi-hourly data for each day)

[Stefano Sofia]

Thank you for your answer. Very clear.
(I don't like the second solution either.)
Let me then ask a final question.
From an initial data frame with semi-hourly data (df_snow, with two columns, data_POSIX of type "POSIXct" "POSIXt" and snow of type "numeric"), I need to evaluate the mean of for each day.

data_POSIX snow
2004-11-01 00:00:00 50
2004-11-01 00:30:00 55
2004-11-01 01:00:00 60
...

I first created a new column of type "Date"
df_snow$day <- as.Date(df_snow$data_POSIX,"%Y-%m-%d")

then I created a new data frame called df_snow_day to store the mean of data grouped by day:
list_days <- unique(df_snow$day)
df_snow_day <- data.frame(day=list_days)

Finally I applied lapply in this way:
df_snow_day$snow <- lapply(df_snow_day$day, function(x) round(mean(df_snow$snow[df_snow$day == x], na.rm=T)))

This does not work. I do not understand why the class of df_snow_day$snow is of type list either:

       day snow
NA    <NA>       NULL
NA.1  <NA>       NULL
NA.2  <NA>       NULL

Where is my mistake?

Thank you for all your help
Stefano


_____________________________________________

Da: Duncan Murdoch [murdoch.duncan at gmail.com]
Inviato: giovedì 9 giugno 2016 12.36
A: Stefano Sofia; r-help at r-project.org
Oggetto: Re: [R] create an empty data frame and then fill in it

On 09/06/2016 6:22 AM, Stefano Sofia wrote:
> Dear R list users,
> sorry for this simple question, but I already spent many efforts to solve it.
>
> I create an empty data frame called df_year like
>
> df_year <- data.frame(day=as.Date(character()), hs_MteBove=integer(), hs_MtePrata=integer(), hs_Pintura=integer(), hs_Pizzo=integer(), hs_Sassotetto=integer(), hs_Sibilla=integer(), stringsAsFactors=FALSE)
>
> and then I start to fill in it with
>
> df_year$day <- seq(as.Date("2004-11-01-00-00","%Y-%m-%d"), as.Date("2005-05-01-00-00","%Y-%m-%d"), by="day")
>
> but I get the following error:
> "replacement has 182 rows, data has 0"
>
> Where is my silly mistake?

Your dataframe has 0 rows, so you can't put a 182 row vector into the
first column.

Unlike vectors, dataframes won't grow if you make assignments beyond the
end of the rows.

There are at least a couple of solutions:

1.  Don't create columns until you have data ready for them.

You can wait to create the dataframe until your "day" column is ready:

df_year <- data.frame(day = seq(...))

As you compute other columns of the same length, you can add them, e.g.

df_year$hs_MteBove <- ...

2.  Create your columns with the right length from the beginning:

df_year <- data.frame(day = rep(as.Date(NA), 182), ...)

I don't like this solution as much.

Duncan Murdoch


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