R: Find first occurence of value for each day

Question

I have a data.frame with datetimes and values (between 0 and 1) and i would like to find the first occurence of value=1 per day.

df <- read.table(header = TRUE, text = '
Datetime                   Value
"2016-12-01 23:45:00"      0
"2016-12-01 23:50:00"      1
"2016-12-02 00:05:00"      1
"2016-12-02 00:10:00"      0
"2016-12-03 04:10:00"      0
"2016-12-03 04:15:00"      0
"2016-12-04 12:10:00"      1
"2016-12-04 12:15:00"      1
')
df$Datetime <- as.POSIXct(df$Datetime, "%Y-%m-%d %H:%M:%S", tz="UTC")
View(df)

What i would like to have is:

2016-12-01 23:50:00      1
2016-12-02 00:05:00      1
2016-12-04 12:10:00      1

I tried to solve the problem with match() and aggregate() but had no luck so far. Furthermore i was able to solve the problem with a for loop but it is was a) very slow and b) probably not the way it is meant to be.

Answer 1

An alternative with dplyr:

library(dplyr)
df %>%
 #group
 group_by(as.Date(Datetime)) %>%
 #select only those where value equals 1
 filter(Value == 1) %>%
 #get only the first row
 slice(1) %>%
 #ungroup
 ungroup %>%
 #select columns
 select(Datetime, Value)

Ouput:

# A tibble: 3 x 2
             Datetime Value
               <time> <int>
1 2016-12-01 23:50:00     1
2 2016-12-02 00:05:00     1
3 2016-12-04 12:10:00     1

Or as per @Akrun 's comment:

df %>% 
  group_by(Date = as.Date(Datetime)) %>% 
  slice(which(Value==1)[1])