count number of unique elements in each columns with dplyr in sparklyr

问题

I'm trying to count the number of unique elements in each column in the spark dataset s.

However It seems that spark doesn't recognize tally() k<-collect(s%>%group_by(grouping_type)%>%summarise_each(funs(tally(distinct(.))))) Error: org.apache.spark.sql.AnalysisException: undefined function TALLY

It seems that spark doesn't recognize simple r functions either, like "unique" or "length". I can run the code on local data, but when I try to run the exact same code on spark table it doesn't work.

```

d<-data.frame(cbind(seq(1,10,1),rep(1,10)))
d$group<-rep(c("a","b"),each=5)
d%>%group_by(group)%>%summarise_each(funs(length(unique(.))))
A tibble: 2 × 3
  group    X1    X2
  <chr> <int> <int>
1     a     5     1
2     b     5     1
k<-collect(s%>%group_by(grouping_type)%>%summarise_each(funs(length(unique(.)))))
Error: org.apache.spark.sql.AnalysisException: undefined function UNIQUE;

```

回答1:

Remember when you are writing sparlyr you are really transpiling to spark-sql, so you may need to use spark-sql verbs from time to time. This is one of those times where spark-sql verbs like count and distinct come in handy.

library(sparkylr)

sc <- spark_connect()
iris_spk <- copy_to(sc, iris)

# for instance this does not work in R, but it does in sparklyr
iris_spk %>%
  summarise(Species = distinct(Species))
# or
iris_spk %>%
  summarise(Species = approx_count_distinct(Species))

# this does what you are looking for
iris_spk %>% 
    group_by(species) %>%
    summarise_all(funs(n_distinct))

# for larger data sets this is much faster
iris_spk %>% 
    group_by(species) %>%
    summarise_all(funs(approx_count_distinct))

回答2:

library(sparklyr)
library(dplyr)
#I am on Spark V. 2.1

#Building example input (local)
d <- data.frame(cbind(seq(1, 10, 1), rep(1,10)))
d$group <- rep(c("a","b"), each = 5)
d

#Spark tbl 
sdf <- sparklyr::sdf_copy_to(sc, d)

# The Answer
sdf %>% 
    group_by(group) %>% 
    summarise_all(funs(n_distinct)) %>%
    collect()

#Output
  group    X1    X2
  <chr> <dbl> <dbl>
1     b     5     1
2     a     5     1

NB: Given that we are using sparklyr I went for dplyr::n_distinct(). Minor: dplyr::summarise_each is deprecated. Thus, dplyr::summarise_all.

来源：https://stackoverflow.com/questions/49930004/count-number-of-unique-elements-in-each-columns-with-dplyr-in-sparklyr

标签

apache-spark

statistics

dplyr

sparklyr