# Group By¶

The `group_by`

function allows you to group one or more columns and apply a function to the result. Specifically, the `group_by`

function performs the following actions on an H2O Frame:

- splits the data into groups based on some criteria
- applies a function to each group independently
- combines the results into an H2OFrame

The result is a new H2OFrame with columns equivalent to the number of groups created. The returned groups are sorted by the natural group-by column sort.

The `group_by`

function accepts the following parameters:

**Python and R**

- H2O Frame: This specifies the H2OFrame that you want the group by operation to be performed on.
`by`

: The`by`

option can take a list of columns if you want to group by more than one column to compute the summary.

**Python Only**

`na`

, which controls treatment of NA values during the calculation. It can be one of:`all`

(default): any NAs are used in the calculation as-is; which usually results in the final result being NA too.`ignore`

: NA entries are not included in calculations, but the total number of entries is taken as the total number of rows. For example,`mean([1, 2, 3, nan], na="ignore")`

will produce`1.5`

.`rm`

: entries are skipped during the calculations, reducing the total effective count of entries. For example,`mean([1, 2, 3, nan], na="rm")`

will produce`2`

.

**R Only**

`gb.control`

: In R, the`gb.control`

option specifies how to handle NA values in the dataset as well as how to name output columns. Note that to specify a list of column names in the`gb.control`

list, you must add the`col.names`

argument.`nrow`

: Specify the name of the generated column.`na.methods`

, which controls treatment of NA values during the calculation. It can be one of:

`all`

(default): any NAs are used in the calculation as-is; which usually results in the final result being NA too.`ignore`

: NA entries are not included in calculations, but the total number of entries is taken as the total number of rows. For example,`mean([1, 2, 3, nan], na="ignore")`

will produce`1.5`

.`rm`

: entries are skipped during the calculations, reducing the total effective count of entries. For example,`mean([1, 2, 3, nan], na="rm")`

will produce`2`

.

Note: If a list smaller than the number of columns groups is supplied, then the list will be padded by`ignore`

.

In addition to the above parameters, any number of the following aggregations can be chained together in the `group_by`

function:

`count`

: Count the number of rows in each group of a GroupBy object.`max`

: Calculate the maximum of each column specified in`col`

for each group of a GroupBy object.`mean`

: Calculate the mean of each column specified in`col`

for each group of a GroupBy object.`min`

: Calculate the minimum of each column specified in`col`

for each group of a GroupBy object.`mode`

: Calculate the mode of each column specified in`col`

for each group of a GroupBy object.`sd`

: Calculate the standard deviation of each column specified in`col`

for each group of a GroupBy object.`ss`

: Calculate the sum of squares of each column specified in`col`

for each group of a GroupBy object.`sum`

: Calculate the sum of each column specified in`col`

for each group of a GroupBy object.`var`

: Calculate the variance of each column specified in`col`

for each group of a GroupBy object.

If no arguments are given to the aggregation (e.g.,`max()`

in`grouped.sum(col="X1", na="all").mean(col="X5", na="all").max()`

), then it is assumed that the aggregation should apply to all columns except the GroupBy columns.

Note that once the aggregation operations are complete, calling the GroupBy object with a new set of aggregations will yield no effect. You must generate a new GroupBy object in order to apply a new aggregation on it. In addition, certain aggregations are only defined for numerical or categorical columns. An error will be thrown for calling aggregation on the wrong data types.

```
> library(h2o)
> h2o.init()
# Import the airlines data set and display a summary.
> airlinesURL <- "https://s3.amazonaws.com/h2o-airlines-unpacked/allyears2k.csv"
> airlines.hex <- h2o.importFile(path = airlinesURL, destination_frame = "airlines.hex")
> summary(airlines.hex)
# Find number of flights by airport
> originFlights <- h2o.group_by(data = airlines.hex, by = "Origin", nrow("Origin"), gb.control=list(na.methods="rm"))
> originFlights.R <- as.data.frame(originFlights)
> originFlights.R
Origin nrow_Origin
1 ABE 59
2 ABQ 876
3 ACY 31
...
# Find number of flights per month
> flightsByMonth <- h2o.group_by(data = airlines.hex, by = "Month", nrow("Month"), gb.control=list(na.methods="rm"))
> flightsByMonth.R <- as.data.frame(flightsByMonth)
> flightsByMonth.R
Month nrow_Month
1 1 41979
2 10 1999
# Find the number of flights in a given month based on the origin
> cols <- c("Origin","Month")
> flightsByOriginMonth <- h2o.group_by(data=airlines.hex, by=cols, nrow("Month"), gb.control=list(na.methods="rm"))
> flightsByOriginMonth.R <- as.data.frame(flightsByOriginMonth)
> flightsByOriginMonth.R
Origin Month nrow_NumberOfFlights
1 ABE 1 59
2 ABQ 1 846
3 ABQ 10 30
4 ACY 1 31
5 ALB 1 75
...
# Find months with the highest cancellation ratio
> which(colnames(airlines.hex)=="Cancelled")
[1] 22
> cancellationsByMonth <- h2o.group_by(data = airlines.hex, by = "Month", sum("Cancelled"), gb.control=list(na.methods="rm"))
> cancellation_rate <- cancellationsByMonth$sum_Cancelled/flightsByMonth$nrow_Month
> rates_table <- h2o.cbind(flightsByMonth$Month,cancellation_rate)
> rates_table.R <- as.data.frame(rates_table)
> rates_table.R
Month sum_Cancelled
1 1 0.025417471
2 10 0.009504752
# Use group_by with multiple columns. Summarize the destination, arrival delays, and departure delays for an origin
> cols <- c("Dest", "IsArrDelayed", "IsDepDelayed")
> originFlights <- h2o.group_by(data = airlines.hex[c("Origin",cols)], by = "Origin", sum(cols),gb.control = list(na.methods = "ignore", col.names = NULL))
# Note a warning because col.names null
> res <- h2o.cbind(lapply(cols, function(x){h2o.group_by(airlines.hex,by="Origin",sum(x))}))[,c(1,2,4,6)]
> res
Origin sum_Dest sum_IsArrDelayed sum_IsDepDelayed
1 ABE 5884 40 30
2 ABQ 84505 545 370
3 ACY 3131 9 7
4 ALB 3646 49 50
5 AMA 317 4 6
6 ANC 100 0 1
```

```
>>> import h2o
>>> h2o.init()
# Upload the airlines dataset
>>> air = h2o.import_file("https://s3.amazonaws.com/h2o-airlines-unpacked/allyears2k.csv")
>>> air.dim
[43978, 31]
# Find number of flights by airport
>>> originFlights = air.group_by("Origin")
>>> originFlights.count()
>>> originFlights.get_frame()
Origin nrow
-------- ------
ABE 59
ABQ 876
ACY 31
...
# Find number of flights per month based on the origin
>>> cols = ["Origin","Month"]
>>> flights_by_origin_month = air.group_by(by=cols).count(na ="all")
>>> flights_by_origin_month.get_frame()
Origin Month nrow
-------- ------- ------
ABE 1 59
ABQ 1 846
ABQ 10 30
...
# Find months with the highest cancellation ratio
>>> cancellation_by_month = air.group_by(by='Month').sum('Cancelled', na ="all")
>>> flights_by_month = air.group_by('Month').count(na ="all")
>>> canceled = cancellation_by_month.get_frame()['sum_Cancelled']
>>> flights = flights_by_month.get_frame()['nrow']
>>> month_count = flights_by_month.get_frame()['Month']
>>> ratio = canceled/flights
>>> month_count.cbind(ratio)
Month sum_Cancelled
------- ---------------
1 0.0254175
10 0.00950475
[2 rows x 2 columns]
# Use group_by with multiple columns. Summarize the destination, arrival delays, and departure delays for an origin
>>> cols_1 = ['Origin', 'Dest', 'IsArrDelayed', 'IsDepDelayed']
>>> cols_2 = ["Dest", "IsArrDelayed", "IsDepDelayed"]
>>> air[cols_1].group_by(by='Origin').sum(cols_2, na ="ignore").get_frame()
Origin sum_Dest sum_IsDepDelayed sum_IsArrDelayed
-------- ---------- ------------------ ------------------
ABE 5884 30 40
ABQ 84505 370 545
ACY 3131 7 9
ALB 3646 50 49
AMA 317 6 4
ANC 100 1 0
...
```