Table of Contents

The dcast function from the data.table package in R is a convenient and powerful tool for reshaping data. It can be used to transform a data frame from a wide to a long format, or from a long to a wide format, while preserving the values of the data. It is also a useful tool for aggregating data, as it can be used to calculate summary statistics such as means, medians, and counts. It is very flexible and can be used to reshape data for many different applications.

You can use the **dcast** function from the **data.table** package in R to reshape a data frame from a to a wide format.

This function is particularly useful when you want to summarize specific variables in a data frame, grouped by other variables.

The following examples show how to use the **dcast** function in practice with the following data frame in R:

library(data.table) #create data frame df <- data.frame(team=c('A', 'A', 'A', 'A', 'B', 'B', 'B', 'B'), position=c('G', 'G', 'F', 'F', 'G', 'G', 'F', 'F'), points=c(18, 13, 10, 12, 16, 25, 24, 31), assists=c(9, 8, 8, 5, 12, 15, 10, 7)) #convert data frame to data table dt <- setDT(df) #view data table dt team position points assists 1: A G 18 9 2: A G 13 8 3: A F 10 8 4: A F 12 5 5: B G 16 12 6: B G 25 15 7: B F 24 10 8: B F 31 7

**Example 1: Calculate Metric for One Variable, Grouped by Other Variables**

The following code shows how to use the **dcast** function to calculate the mean **points** value, grouped by the **team** and **position** variables:

library(data.table) #calculate mean points value by team and position dt_new <- dcast(dt, team + position ~ ., fun.aggregate = mean, value.var = 'points') #view results dt_new team position . 1: A F 11.0 2: A G 15.5 3: B F 27.5 4: B G 20.5

**Example 2: ****Calculate Multiple Metrics for One Variable, Grouped by Other Variables**

The following code shows how to use the **dcast** function to calculate the mean **points** value and the max **points** value, grouped by the **team** and **position** variables:

library(data.table) #calculate mean and max points values by team and position dt_new <- dcast(dt, team + position ~ ., fun.aggregate = list(mean, max), value.var = 'points') #view results dt_new team position points_mean points_max 1: A F 11.0 12 2: A G 15.5 18 3: B F 27.5 31 4: B G 20.5 25

**Example 3: ****Calculate Metric for Multiple Variables, Grouped by Other Variables**

The following code shows how to use the **dcast** function to calculate the mean **points** value and mean **assists** value, grouped by the **team** and **position** variables:

library(data.table) #calculate mean and max points values by team and position dt_new <- dcast(dt, team + position ~ ., fun.aggregate = mean, value.var = c('points', 'assists')) #view results dt_new team position points assists 1: A F 11.0 6.5 2: A G 15.5 8.5 3: B F 27.5 8.5 4: B G 20.5 13.5

The following tutorials provide additional information about data tables: