Practical Data Wrangling

Summarizing data by category

The summarize() function reduces the columns of a dataframe to a summary. The arguments to the summarize() function are expressions which create new variables a function of the rows of other columns. Here are a couple examples of possible arguments to the summarize() function:

• avg.column.1 = mean(column.1) 
• sum.column.2 = sum(column.2)

The group_by() function causes all of the subsequent operations to be performed by group. The arguments to the group_by() function are the names of columns that the result should be grouped by. When the group_by() function is followed by the summarize() function, the summary is applied to each unique group. 

The best way to understand the group_by() function is with a demonstration. ...