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R Data Analysis Cookbook - Second Edition by Kuntal Ganguly

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How to do it...

To use the split-apply-combine strategy for data analysis with plyr, follow these steps:

  1. Calculate mean mpg for each cylinder type (two versions):
> ddply(auto, "cylinders", function(df) mean(df$mpg)) 
> ddply(auto, ~ cylinders, function(df) mean(df$mpg)) 
cylinders       V1 
1      3cyl 20.55000 
2      4cyl 29.28676 
3      5cyl 27.36667 
4      6cyl 19.98571 
5      8cyl 14.96311 
  1. Calculate the mean, minimum, and maximum mpg for each cylinder type and model year:
> ddply(auto, c("cylinders","model_year"), function(df) c(mean=mean(df$mpg), min=min(df$mpg), max=max(df$mpg))) > ddply(auto, ~ cylinders + model_year, function(df) c(mean=mean(df$mpg), min=min(df$mpg), max=max(df$mpg))) cylinders model_year mean min max 1 3cyl 72 19.00000 19.0 19.0 2 3cyl 73 18.00000 ...

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