Use group_ Dplyr issues when using group_ by(multiple variables)

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problem ：

I want to start using dplyr in place of ddply but I can't get a handle on how it works (I've read the documentation). I want to start using dplyr Instead of ddply But I can't understand how it works （ I have read the document ）.

For example, why when I try to mutate() something does the "group_by" function not work as it's supposed to? for example , Why when I try mutate() when ,“group_by” Function does not work properly ？

Looking at mtcars: see mtcars：

library(car) The library （ automobile ）

Say I make a data.frame which is a summary of mtcars, grouped by "cyl" and "gear": Suppose I make a data.frame, It is mtcars A summary of the , Press “cyl” and “gear” grouping ：

df1 <- mtcars %.%            group_by(cyl, gear) %.%            summarise(                newvar = sum(wt)            )

Then say I want to further summarise this dataframe. Then say I want to summarize this data frame further .With ddply, it'd be straightforward, but when I try to do with with dplyr, it's not actually "grouping by": Use ddply, It would be simple , But when I try to use dplyr when , It's not really “ Group by ”：

df2 <- df1 %.%            group_by(cyl) %.%            mutate(                newvar2 = newvar + 5            )

Still yields an ungrouped output: Still produce ungrouped output ：

  cyl gear newvar newvar21   6    3  6.675  11.6752   4    4 19.025  24.0253   6    4 12.375  17.3754   6    5  2.770   7.7705   4    3  2.465   7.4656   8    3 49.249  54.2497   4    5  3.653   8.6538   8    5  6.740  11.740

Am I doing something wrong with the syntax? Did I make a mistake in grammar ？

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Edit: edit ：

If I were to do this with plyr and ddply: If I want to use plyr and ddply Do this ：

df1 <- ddply(mtcars, .(cyl, gear), summarise, newvar = sum(wt))

and then to get the second df: Then get the second df：

df2 <- ddply(df1, .(cyl), summarise, newvar2 = sum(newvar) + 5)

But that same approach, with sum(newvar) + 5 in the summarise() function doesn't work with dplyr... But the same way , stay summarise() Use in a function sum(newvar) + 5 Do not apply to dplyr ...

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Solution ：

Reference resources ： https://stackoom.com/en/question/1Sr0h