Count number of rows per group and add result to original data frame

problem ：

Say I have a data.frame object: Let's say I have one data.frame object ：

df <- data.frame(name=c('black','black','black','red','red'),                 type=c('chair','chair','sofa','sofa','plate'),                 num=c(4,5,12,4,3))

Now I want to count the number of rows (observations) of for each combination of name and type . Now I want to calculate name and type Rows per combination （ Observe ）.This can be done like so: This can be done like this ：

table(df[ , c("name","type")])

or possibly also with plyr , (though I am not sure how). Or you can use plyr ,（ Although I'm not sure how ）.

However, how do I get the results incorporated into the original data frame? however , How to merge the results into the original data frame ？So that the results will look like this: The result will be as follows ：

df#    name  type num count# 1 black chair   4     2# 2 black chair   5     2# 3 black  sofa  12     1# 4   red  sofa   4     1# 5   red plate   3     1

where count now stores the results from the aggregation. Now? count Store aggregated results .

A solution with plyr could be interesting to learn as well, though I would like to see how this is done with base R. Use plyr Your solution can also be interesting , Although I want to see how this is used on the basis R Accomplished .

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Solution ：

Reference resources ： https://stackoom.com/en/question/VGEy