R语言学习笔记

如何对dataframe做group by ，需要使用data.frame table等等

http://stackoverflow.com/questions/25293045/count-number-of-rows-in-a-data-frame-in-r-based-on-group

mydf
#    ID MONTH.YEAR VALUE
# 1 110  JAN. 2012  1000
# 2 111  JAN. 2012  2000
# 3 121  FEB. 2012  3000
# 4 131  FEB. 2012  4000
# 5 141  MAR. 2012  5000

Here's the calculation of the number of rows per group, in two output display formats:

table(mydf$MONTH.YEAR)
# 
# FEB. 2012 JAN. 2012 MAR. 2012 
#         2         2         1

data.frame(table(mydf$MONTH.YEAR))
#        Var1 Freq
# 1 FEB. 2012    2
# 2 JAN. 2012    2
# 3 MAR. 2012    1


第一列升序，第二列降序
> a[order(a[,1],-a[,2]),]
     [,1] [,2] [,3]
[1,]    2    7   14
[2,]    2    6   13
[3,]    3    8   10
[4,]    4    9   11
[5,]    5    6   12

reshape做融合
dfm <- melt(mtcars,measure.vars=c("mpg","hp","wt"),id.vars=c("am","cyl"))
cast(dfm, am + cyl + variable ~., func)