CODEWITHSUNDEEP
rdataframe
James
James

Reputation: 1457

Calculate frequencies

I have a data frame like this:

Expt    Replicate
A       1
A       2
A       3
B       1
B       2
B       3
C       1
C       2
C       3
C       4

I want to return the number of replicates for each experiment. Like this:

Expt    #Reps
A       3
B       3
C       4

This has got to be super simple, but I have tried some things like

ddply(df, Expt, .fun=max(Replicate))

with no luck. Please help.

Upvotes: 1

Views: 3298

Answers (5)

mnel
mnel

Reputation: 115392

A data.table solution (to counting the number of replicates)

library(data.table)
df <- data.table(df)
df[,list(max = .N),by='Expt']

If you want the maximum a column then use

df[,list(max = max(Replicate)),by='Expt']

Upvotes: 4

Jilber Urbina
Jilber Urbina

Reputation: 61154

Another way to achieve your goal is

> df <- data.frame(Expt = rep(c('A', 'B', 'C'), c(3,3,4)), Replicate = append(rep(1:3, 3), 4, 10))
> tapply(df$Replicate, df$Expt, max)
A B C 
3 3 4

But if you really want to use ddply, you can try:

library(plyr)
ddply(df,.(Expt),numcolwise(max))

Upvotes: 2

James
James

Reputation: 1457

Thanks everyone. I also found this works:

aggregate(as.numeric(df$Replicate), list(exptCount=df$expt),max)

Upvotes: 3

Subs
Subs

Reputation: 529

another simple way:

 summary(df[,1]) #where df is your data frame and you want the 1st column counts ("Expt")

Note: the 1st column is a factor and this applies to any columns which are factor

Upvotes: -1

ffriend
ffriend

Reputation: 28492

table is your friend. You even need not second column, table will count items in first column for you: 

> x <- c('A', 'A', 'A', 'B', 'B', 'B', 'C', 'C', 'C', 'C')
> x
 [1] "A" "A" "A" "B" "B" "B" "C" "C" "C" "C"
> table(x)
x
A B C 
3 3 4

Upvotes: 4

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