HI,
Sorry, my previous solution doesn't work.
This should work for your dataset:
set.seed(1851)
x<-
data.frame(item=sample(letters[1:5],20,replace=TRUE),a=sample(1:15,20,replace=TRUE),b=sample(20:30,20,replace=TRUE),stringsAsFactors=F)
y<- data.frame(item="f",a=3,b=10,stringsAsFactors=F)
x[x$a%in%which.min(x[x$a<y$a,]$a),]<- y #if there are multiple minimum values
set.seed(1241)
x1<-
data.frame(item=sample(letters[1:10],1e4,replace=TRUE),a=sample(1:30,1e4,replace=TRUE),b=sample(1:100,1e4,replace=TRUE),stringsAsFactors=F)
y1<- data.frame(item="f",a=3,b=10,stringsAsFactors=F)
length(x1$a[x1$a==1])
#[1] 330
system.time({x1[x1$a%in%which.min(x1[x1$a<y1$a,]$a),]<- y1})
# user system elapsed
# 0.000 0.000 0.001
length(x1$a[x1$a==1])
#[1] 0
#For some reason, it is not working when the multiple number of minimum values
> some value
set.seed(1241)
x1<-
data.frame(item=sample(letters[1:10],1e5,replace=TRUE),a=sample(1:30,1e5,replace=TRUE),b=sample(1:100,1e5,replace=TRUE),stringsAsFactors=F)
y1<- data.frame(item="f",a=3,b=10,stringsAsFactors=F)
length(x1$a[x1$a==1])
#[1] 3404
x1[x1$a%in%which.min(x1[x1$a<y1$a,]$a),]<- y1
length(x1$a[x1$a==1])
#[1] 3404 #not getting replaced
#However, if I try:
set.seed(1241)
x1<-
data.frame(item=sample(letters[1:10],1e6,replace=TRUE),a=sample(1:5000,1e6,replace=TRUE),b=sample(1:100,1e6,replace=TRUE),stringsAsFactors=F)
y1<- data.frame(item="f",a=3,b=10,stringsAsFactors=F)
length(x1$a[x1$a==1])
#[1] 208
system.time(x1[x1$a%in%which.min(x1[x1$a<y1$a,]$a),]<- y1)
#user system elapsed
# 0.124 0.016 0.138
length(x1$a[x1$a==1])
#[1] 0
#Tried Jessica's solution:
set.seed(1851)
x<-
data.frame(item=sample(letters[1:5],20,replace=TRUE),a=sample(1:15,20,replace=TRUE),b=sample(20:30,20,replace=TRUE),stringsAsFactors=F)
y<- data.frame(item="f",a=3,b=10,stringsAsFactors=F)
x[intersect(which(x$a < y$a),which.min(x$a)),] <- y
x
# item a b
#1 a 8 25
#2 a 10 26
#3 f 3 10 #replaced
#4 e 15 26
#5 b 13 20
#6 a 5 23
#7 d 4 29
#8 e 2 24
#9 c 7 30
#10 e 14 24
#11 d 2 20
#12 e 10 21
#13 c 13 27
#14 d 12 23
#15 b 11 26
#16 e 5 22
#17 c 1 26 #it is not replaced
#18 a 8 21
#19 e 10 26
#20 c 2 22
A.K.
----- Original Message -----
From: Dimitri Liakhovitski <dimitri.liakhovit...@gmail.com>
To: r-help <r-help@r-project.org>
Cc:
Sent: Tuesday, January 29, 2013 4:11 PM
Subject: [R] Fastest way to compare a single value with all values in one
column of a data frame
Hello!
I have a large data frame x:
x<-data.frame(item=letters[1:5],a=1:5,b=11:15) # in actuality, x has 1000
rows
x$item<-as.character(x$item)
I also have a small data frame y with just 1 row:
y<-data.frame(item="f",a=3,b=10)
y$item<-as.character(y$item)
I have to decide if y$a is larger than the smallest of all the values in
x$a. If it is, I want y to replace the whole row in x that has the lowest
value in column a.
This is how I'd do it.
if(y$a>min(x$a)){
whichmin<-which(x$a==min(x$a))
x[whichmin,]<-y[1,]
}
I am wondering if there is a faster way of doing it. What would be the
fastest possible way? I'd have to do it, unfortunately, many-many times.
Thank you very much!
--
Dimitri Liakhovitski
gfk.com <http://marketfusionanalytics.com/>
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