Hi,
You could use this, but it would be slow.
"""
# I know functions like e.g. 'apply' but don't know how to use them for
the solution of the following problem:
"""
lst1<-split(seq_along(b),rep(unlist(a),unlist(a)))
res<-cbind(r,sapply(lst1,function(i)
{x1<-b[,i];sapply(split(as.matrix(r),row(r)),function(x){ x2<-x1[unlist(x),];
sum(abs(apply(x2,2,diff)))/2}) }))
colnames(res)[3:4]<-colnames(a)[match(colnames(res), unlist(a),nomatch=0)]
res
# V1 V2 group2 group1
#1 1 2 0.6666667 1.0000000
#2 1 3 0.5000000 0.6666667
#3 2 3 0.5000000 0.5000000
all.equal(res[,c(1:2,4,3)],result,check.attributes=FALSE)
#[1] TRUE
I guess Jean's solution would be faster and elegant than using ?apply() based
solution.
A.K.
----- Original Message -----
From: Kulupp <kul...@online.de>
To: r-help@r-project.org
Cc:
Sent: Monday, August 5, 2013 2:39 AM
Subject: [R] Avoiding slow for-loops (once again)
# Dear R-experts,
# (Once again) I want to avoid the usage of for-loops but unfortunately
I don't know how.
# I know functions like e.g. 'apply' but don't know how to use them for
the solution of the following problem:
# I have a data frame 'a' giving the number of columns in data frame 'b'
that belong to one group
a <- data.frame(group1=5, group2=4)
b <- data.frame(col1=c(0,0,0), col2=c(0,1,0.5), col3=c(0,0,0),
col4=c(1/3,0,0.5), col5=c(2/3,0,0),
col6=c(0,0,0), col7=c(1,1/3,0), col8=c(0,2/3,0),
col9=c(0,0,0))
# ... thus columns 1-5 in 'b' belong to group 1 and columns 6-9 in 'b'
belong to group 2
# then I created a data frame giving all possible row combinations of 'b'
r <- as.data.frame(t(combn(nrow(b), 2)))
# .. so e.g. the second row of 'r' tells me that I have to perform an
equation with the values of the
# first and third row of table 'b'. The equation has to be calculated
for each group seperately.
# e.g. within group 2 (columns 6-9 in 'b') I have to calculate e.g. for
rows 1 and 3 in 'b'
# (abs(b[row1,col6] - b[row3, col6]) + abs(b[row1, col7] - b[row3,
col7]) + .... + abs(b[row1, col9] - b[row3, col9]))/2
# the resulting data frame shall look as follows:
result <- cbind(r, data.frame(group1=c(1,2/3,0.5), group2=c(2/3,0.5,0.5)))
# The original tables are much larger and I don't know how to solve this
problem w/o a lot of very slow for-loops.
# Is there any possible solution w/o using 'for'-loops?
# I'd be happy for any suggestions
# Thank you very much in anticipation
# Best regards
# Thomas
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
