Thank you for the reproducible example, but it is not clear what cross product you want. Jim's solution gives you the cross product of the 2-column matrix with itself. If you want the cross product between the columns you need something else. The aggregate function will not work since it will treat the columns separately:> A <- as.matrix(myData[myData$Z=="A", 1:2]) > AX Y 1 1 4 2 2 3> crossprod(A) # Same as t(A) %*% AX Y X 5 10 Y 10 25> crossprod(A[, 1], A[, 2]) # Same as t(A[, 1] %*% A[, 2][,1] [1,] 10> > # For all the groups > lapply(split(myData, myData$Z), function(x) crossprod(as.matrix(x[, 1:2])))$A X Y X 5 10 Y 10 25 $B X Y X 25 10 Y 10 5> lapply(split(myData, myData$Z), function(x) crossprod(x[, 1], x[, 2]))$A [,1] [1,] 10 $B [,1] [1,] 10 ------------------------------------- David L Carlson Department of Anthropology Texas A&M University College Station, TX 77840-4352 -----Original Message----- From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Jim Lemon Sent: Tuesday, August 23, 2016 6:02 PM To: Gang Chen; r-help mailing list Subject: Re: [R] aggregate Hi Gang Chen, If I have the right idea: for(zval in levels(myData$Z)) crossprod(as.matrix(myData[myData$Z==zval,c("X","Y")])) Jim On Wed, Aug 24, 2016 at 8:03 AM, Gang Chen <gangchen6 at gmail.com> wrote:> This is a simple question: With a dataframe like the following > > myData <- data.frame(X=c(1, 2, 3, 4), Y=c(4, 3, 2, 1), Z=c('A', 'A', 'B', 'B')) > > how can I get the cross product between X and Y for each level of > factor Z? My difficulty is that I don't know how to deal with the fact > that crossprod() acts on two variables in this case. > > ______________________________________________ > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see > stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code.______________________________________________ R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Thank you all for the suggestions! Yes, I'm looking for the cross product between the two columns of X and Y. A follow-up question: what is a nice way to merge the output of lapply(split(myData, myData$Z), function(x) crossprod(x[, 1], x[, 2])) with the column Z in myData so that I would get a new dataframe as the following (the 2nd column is the cross product between X and Y)? Z CP A 10 B 10 Is the following legitimate? data.frame(Z=levels(myData$Z), CP= unlist(lapply(split(myData, myData$Z), function(x) crossprod(x[, 1], x[, 2])))) On Wed, Aug 24, 2016 at 10:37 AM, David L Carlson <dcarlson at tamu.edu> wrote:> Thank you for the reproducible example, but it is not clear what cross product you want. Jim's solution gives you the cross product of the 2-column matrix with itself. If you want the cross product between the columns you need something else. The aggregate function will not work since it will treat the columns separately: > >> A <- as.matrix(myData[myData$Z=="A", 1:2]) >> A > X Y > 1 1 4 > 2 2 3 >> crossprod(A) # Same as t(A) %*% A > X Y > X 5 10 > Y 10 25 >> crossprod(A[, 1], A[, 2]) # Same as t(A[, 1] %*% A[, 2] > [,1] > [1,] 10 >> >> # For all the groups >> lapply(split(myData, myData$Z), function(x) crossprod(as.matrix(x[, 1:2]))) > $A > X Y > X 5 10 > Y 10 25 > > $B > X Y > X 25 10 > Y 10 5 > >> lapply(split(myData, myData$Z), function(x) crossprod(x[, 1], x[, 2])) > $A > [,1] > [1,] 10 > > $B > [,1] > [1,] 10 > > ------------------------------------- > David L Carlson > Department of Anthropology > Texas A&M University > College Station, TX 77840-4352 > > > -----Original Message----- > From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Jim Lemon > Sent: Tuesday, August 23, 2016 6:02 PM > To: Gang Chen; r-help mailing list > Subject: Re: [R] aggregate > > Hi Gang Chen, > If I have the right idea: > > for(zval in levels(myData$Z)) > crossprod(as.matrix(myData[myData$Z==zval,c("X","Y")])) > > Jim > > On Wed, Aug 24, 2016 at 8:03 AM, Gang Chen <gangchen6 at gmail.com> wrote: >> This is a simple question: With a dataframe like the following >> >> myData <- data.frame(X=c(1, 2, 3, 4), Y=c(4, 3, 2, 1), Z=c('A', 'A', 'B', 'B')) >> >> how can I get the cross product between X and Y for each level of >> factor Z? My difficulty is that I don't know how to deal with the fact >> that crossprod() acts on two variables in this case. >> >> ______________________________________________ >> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see >> stat.ethz.ch/mailman/listinfo/r-help >> PLEASE do read the posting guide R-project.org/posting-guide.html >> and provide commented, minimal, self-contained, reproducible code. > > ______________________________________________ > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see > stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code.
Your is fine, but it will be a little simpler if you use sapply() instead:> data.frame(Z=levels(myData$Z), CP=sapply(split(myData, myData$Z),+ function(x) crossprod(x[, 1], x[, 2]))) Z CP A A 10 B B 10 David C -----Original Message----- From: Gang Chen [mailto:gangchen6 at gmail.com] Sent: Wednesday, August 24, 2016 10:17 AM To: David L Carlson Cc: Jim Lemon; r-help mailing list Subject: Re: [R] aggregate Thank you all for the suggestions! Yes, I'm looking for the cross product between the two columns of X and Y. A follow-up question: what is a nice way to merge the output of lapply(split(myData, myData$Z), function(x) crossprod(x[, 1], x[, 2])) with the column Z in myData so that I would get a new dataframe as the following (the 2nd column is the cross product between X and Y)? Z CP A 10 B 10 Is the following legitimate? data.frame(Z=levels(myData$Z), CP= unlist(lapply(split(myData, myData$Z), function(x) crossprod(x[, 1], x[, 2])))) On Wed, Aug 24, 2016 at 10:37 AM, David L Carlson <dcarlson at tamu.edu> wrote:> Thank you for the reproducible example, but it is not clear what cross product you want. Jim's solution gives you the cross product of the 2-column matrix with itself. If you want the cross product between the columns you need something else. The aggregate function will not work since it will treat the columns separately: > >> A <- as.matrix(myData[myData$Z=="A", 1:2]) >> A > X Y > 1 1 4 > 2 2 3 >> crossprod(A) # Same as t(A) %*% A > X Y > X 5 10 > Y 10 25 >> crossprod(A[, 1], A[, 2]) # Same as t(A[, 1] %*% A[, 2] > [,1] > [1,] 10 >> >> # For all the groups >> lapply(split(myData, myData$Z), function(x) crossprod(as.matrix(x[, 1:2]))) > $A > X Y > X 5 10 > Y 10 25 > > $B > X Y > X 25 10 > Y 10 5 > >> lapply(split(myData, myData$Z), function(x) crossprod(x[, 1], x[, 2])) > $A > [,1] > [1,] 10 > > $B > [,1] > [1,] 10 > > ------------------------------------- > David L Carlson > Department of Anthropology > Texas A&M University > College Station, TX 77840-4352 > > > -----Original Message----- > From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Jim Lemon > Sent: Tuesday, August 23, 2016 6:02 PM > To: Gang Chen; r-help mailing list > Subject: Re: [R] aggregate > > Hi Gang Chen, > If I have the right idea: > > for(zval in levels(myData$Z)) > crossprod(as.matrix(myData[myData$Z==zval,c("X","Y")])) > > Jim > > On Wed, Aug 24, 2016 at 8:03 AM, Gang Chen <gangchen6 at gmail.com> wrote: >> This is a simple question: With a dataframe like the following >> >> myData <- data.frame(X=c(1, 2, 3, 4), Y=c(4, 3, 2, 1), Z=c('A', 'A', 'B', 'B')) >> >> how can I get the cross product between X and Y for each level of >> factor Z? My difficulty is that I don't know how to deal with the fact >> that crossprod() acts on two variables in this case. >> >> ______________________________________________ >> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see >> stat.ethz.ch/mailman/listinfo/r-help >> PLEASE do read the posting guide R-project.org/posting-guide.html >> and provide commented, minimal, self-contained, reproducible code. > > ______________________________________________ > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see > stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code.