# R: Zip fastener for two data frames / combining rows or columns of two dataframes in an alternating manner

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Sometimes I find it useful to merge two data frames like the following ones

X1 X2 X3 X4 Y1 Y2 Y3 Y4 1 o o o o X X X X 2 o o o o X X X X 3 o o o o X X X X

by using zip feeding either along the columns

X1 Y1 X2 Y2 X3 Y3 X4 Y4 1 o X o X o X o X 2 o X o X o X o X 3 o X o X o X o X

or along the rows of the data frames.

V1 V2 V3 V4 1 o o o o 4 X X X X 2 o o o o 5 X X X X 3 o o o o 6 X X X X

The following function acts like a *“zip fastener” *for combining two dataframes. It takes the first column (or row) of the first data frame and places it next to the first column (or row) of the second data frame and so on. Only one dimension of the data frame has to be equal to do this. E.g. to combine the *columns *by zip feeding the number of *rows *must be equal and vice versa.

So here comes the code for the `zipFastener()` function. Actually its only the last few lines (from `#zip fastener operations` on) that do the job, but as I did not want to restrict the function to equal dimensions there is a little prelude.

############################################################### # zipFastener for TWO dataframes of unequal length zipFastener <- function(df1, df2, along=2) { # parameter checking if(!is.element(along, c(1,2))){ stop("along must be 1 or 2 for rows and columns respectively") } # if merged by using zip feeding along the columns, the # same no. of rows is required and vice versa if(along==1 & (ncol(df1)!= ncol(df2))) { stop ("the no. of columns has to be equal to merge them by zip feeding") } if(along==2 & (nrow(df1)!= nrow(df2))) { stop ("the no. of rows has to be equal to merge them by zip feeding") } # zip fastener preperations d1 <- dim(df1)[along] d2 <- dim(df2)[along] i1 <- 1:d1 # index vector 1 i2 <- 1:d2 + d1 # index vector 2 # set biggest dimension dMax if(d1==d2) { dMax <- d1 } else if (d1 > d2) { length(i2) <- length(i1) # make vectors same length, dMax <- d1 # fill blanks with NAs } else if(d1 < d2){ length(i1) <- length(i2) # make vectors same length, dMax <- d2 # fill blanks with NAs } # zip fastener operations index <- as.vector(matrix(c(i1, i2), ncol=dMax, byrow=T)) index <- index[!is.na(index)] # remove NAs if(along==1){ colnames(df2) <- colnames(df1) # keep 1st colnames res <- rbind(df1,df2)[ index, ] # reorder data frame } if(along==2) res <- cbind(df1,df2)[ , index] return(res) } ###############################################################

Here come some examples.

############################################################### ### examples ### require(plyr) # data frames equal dimensions df1 <- rdply(3, rep("o",4))[ ,-1] # from plyr package df2 <- rdply(3, rep("X",4))[ ,-1] zipFastener(df1, df2) zipFastener(df1, df2, 2) zipFastener(df1, df2, 1) # data frames unequal in no. of rows df1 <- rdply(10, rep("o",4))[ ,-1] zipFastener(df1, df2, 1) zipFastener(df2, df1, 1) # data frames unequal in no. of columns df2 <- rdply(10, rep("X",3))[ ,-1] zipFastener(df1, df2) zipFastener(df2, df1, 2) ###############################################################

I hope you find that useful.

Ciao, Mark

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