Merge {Hmisc} | R Documentation

## Merge Multiple Data Frames or Data Tables

### Description

Merges an arbitrarily large series of data frames or data tables containing common `id` variables (keys for data tables). Information about number of observations and number of unique `id`s in individual and final merged datasets is printed. The first data frame has special meaning in that all of its observations are kept whether they match `id`s in other data frames or not. For all other data frames, by default non-matching observations are dropped. The first data frame is also the one against which counts of unique `id`s are compared. Sometimes `merge` drops variable attributes such as `labels` and `units`. These are restored by `Merge`. If all objects are of class `data.table`, faster merging will be done using the `data.table` package's join operation. This assumes that all objects have identical key variables and those of the variables on which to merge.

### Usage

    Merge(..., id, all = TRUE, verbose = TRUE)

### Arguments

`id` | a formula containing all the identification variables such that the combination of these variables uniquely identifies subjects or records of interest. May be omitted for data tables; in that case the `key` function retrieves the id variables.
`all` | set to `FALSE` to drop observations not found in second and later data frames (only applies if not using `data.table`)
`verbose` | set to `FALSE` to not print information about observations
`...` | two or more dataframes or data tables

### Examples

    a <- data.frame(sid=1:3, age=c(20,30,40))
    b <- data.frame(sid=c(1,2,2), bp=c(120,130,140))
    d <- data.frame(sid=c(1,3,4), wt=c(170,180,190))
    all <- Merge(a, b, d, id = ~ sid)
    all
    ## Not run:
    # For data.table, first file must be the master file and must
    # contain all ids that ever occur.  ids not in the master will
    # not be merged from other datasets.
    require(data.table)
    a <- data.table(a); setkey(a, sid)
    # data.table also does not allow duplicates without allow.cartesian=TRUE
    b <- data.table(sid=1:2, bp=c(120,130)); setkey(b, sid)
    d <- data.table(d); setkey(d, sid)
    all <- Merge(a, b, d)
    all

    ## End(Not run)

[Package Hmisc version 4.0-3 Index]