R：添加具有在不同数据框中找到的值的列

Question

Sorry if this has been asked before. I am fairly new to R, and strugling a tad with formulating the question correctly.

I have two dataframes imported from a csv, looking similar to this:

df1: 
    date time vdd temp seat volume output
    ...  ...  1.8 25   1    ...    ... 
    ...  ...  1.8 -40  0    ...    ... 
    ...  ...  ... ...  ...  ...    ... 

And

df2:
    seat temp vdd noise
    0    -40  1.8 5.3E-09
    1    25   1.8 4.9E-09
    .     .    .   .  

I want the second dataframe to work as a lookup table, to create a new column in the first dataframe, producing a result like this:

df1: 
    date time vdd temp seat volume output noise
    ...  ...  1.8 25   1    ...    ...    4.9E-09
    ...  ...  1.8 -40  0    ...    ...    5.3E-09
    ...  ...  ... ...  ...  ...    ...    ...

I have tried looking at merge and match, but I can't wrap my head around how to produce the result I want.

Thanks for any help I can get.

Answer 1

You can easily use merge() as you mentioned in your post.

merge(df1, df2, by=c("seat", "temp", "vdd"))

By default this will keep all records from the first data frame and only corresponding records from the second. To keep all records from both, use all=T .

Alternatively you can omit the by= argument and it will merge on all common columns.

Answer 2

There are several ways you could approach this, but I find this to be the easiest:

1. Install this package if you don't already have it:

install.packages("sqldf")

It lets you use SQL commands to transform data.

2. Join the data as you see fit:

require(sqldf)

output_table <- sqldf("select df1.*, df2.noise from df1 join df2 on df1.seat = df2.seat")