combine datasets by the value of multiple columns - r

Question

I'm trying to enter values based on the value of multiple columns from two datasets.
I have my main dataset (df1), with lists of a location and corresponding dates and df2 consists of a list of temperatures at all locations on every possible date. Eg:
df1
Location Date
A 2
B 1
C 1
D 3
B 3
df2
Location Date1Temp Date2Temp Date3Temp
A -5 -4 0
B 2 0 2
C 4 4 5
D 6 3 4
I would like to create a temperature variable in df1, according to the location and date of each observation. Preferably I would like to carry this out with all Temperature data in the same dataframe, but this can be separated and added 'by date' if necessary. With the example data, I would want this to create something like this:
Location Date Temp
A 2 -4
B 1 2
C 1 4
D 3 4
B 3 2
I've been playing around with merge and ifelse, but haven't figured anything out yet.

Answer 1

is it what you need?
library(reshape2)
library(magrittr)
df1 <- data.frame(Location= c("A","B","C","D","B"),Date=c(2,1,1,3,3))
df2 <- data.frame(Location= c("A","B","C","D"),d1t=c(-5,5,4,6),d2t=c(-4,0,4,3),d3t=c(0,2,5,4))
merge(df1,df2) %>% melt(id.vars=c("Location","Date"))

Answer 2

Here's how to do that with dplyr and tidyr.
Basically, you want to use gather to melt the DateXTemp columns from df2 into two columns. Then, you want to use gsub to remove the "Date" and "Temp" strings to get numbers that are comparable to what you have in df1. Since DateXTemp were initially characters, you need to transform the remaining numbers to numeric with as.numeric. I then use left_join to join the tables.
library(dplyr);library(tidyr)
df1 <- data.frame(Location= c("A","B","C","D","B"),Date=c(2,1,1,3,3))
df2 <- data.frame(Location= c("A","B","C","D"),Date1Temp=c(-5,5,4,6),
Date2Temp=c(-4,0,4,3),Date3Temp=c(0,2,5,4))
df2_new <- df2%>%
gather(Date,Temp,Date1Temp:Date3Temp)%>%
mutate(Date=gsub("Date|Temp","",Date))%>%
mutate(Date=as.numeric(Date))
df1%>%left_join(df2_new)
Joining, by = c("Location", "Date")
Location Date Temp
1 A 2 -4
2 B 1 5
3 C 1 4
4 D 3 4
5 B 3 2
EDIT
As suggested by #Sotos, you can do that in one piping like so:
df2%>%
gather(Date,Temp,Date1Temp:Date3Temp)%>%
mutate(Date=gsub("Date|Temp","",Date))%>%
mutate(Date=as.numeric(Date))%>%
left_join(df1,.)
Joining, by = c("Location", "Date")
Location Date Temp
1 A 2 -4
2 B 1 5
3 C 1 4
4 D 3 4
5 B 3 2