r - How do I select a value based on two conditions from a data frame and then paste it into a cell in another data frame? -

i have 2 data frames. 1 has trials in 2 objects appeared. has information on how 2 objects relate (visual similarity). each row (trial) in first data frame, pull names of objects , use them specific rows in second data frame. this:

  trial probepic target_pic vissim 1     1  robot1    robot6     na 2     2  robot1    robot3     na 3     3  robot5    robot6     na 4     4  robot2    robot1     na 5     5  robot3    robot9     na 6     6  robot14   robot9     na      rob1   rob2  sim 1 robot1 robot1   na 2 robot2 robot1 2.88 3 robot3 robot1 3.75 4 robot4 robot1 1.63 5 robot5 robot1 3.63 6 robot6 robot1 2.50 

etc. want use probepic , target_pic variables select value of sim second data frame , paste vissim in first data frame. i've been playing around subset , can correct value using code:

subset(vissim, rob1=="robot1" & rob2=="robot2")$sim 

but want use probepic , target_pic variables each row instead of "robot1" , "robot2." , apply function each row in first data frame. so, in each row, function in probepic column , match value rob1 column , in target_pic column , match value rob2 column. specify unique row in second data frame. then, take value of sim unique row , paste vissim.

i think want merge data sets together. try following:

newdf <- merge(df1, df2, by.x=c("probepic", "target_pic"), by.y=c("rob1", "rob2"), all=t) 

data

df1 <- read.table(header=t, text="  trial probepic target_pic vissim 1  robot1    robot6     na 2  robot1    robot3     na 3  robot5    robot6     na 4  robot2    robot1     na 5  robot3    robot9     na 6  robot14   robot9     na")  df2 <- read.table(header= t, text="    rob1   rob2  sim robot1 robot1   na robot2 robot1 2.88 robot3 robot1 3.75 robot4 robot1 1.63 robot5 robot1 3.63 robot6 robot1 2.50")