Question

I am relatively new with R and I have a problem with a dataframe.
I have a very long dataframe (df1) with some coordinates xy and a value z. I have a shorter dataframe (df2) with the same columns but smaller number of rows. I want to replace values in df1 when xy are equal in df2.

x = c(1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4) 
y = c(1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4)
z = c(8, 5, 3, 1, 2, 6, 8, 5, 3, 2, 8, 4, 4, 6, 2, 1) 
df1 = data.frame(x, y, z)      

x1=c(1,3,4)
y1=c(2,1,4)
z1=c(58,37,23)

df2=data.frame(x1,y1,z1)
names(df2) <- c("x", "y", "z")

I thought that I might use ifelse function as:

df1$znew<-ifelse((df1[,1]== df2[,1])&(df1[,2]==df2[,2]), df2[,3], df1[,3])

But the two objects are not the same dimensions.

I have tried to use loops so it analyse each row to compare x and y and then decide what z to use but I can’t make it work.

At the end I would like to have a dataframe with a new variable of z to compare the values and corroborate that it really changed the values. My final dataframe would look like:

 znew = c(8,58,3,1,2,6,8,5,37,2,8,4,4,6,2,23)

I really appreciate any help and I am sorry if somebody else posted similar questions, I have been all day trying to figure it out and I can’t find any example that suits my case.

Answer 1

Assuming the two data frames do in fact have the same column names (probably just a typo in your question), you might do this with merge:

tmp <- merge(df1,df2,all.x = TRUE,by = c('x','y'))
tmp$z.x[!is.na(tmp$z.y)] <- tmp$z.y[!is.na(tmp$z.y)]
> tmp
   x y z.x z.y
1  1 1   8  NA
2  1 2   4   4
3  1 3   3  NA
4  1 4   1  NA
5  2 1   2  NA
6  2 2   6  NA
7  2 3   8  NA
8  2 4   5  NA
9  3 1   4   4
10 3 2   2  NA
11 3 3   8  NA
12 3 4   4  NA
13 4 1   4  NA
14 4 2   6  NA
15 4 3   2  NA
16 4 4   3   3

Then just remove the extra column and rename the columns.