how to see if random paired sample is in dataframe (with conditions) - r

Question

Say I have a df like so:
T1 <- c("a","b","c","d","e")
T2 <- c("f","g","h","i","j")
score1 <- c(NA,0.01,0.5,0.78,NA)
score2 <- c(1, 2, 3, NA, 6)
df <- data.frame(T1, T2, score1, score2)
df
T1 T2 score1 score2
1 a f NA 1
2 b g 0.01 2
3 c h 0.50 3
4 d i 0.78 NA
5 e j NA 6
If I want to randomly create new T1-T2 pairs, how can I see if these new pairs are in the df but only if score1 column is not NA?
In other words, I randomly sample, say, 2 values from T1 and T2:
(l1 <- sample(df$T1, 2))
(l2 <- sample(df$T2, 2))
and get:
> l1
[1] "c" "d"
> l2
[1] "h" "g"
How would one go about to get the score2 of the c-h and d-g pairs from df but only if score1 is not NA?
My first instinct would be to create a new df2 without NAs in the score1 column:
df2 <- df[which(!is.na(df$score1)), ]
Then I can create a new df for the new pairs:
df3$X1 <- l1
df3$X2 <- l2
df3$X3 <- l2
df3$X4 <- l1
#stack X3 with X1 and X4 with X2 (considering that T1-T2 pair is the same as T2-T1 pair)
df4 <- data.frame(T1 = c(df3[,"X1"], df3[,"X3"]),
T2 = c(df3[,"X2"], df3[,"X4"]))
> df4
T1 T2
1 c h
2 d g
3 h c
4 g d
But I'm missing the last step of how to get see if the paired columns from df4 match the paired columns in df2. In the end, I want to get something like:
df
T1 T2 score1 score2
1 c h 0.50 3
2 d g NA NA

Answer 1

I think a merge/join operation makes sense here:
res <- merge(df, data.frame(T1=l1, T2=l2, found=TRUE), by = c("T1","T2"), all = TRUE)
subset(res, found, select = -found)
# T1 T2 score1 score2
# 3 c h 0.5 3
# 4 d g NA NA
Data
df <- structure(list(T1 = c("a", "b", "c", "d", "e"), T2 = c("f", "g", "h", "i", "j"), score1 = c(NA, 0.01, 0.5, 0.78, NA), score2 = c(1, 2, 3, NA, 6)), class = "data.frame", row.names = c(NA, -5L))
l1 <- c("c", "d"); l2 <- c("h", "g")

Answer 2

Something like this?
set.seed(2022)
(l1 <- sample(df$T1, 2))
#> [1] "d" "c"
(l2 <- sample(df$T2, 2))
#> [1] "h" "i"
mapply(\(x1, x2, data){
i <- match(x1, data$T1)
j <- match(x2, data$T2)
if(any(is.na(c(data$score1[i], data$score1[i])))) {
NA_real_
} else {
sum(c(data$score2[i], -1*data$score2[j]), na.rm = TRUE)
}
}, l1, l2, MoreArgs = list(data = df))
#> d c
#> -3 3
Created on 2022-01-30 by the reprex package (v2.0.1)