The evaluation of condition resulted in an NA. The if conditional must have either a TRUE or FALSE result.
D[i, j] < dmin 如果任何一個值是NA，就會有這結果。
可以先用is.na(D[i,j] 作過濾動作。
---
> b = NA
> is.na(b)
[1] TRUE
> if (b < 0 ) { a = 1}
Error in if (b < 0) { : missing value where TRUE/FALSE needed