ex.R

# Rather trivial example of how dataframe row indexing could be used.
# I would not use this method, but it's not terrible design — access
# is fast compared to say genotype == 1L. Why limit these cases

> genotypes <- sample(0:2, 10, replace=TRUE)
> freq <- 0.3
> df <- data.frame(name=c("aa", "Aa", "AA"), type=c("homozygote", "heterozygote", "homozygote"),
        freq=c((1-freq)^2, 2*freq*(1-freq), freq^2))
> df
  name         type freq
1   aa   homozygote 0.49
2   Aa heterozygote 0.42
3   AA   homozygote 0.09
> df$type[genotypes+1L]
 [1] homozygote   homozygote   heterozygote homozygote   heterozygote
 [6] homozygote   homozygote   homozygote   heterozygote heterozygote
Levels: heterozygote homozygote
> df$name[genotypes+1L]
 [1] AA aa Aa aa Aa AA aa aa Aa Aa
Levels: aa Aa AA
> df$freq[genotypes+1L]
 [1] 0.09 0.49 0.42 0.49 0.42 0.09 0.49 0.49 0.42 0.42