r - 有效地访问成对距离

Question

我有一个距离矩阵:

> mat
          hydrogen   helium  lithium beryllium    boron
hydrogen  0.000000 2.065564 3.940308  2.647510 2.671674
helium    2.065564 0.000000 2.365661  1.697749 1.319400
lithium   3.940308 2.365661 0.000000  3.188148 2.411567
beryllium 2.647510 1.697749 3.188148  0.000000 2.499369
boron     2.671674 1.319400 2.411567  2.499369 0.000000

和一个数据框:

> results

El1      El2    Score
Helium Hydrogen   92
Boron   Helium    61
Boron  Lithium    88

我想计算 results$El1 中单词之间的所有成对距离和 results$El2得到以下内容:

> results

El1      El2    Score   Dist
Helium Hydrogen   92    2.065564
Boron   Helium    61    1.319400
Boron  Lithium    88    2.411567

我用 for 循环做了这个，但它看起来很笨重。有没有更优雅的方法来用更少的代码行搜索和提取距离？

这是我当前的代码:

names = row.names(mat) 
num.results <- dim(results)[1]   
El1 =  match(results$El1, names)  
El2 = match(results$El2, names)    
el.dist <- matrix(0, num.results, 1)        
for (i1 in c(1:num.results)) {             
el.dist[i1, 1] <- mat[El1[i1], El2[i1]]
}
results$Dist = el.dist[,1] 

Answer 1

cols <- match(tolower(results$El1), colnames(mat))
rows <- match(tolower(results$El2), colnames(mat))
results$Dist <- mat[cbind(rows, cols)]
results
     El1      El2 Score     Dist
1 Helium Hydrogen    92 2.065564
2  Boron   Helium    61 1.319400
3  Boron  Lithium    88 2.411567

你会认出大部分代码。要重点关注的是mat[cbind(rows, cols)] .对于矩阵，我们可以通过另一个列数与维度相同的矩阵进行子集化。来自 ?`[`帮助:

When indexing arrays by [ a single argument i can be a matrix with as many columns as there are dimensions of x; the result is then a vector with elements corresponding to the sets of indices in each row of i.