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When I have a distance matrix (or a data frame based on a matrix), how do I get the row and column that corresponds to a given value?

Example:

df <- data.frame(x = c(11:20), y= c(12:21))
dst <- dist(df)

Output: 

          1         2         3         4         5         6         7         8         9
2   1.414214                                                                                
3   2.828427  1.414214                                                                      
4   4.242641  2.828427  1.414214                                                            
5   5.656854  4.242641  2.828427  1.414214                                                  
6   7.071068  5.656854  4.242641  2.828427  1.414214                                        
7   8.485281  7.071068  5.656854  4.242641  2.828427  1.414214                              
8   9.899495  8.485281  7.071068  5.656854  4.242641  2.828427  1.414214                    
9  11.313708  9.899495  8.485281  7.071068  5.656854  4.242641  2.828427  1.414214          
10 12.727922 11.313708  9.899495  8.485281  7.071068  5.656854  4.242641  2.828427  1.414214

Now I want to input e.g. 11.313708 and get as output (9, 1)

Jeromy Anglim
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Azrael
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  • Be aware of this when comparing floating point values: http://stackoverflow.com/questions/9508518/why-are-these-numbers-not-equal – nicola May 24 '16 at 09:28

1 Answers1

12

We convert to a matrix and get the index with which with arr.ind=TRUE (assuming that it is what you wanted).

 m1 <- as.matrix(dst)
 which(m1==val, arr.ind=TRUE)

Otherwise, we can use the regular subsetting by row, column if we already know the index of the value. As @nicola mentioned in the comments, there is a chance for floating point issues. To avoid that may be round it and then do the comparison. i.e. 

 which(round(m1, 3)== 11.314, arr.ind=TRUE)
 #    row col
 #9    9   1
 #10  10   2
 #1    1   9
 #2    2  10
akrun
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    I'd warn about floating point issues. For instance, `which(m1==11.313708, arr.ind=TRUE)` won't produce any result and don't know if they are aware of this. – nicola May 24 '16 at 09:27
  • With this I only get `row col` – Azrael May 24 '16 at 09:30
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    @Azrael Read the link I provided as a comment to your question. Floating point comparisons should be avoided. Furthermore, keep in mind that when R print objects, it rounds the values; so the `m1[9,1]` is not actually `11.313708`, but something close to it. – nicola May 24 '16 at 09:35