Help on calculating spearman rank correlation for a data frame with conditions

id  price  distance
1   2     4
1   3    5
...
2  4   8
2  5   9
I would like to calculate the rank-order correlation between 
price and distance for each id.

cor(price,distance,method = "spearman") calculate a 
correlation for all.

Try by()

#Example
d <- data.frame(g=gl(5, 10), x=rnorm(50), y=rnorm(50))
by(d[,2:3], d$g, cor, method="spearman")
 

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