How can I map an integer to an integer?

Question

I need to map an integer to an integer in R. In python this is the job of a dictionary

>>> a = { 4: 1, 5: 2, 6:3 }
>>> a[5]
2

but no such thing exists in R. A vector does not work:

 a<- c(1,2,3)
> a
[1] 1 2 3
> names(a) <- c(5,6,7)
> a
5 6 7 
1 2 3 
> a[[5]]
Error in a[[5]] : subscript out of bounds

a list doesn't work either

> a<- list(1,2,3)
> a
[[1]]
[1] 1

[[2]]
[1] 2

[[3]]
[1] 3

> names(a) <- c(4, 5, 6)
> a
$`4`
[1] 1

$`5`
[1] 2

$`6`
[1] 3

> a[[6]]
Error in a[[6]] : subscript out of bounds

You just called it wrong. Try `a['5']` or `a[['5']]`. This should work in both lists and named vectors — Sotos, Feb 06 '20 at 13:44
Oh, you need to call it as integer? Then you can't because R will take the integer as the index, not as the name — Sotos, Feb 06 '20 at 13:47
@sotos I added a python example. Is there any data structure I can use for this purpose? i'd prefer not to pass through string if possible. — Stefano Borini, Feb 06 '20 at 13:48
No but you should be able to hack it. Maybe build a vector of the form `x — Sotos, Feb 06 '20 at 13:51
@Sotos I can't. I am mapping id numbers to values, and the id numbers can be huge. Unless the vector code does run length encoding I would waste a massive amount of memory — Stefano Borini, Feb 06 '20 at 13:53
Then definitely not. Answer below with `hashmap` should suit you — Sotos, Feb 06 '20 at 13:55

score 5 · Accepted Answer · answered Feb 06 '20 at 13:54

There are some dictionaries in R.

I would suggest the hashmap package for your case.

library(hashmap)
H <- hashmap(c(2, 4, 6), c(99, 999, 9999))
H
##   (numeric) => (numeric)     
## [+2.000000] => [+99.000000]  
## [+4.000000] => [+999.000000] 
## [+6.000000] => [+9999.000000]
H[[4]]
# [1] 999

If you want "true" integers:

H <- hashmap(c(2L, 4L, 6L), c(99L, 999L, 9999L))
H
## (integer) => (integer)
##       [2] => [99]     
##       [4] => [999]    
##       [6] => [9999]

Der Gnirreh · Answer 2 · 2020-02-06T13:59:48.053

2

For good performance you use match.

k<-c(5,6,7)
v<-c(1,2,3)

x<-c(5,5,6)

v[match(x,k)]

In fact, if your v is just 1:n it is enough to do

match(x, k)

as match returns the index in the array you are doing lookup in.

edited Feb 06 '20 at 13:59

answered Feb 06 '20 at 13:50

Der Gnirreh

188
10

score 1 · Answer 3 · answered Feb 06 '20 at 13:44

1

In your last example this should work (string not numeric):

a <- list(1,2,3)
names(a) <- c(4, 5, 6)    

a[["6"]]
a[[as.character(6)]]

answered Feb 06 '20 at 13:44

Marta

2,864
2
15
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So the only way is to cast the key number into a string? – Stefano Borini Feb 06 '20 at 13:46
I think that there is no strict solution (object) for this. – Marta Feb 06 '20 at 13:52
3

integer comparison is faster than string comparison, and I would have to go back and forth between strings and integers, which is also a performance burden – Stefano Borini Feb 06 '20 at 13:54

score 1 · Answer 4 · answered Feb 06 '20 at 13:53

1

A less than optimal possible solution would be to assign the integers into position of their integers in a vector:

a[c(5:7)]<-1:3
a[6]
> [1] 2

A drawback (or benefit, depends on your needs) is that a[1] would yield an NA

answered Feb 06 '20 at 13:53

iod

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How can I map an integer to an integer?

4 Answers4