How replace just leading number for a string where those characters appear also as unit

I have the following vector

vector <- c(31, 41, 51, 61, 32, 42, 52, 62, 33, 43, 53, 63,
            34, 44, 54, 64, 35, 45, 55, 65, 36, 46, 56, 66)

I would like just to replace the leading number without convert that number when it is found as unit digit

str_replace_all(vector, c('3' = 'Air', '4' = 'Car',
                          '5' = 'Polluted', '6' = 'Gas'))

The desired out is just is to use each final/unit digit as a marker meaning that for example:

wherever there is a 3(unit/digit), the code should convert it as:

'Air_(unit/digit)'

more clearly as:

#  [1] "Air_1"      "Car_1"      "Polluted_1"
#  [4] "Gas_1"      "Air_2"      "Car_2"     
#  [7] "Polluted_2" "Gas_2"      "Air_3"     
# [10] "Car_3"      "Polluted_3" "Gas_3"     
# [13] "Air_4"      "Car_4"      "Polluted_4"
# [16] "Gas_4"      "Air_5"      "Car_5"     
# [19] "Polluted_5" "Gas_5"      "Air_6"     
# [22] "Car_6"      "Polluted_6" "Gas_6" 

Would you suggest any quick way to do this? Thanks

>Solution :

Assuming we always have 2 digits, using arithmetic operators %% to get 1st digit and %/% to get remainder – the 2nd digit. Then paste all as desired:

paste(
  c('3' = 'Air', '4' = 'Car', '5' = 'Polluted', '6' = 'Gas')[
    as.character(vector %/% 10) ],
  vector %% 10, sep = "_")
#  [1] "Air_1"      "Car_1"      "Polluted_1"
#  [4] "Gas_1"      "Air_2"      "Car_2"     
#  [7] "Polluted_2" "Gas_2"      "Air_3"     
# [10] "Car_3"      "Polluted_3" "Gas_3"     
# [13] "Air_4"      "Car_4"      "Polluted_4"
# [16] "Gas_4"      "Air_5"      "Car_5"     
# [19] "Polluted_5" "Gas_5"      "Air_6"     
# [22] "Car_6"      "Polluted_6" "Gas_6"