convert integers to characters in string

[h7dra]

hello

i want to convert integers into characters,
with the rule 0>a,1>b,2>c,3>d,4>d,....

i mean: for example

int i=0
int j=3
str s

i want to have s="ad" !!!

how can i do that?
maybe something with character codes ( i saw that a~97,b~98,...)

Thanks