#C, 48 chars

i(n,b){n?i(n/b,b),putchar(n%b+48+(n%b>9)*39):0;}

• input of 0 produces empty string

#C, 77 49 chars, prints 0 for zero

I(n,b){putchar(n?i(n/b,b),48+n%b+(n%b>9)*39:48);}

To run, call i for first version, I for second.

C, 48 chars

i(n,b){n?i(n/b,b),putchar(n%b+48+(n%b>9)*39):0;}

• input of 0 produces empty string

C, 77 49 chars, prints 0 for zero

I(n,b){putchar(n?i(n/b,b),48+n%b+(n%b>9)*39:48);}

To run, call i for first version, I for second.

#C, 48 chars

i(n,b){n?i(n/b,b),putchar(n%b+48+(n%b>9)*39):0;}

• input of 0 produces empty string

#C, 77 chars, prints 0 for zero

j(n,b){n?j(n/b,b),putchar(n%b+48+(n%b>9)*39):0;}I(n,b){n?j(n,b):putchar(48);}

To run, call i for first version, I for second.

#C, 48 chars

i(n,b){n?i(n/b,b),putchar(n%b+48+(n%b>9)*39):0;}

• input of 0 produces empty string

#C, 77 49 chars, prints 0 for zero

I(n,b){putchar(n?i(n/b,b),48+n%b+(n%b>9)*39:48);}

To run, call i for first version, I for second.