How to get the digits of a number without converting it to a string/ char array?

Question

How do I get what the digits of a number are in C++ without converting it to strings or character arrays?

Answer 1

Years ago, in response to the above questions I would write the following code:

int i2a_old(int n, char *s)
{
    char d,*e=s;//init begin pointer
    do{*e++='0'+n%10;}while(n/=10);//extract digits
    *e--=0;//set end of str_number
    int digits=e-s;//calc number of digits
    while(s

I think that the function printf(...) does something like that.

Now I will write this:

int i2a_new(int n, char *s)
{
    int digits=n<100000?n<100?n<10?1:2:n<1000?3:n<10000?4:5:n<10000000?n<1000000?6:7:n<100000000?8:n<1000000000?9:10;
    char *e=&s[digits];//init end pointer
    *e=0;//set end of str_number
    do{*--e='0'+n%10;}while(n/=10);//extract digits
    return digits;//return number of digits
}

Advantages: lookup table indipendent; C,C++,Java,JavaScript,PHP compatible; get number of digits, min comparisons: 3; get number of digits, max comparisons: 4; fast code; a comparison is very simple and fast: cmp reg, immediate_data --> 1 CPU clock.