CareerCup

int num_digits(int n) {
int count = 0;
do {
count++;
n /= 10;
} while(n);
}

Convert the number to a string and then call the string's length function to give the number of digits

dont forget the "return count;" =P

int count =1;
while(n){
n = n/10;
count++
}

What about this:
void num_digits(int n) {
char line[35];
sprintf(line,"%d",n);
return strlen(line);
}

what if the number is a float number?
it is safe to convert it into a string, and use string length function. In addition, we need to detect if there is a float point.

log(n)/log(2)

log(n), log shud be taken in base 10

For the log, it should be int( log(n) / log(10)) + 1

int countDigit(int n){
if(n==0)
return 1;

int abs_n=n>0?n:-n;
int re=0;
while(abs_n>0){
asb_n/=10;
re++;
}

return re;
}