Getting the number of digits of an integer - c

Question

bool isValidId(int* id)
{
if(log10(*id) != 6)
{
return false;
}
return true;
}
printf("Enter ID: ");
gets(input);
c.id = atoi(input);
validID= isValidId(c.id);
if(!validID)
{
printf("Invalid ID format -(Use example 123456 format). \n");
}
This is how it looks now.I ask the user to enter an ID and check it if is valid with the isValidId method but my program is crashing when I enter an ID. Please help! Thanks

Answer 1

return *id >= 100000 && *id < 1000000;
I think this may be a good solution, both easy to read and efficient.
There is no need to acquire its length if you just want to judge if it is a valid id
Program crashes because the parameter of isValidId is pointer to int, not int, so
validID = isValidId(c.id);
should be
validID = isValidId(&c.id);

Answer 2

First of all, I don't see any reason to pass a pointer to isValidId function. You can pass an integer and calculate the number of digits.
bool isValidId(int id) {
// count digits here
}
Now there are at least two ways to calculate the number of digits. First one is to use log10. The number of digits in a 10-base integer n is (int)(log10(n) + 1). You will need to import math.h to use log10. You may check whether n <= 0 before calling log10.
The second way is to loop through n.
int count = 0;
while (n > 0) {
count++;
n /= 10;
}

Answer 3

You've declared isValidId to take a pointer to an int, but you're passing it a plain int; in this case, there's no reason to use a pointer, so you'd be better off changing isValidId to use a regular int.
NEVER NEVER NEVER NEVER NEVER USE GETS -- IT WILL INTRODUCE A POINT OF FAILURE/MAJOR SECURITY HOLE IN YOUR CODE. Use fgets(input, sizeof input, stdin) instead.
How is input declared? Is it large enough to hold as many digits as int will allow, plus a sign, plus a 0 terminator?
log10 returns a double, not an int. To properly count digits with log10, you will need to write something like (int)floor(log10(id)) + 1.
You can simplify your isValidId function a little:
bool isValidId(int id)
{
return (int) floor(log10(id)) + 1 == 6;
}
The Boolean data type is a latecomer to the C language (introduced in C99), so a lot of us older types tend to avoid using Boolean constants in our code.

Answer 4

I've not done C for eons however try something like this
bool isValidId(int* id)
{
char str[15];
sprintf(str, "%d", id)
if(strlen(str) != 6)
{
return false;
}
return true;
}

Answer 5

It's way easier like this:
#include <math.h>
bool isValidId(int *id)
{
return (int)log10(*id) == 6;
}