Every 2nd digit on Pset1 Credit problem: Tldr bolded

Question

Im struggling with manipulating the string of numbers in the credit problem. I've figured out how to divide the initial input number by 10 (*100 each iteration) then mod % by 10 in order to access the 2nd to last digit, then every other digit following. However Im having trouble figuring out how to effectively store those outputs so that I can double them in order to do the check sum. Right now I have each one being transfered to its own variable in a series of 7 if statements within the loop, but I feel like there has to be a more efficient way to do so. (I also have it mapped to 7 if statements, and the loop repeating 7 times based on the sample number given, but since cc numbers can range from 14-16 characters I think that may cause problems down the road. I ideally would like int i to represent the number of figures in the input string but so far havent been able to find out how to take the length of a long long and assign that length to a variable. Is there a check length() function of some kind?)

Im also struggling with how to split the individual digits of a double digit number to add them (ie cc number =9876, take 7, double it to get 14, but Im trying to add 1+4 to my total rather than adding 14 to my total). I think I can do this by taking mod % 10 of that number then dividing it and taking mod % 10 again to get the separate digits, but since all of my outputs are in individual variables I cant figure out how to set that up without an additional series of 7 if statements. I also then run into the trouble of how to link those two mod and divide functions without assigning their outputs to seperate variables again.

I feel like there is either some sort of operation that I may be missing, or an aspect to the operations Im using that I am not understanding/implementing correctly. I've pasted my code below in case that would help with figuring out where I may be going wrong.`enter code here

#include <cs50.h>
#include <stdio.h>

int b, c, d, e, f, g, h;

int main(void)
{
long long x = get_long_long();  //gets credit card number input
long y = 10;   //is the value that the input will be mod'ed by
long long a = 10; //is the divisor so that mod 10 gives every other digit from 2nd to last

for (int i = 0; i<7;i++)  //repeates 7 times to get every other digit in the sample cc number 
{
    int z = (x/a) % y;  //z is the output, the every other digit so to speak
    printf("%lld divided by %lld mod %ld equals %i\n", x, a, y, z); //just for testing to see values
        if (a==10) //this is a long winded way Ive figured out how to seperate the digits to their own 
                    //values so that they can be multiplied by 2 individually
        {
           b=z*2;
        }
        if (a==1000)
        {
           c=z*2;
        }
        if (a==100000)
        {
           d=z*2;
        }
        if (a==10000000)
        {
           e=z*2;
        }
        if (a==1000000000)
        {
           f=z*2;
        }
        if (a==100000000000)
        {
           g=z*2;
        }
        if (a==10000000000000)
        {
           h=z*2;
        }
    a*=100; //this increments so that the value given by the mod changes by 2 places each loop
}
long long sum_of_doubled=b+c+d+e+f+g+h; //this gives the sum of the digits aquired above
printf("%i, %i, %i, %i, %i, %i, %i \n", b,c,d,e,f,g,h ); //shows the every other digits once doubled
printf("%lld is the sum of every other digit\n", sum_of_doubled); //just to show value during testing

}