I'm gonna explain a little more in detail since I already saw your other thread and you seem motivated but you're lacking the basics. As you already know, C has several "types" for variables, each of which serve a very specific purpose. A number is generally of type "int", which is the short form for integer. An integer is used for mathematical operations (in its pure form, there are other possibilities for it too) and you can use every imaginable mathematical operation on it.

Here's an example:

Code:

int number1 = 243;
int number2;
number2 = number1 + 156;
number1 = number2 * 2;

An array in C is a continuous memory space that hold several items of the same type. You can access the elements seperately by specifying its index within the array.

In the following I'll define an array of integers and do various operations with it:

Code:

int array[3] = {100, 3, 25};
array[0] = array[0] + array[1];
array[2] = array[2] / 5;

Now you need to understand that the possibility to access different elements only works in arrays, it isn't possible to access a specific decimal place within a number like that. If you want to access a decimal place in an integer, you first need to convert it into an array of characters (a string):

Code:

int i = 987654321;
char buf[512];
snprintf(buf, sizeof(buf), "%d", i);

with this code you're actually printing the number into a string, which is an array of characters. Now you are able to access each decimal place just like in the example above.

The first digit "9" would be inside buf[0] etc...

Now, unfortunately there's another problem, namely that the digits are converted to their ASCII value and since the ascii code for 9 is something different than 9, you can't apply the mathematical operations on them just like that.

You have chosen by pure chance a very difficult task which requires quite a bit of understanding