Complicated (for me) function HELP!

[hotshotennis]

Here is the assignment for a function that will be part of my program that I can't figure out how to start even: (its for a program that validates credit card numbers)

Function that checks if the last digit is correct, use the following algorithm:
-In each of the chars with an EVEN index (indices 0,2,4,6, etc)
-get the int value represented by the char (for example 7
from '7') // our teacher mentioned in class that we can do
this by doing something like '2' - '0' but I have no idea how to
put this into a code.
-double it (for example 7*2, which is 14)
-if doubling it has 2 digits, add those digits (ex 1+4, which
is 5) otherwise, just use the doubled value (if it has one
digit) // I'm super confused...
-add that to a sum. //what sum?
-Add the values of each of the chars (converted to the int it represents) n the ODD index positions to the sum. //again, confused
-If a sum is a multiple of 10 (use % operator), its correct (return in a return statement 1) otherwise, its incorrect (return 0)

-I don't want or need anybody to simply hand me the solution, but PLEASE help me out. This seems like the hardest function to do in our program assignment and I can't figure out how to code it.

This is all I have started for this particular code:

char lastChar;
if(lastChar == '0' || lastChar == '2' || lastChar == '4' || lastChar == '6' || lastChar == '8')

-Pathetic, I know. But I'm desperate to finally finish this. Please help me out any way you can!

[laserlight]

Hint: you should use loops, e.g., a for loop.

[hotshotennis]

Should that be instead of the "if" statement I have?

[laserlight]

Personally, I think that is off track to begin with. I would expect one or two loops. For the two loop version, you would be looping over the chars in even indices with one loop and the chars in odd indices in the other loop.

-add that to a sum. //what sum?

A variable that you create.

[hotshotennis]

Ok thank you, I'll try to play around with some loops. If you have any further suggestions, please let me know.