I need a forumla

[Da0omph]

To convert ASCII into a number.
For example, if I had ASCII 50. I need to convert into the number 2.

My prof was saying some thing about sum of crap.. And... well.. idk, I just a formula. ;)

[tntxtnt]

int convertToInt(char c)
{
    if (isdigit(c))
        return c - '0';
    return -1; //c is not a digit
}

http://cplusplus.com/reference/clibrary/cctype/isdigit/

or you can use atoi to convert string to integer
http://cplusplus.com/reference/clibrary/cstdlib/atoi/

[Da0omph]

Could you explain why a character - '0' would convert it to a int?

[Da0omph]

Also, I don't have to use isdigit() because all the ASCII values I'll be working with will be digits. And I can't use any built in functions.

There's gotta some type of formula for calculating it.

[HiteshVaghani1]

you can use
atoi()
function which convert ascii into integer.

[tntxtnt]

because every ASCII char is an integer.
'0' is 48; 48 - 48 == 0
'1' is 49; 49 - 48 == 1
'2' is 50; 50 - 48 == 2
...
'9' is 57; 57 - 48 == 9

so if you want to convert an ASCII char into number, just simply minus 48, or '0' if you can't remember 48. The compiler will treat '0' as 48.

[DesiredNote]

Tntxtnt's code would work perfectly.