Have a really basic easy question..

im trying to get ready for my test tommorow. And well giong over my old labs....I came to this..
I have these to function defs....

Code:

---

void CharSet::Add(char Value)
{
  Contents[Value - 'A'] = true;
}
void CharSet::Remove(char Value)
{
  Contents[Value - 'A'] = false;

---

And Although I used it I never fully understood....the Value -'A'...I was wondering if someone could help me out and give me an explanation into how it is used....I really don't recall using that or being taught that. I used it because the teacher in his instructions hinted for us to use it.
thanks in advance

Assuming that Value holds an uppercase character, the expression Value - 'A' would give you a zero-based index of the uppercase letters, with 'A' being 0 and 'Z' being 25.

The contents of 'A' is the ascii value for the upper-case A... in this case, 'A' is being used as a numeric constant (in hex: 40h in dec: 64d)

oh ok i see....makes more sense now....thanks for the help...

Another take: characters are stored internally as integer codes. An ASCII table will show you the list of integer codes. Since they are integers, you can do mathematical operations on them:

Code:

---

char ch = 'A';
ch += 2;
cout<<ch<<endl;  //C

---

When you output a char type, C++ converts the interger code back to a character. So, if you actually want to output the integer code, then you have to convert a char type to an integer:

Code:

---

char ch = 'A';
int code = ch;
cout<<code<<endl; //65
cout<<ch<<endl;  //A

---

It better style to make an explicit cast, instead, in order to demonstrate that's what you intended:

Code:

---

char ch = 'A';
int code = static_cast<int>(ch);
cout<<code<<endl; //65

---

Another take: characters are stored internally as integer codes. An ASCII table will show you the list of integer codes. Since they are integers, you can do mathematical operations on them:

Code:

---

char ch = 'A';
ch += 2;
cout<<ch<<endl;  //C

---

When you output a char type, C++ converts the interger code back to a character. So, if you actually want to output the integer code, then you have to convert a char type to an integer:

Code:

---

char ch = 'A';
int code = ch;
cout<<code<<endl; //65
cout<<ch<<endl;  //A

---

It's better style to make an explicit cast, instead, in order to demonstrate that's what you intended:

Code:

---

char ch = 'A';
int code = static_cast<int>(ch);
cout<<code<<endl; //65

---

Good luck on your test. :)
Tests :(