can someone please tell why " -'0' "is being done in the fifth line of following function

Question

 m=1e9 + 7;
inline ll rem(char s[],ll m)
{
     ll sum=0 , i;
     for(i=0;S[i]!='\0';i++)
     {
          if(sum>=m)
            sum %= m;
          sum=(sum * 10 + S[i] - '0');
     }
     return sum%m;
}

here S[i] is a string of integer characters. My question is what does -'0' does here, also can a character (here S[i]) be automatically converted to integer form is the above

sum=(sum * 10 + S[i] - '0');

equation.

Subtracting `'0'` from a character will give you its numeric value, assuming the character was representing a numeral. — Christian Gibbons, May 24 '18 at 21:38
Someone please find a duplicate for this, it can't be the first time someone has asked. — Barmar, May 24 '18 at 21:41

score 4 · Answer 1 · answered May 24 '18 at 21:43

First, you have to remember that characters in C are represented as tiny integers corresponding to the character's value in the machine's character set, which is typically ASCII.

For example, 'A' n ASCII is 65, and '0' is 48.

So if you're converting a string of digits to an integer, you want to do something like

int digit = c - 48;

That converts '0' to 0, '1' to 1, etc.

But that magic number 48 is mystifying, and it's theoretically also wrong on a machine using a character set other than ASCII. So the easier (because you don't have to remember that value 48), self-documenting (as long as your reader understands the idiom), and more portable way is to do

int digit = c - '0';

This works because, as I said, '0' is 48 in ASCII. But, more importantly, even on a non-ASCII machine, '0' is whatever value the character '0' has in that machine's character set, so it's always the right value to subtract, no matter what kind of machine you're using.

And C requires `'0' .. '9'` to be represented contiguously. – jxh May 24 '18 at 21:48 — jxh, May 24 '18 at 21:48

can someone please tell why " -'0' "is being done in the fifth line of following function

1 Answers1