What's wrong with sum up two string of integer?

[Mathsniper]

void sumString(char *a, char *b, int len)
{
	int i;
	for (i = 0; i < len; i++)
		a[i] = (char)(((int)a[i]-48 + (int)b[i]-48) % 10);
	for (i = 0; i < len; i++)
		printf("%c", a[i]);
	printf("\n");
}

I wanna implement "1234"+"2345" = "3579" but It displays nothing.

[Tonto]

(a[i]-'0' + b[i]-'0') + '0';
a[i]-'0' + b[i];

Same thing. You should make sure to deal with non-numeric input. And it's a little funky that you just modify the string a, I don't know, unless the user expects that.

[Mathsniper]

yes, I want to modify string a, can you write it in details?

[Tonto]

Well, it's just kind of a peculiar sideeffect. Like, I guess it's just a question of interpretation. I mean, I'll draw an analogous situation in C++.

class number_string 
{ 
    number_string & operator += (const number_string &);
    number_string operator + (const number_string &) const;
};

The first operator += does modify 'this' string, like string 'a' in your case. But the second one gives back a different number_string completely in summing 'this' and 'other' number_string's. To me, that second behavior is what is expected of the sumString, but, I don't know, it's really pretty ambiguous and up to interpretation. You should just make it clear that your function does modify 'a'.

[vart]

don't forget that 9+9=18