A char in C is already a number (the character's ASCII code), no conversion required.

If you want to convert a digit to the corresponding character, you can simply add '0':

Use with AIc = i +'0';

The '0' is a character in the ASCll table.

Depends on what you want to do:

to read the value as an ascii code, you can write

char a = 'a';
int ia = (int)a; 
/* note that the int cast is not necessary -- int ia = a would suffice */

to convert the character '0' -> 0, '1' -> 1, etc, you can write

char a = '4';
int ia = a - '0';
/* check here if ia is bounded by 0 and 9 */

Explanation:
a - '0' is equivalent to ((int)a) - ((int)'0'), which means the ascii values of the characters are subtracted from each other. Since 0 comes directly before 1 in the ascii table (and so on until 9), the difference between the two gives the number that the character a represents.

itoa is the best route:

int num = 1234;
char cstr[16];
itoa(num, cstr, 10);
-> "1234"

You can read more about using it here.

If you want more control over the format of the number (and don't mind the increased PROGMEM usage) you can use sprintf (or the more secure snprintf) to format it:

sprintf(cstr, "%05d", num);
-> "01234"

Or with PROGMEM string constant:

sprintf_P(cstr, (PGM_P)F("%05d"), num);
-> "01234"

Or with more values:

sprintf_P(cstr, (PGM_P)F("%02d:%02d:%02d"), hours, minutes, seconds);
-> "03:23:11"