ascii to integer

I assume you have this one function which can convert a single character to a number. Say it is int convert_one_char(const char c).

Given this function here is the code to extend it to convert a string with multiple chars:

int convert_one_char(const char c)
{
    if( c == '0' ) return 0;
    else if( c == '1' ) return 1;
    else if( c == '2' ) return 2;
    else if( c == '3' ) return 3;
    else if( c == '4' ) return 4;
    else if( c == '5' ) return 5;
    else if( c == '6' ) return 6;
    else if( c == '7' ) return 7;
    else if( c == '8' ) return 8;
    else if( c == '9' ) return 9;
    else return -1 ;
}

long my_atoi( const string& s )
{
    int curr_dec_multiplier = 10 ;
    long ret_number = convert_one_char(s[s.length() - 1]) ;
    for( int i = s.length() - 2; i >= 0; i-- )
    {
        ret_number += convert_one_char(s[i]) * curr_dec_multiplier ;
        curr_dec_multiplier *= 10 ;
    }

    return ret_number ;
}

//------------------------------------------------

int main ()
{
    cout << "523623412 converted = " << my_atoi("523623412") << endl ;
    cout << "623412 converted = " << my_atoi("623412") << endl ;
    cout << "231 converted = " << my_atoi("231") << endl ;

    //-----------------------
    getch() ;
    return 0;
}
//------------------------------------------------

You'll need to put in appropriate error handling code...

To convert a single character to an int just simply subtract '0' from its ascii value

char c = '1';
int num = c - '0';

When converting several characters to int you have to multiply the previous value by 10 to make room for the new digit.
Below num is an integer and digits is a character array that contains all the digits. This code snipped would be placed inside a loop.

num = (num * 10) + digits[i] - '0';

Thanks guys , you were totally helpful .