How does atoi() function in C++ work?

Question

So...I know that the atoi function in the C++ standard library is supposed to convert a string into an integer...how does it work?...(I\'m trying to learn stuff and I was ju

Answer 1

Basically by subtracting ASCII zero ('0') and checking if it is a digit or not. You need to know the position:

#include 
#include 
#include 

int atoi( const char* nptr )
{
    int result   = 0;
    int position = 1;

    const char* p = nptr;
    while( *p )
    {
        ++p;
    }
    for( --p; p >= nptr; p-- )
    {
        if( *p < 0x30 || *p > 0x39 )
        {
            break;
        }
        else
        {
            result += (position) * (*p - 0x30);
            position *= 10;
        }
    }
    result = ((nptr[0] == '-')? -result : result);
    return result;
}

int main()
{
    char buffer[BUFSIZ] = {0};

    printf( "Enter an integer: " );
    fgets( buffer, BUFSIZ, stdin );
    buffer[strlen(buffer)-1] = 0;

    printf( "You entered %d\n", atoi( buffer ) );

    return 0;
}