atoi implementation in C

问题

I can't understand the following atoi implementation code, specifically this line:

k = (k << 3) + (k << 1) + (*p) - '0';

Here is the code:

int my_atoi(char *p) {
    int k = 0;
    while (*p) {
        k = (k << 3) + (k << 1) + (*p) - '0';
        p++;
     }
     return k;
}

Can someone explain it to me ?

Another question: what should be the algorithm of atof implementation ?

回答1:

k = (k << 3) + (k << 1);

means

k = k * 2³ + k * 2¹ = k * 8 + k * 2 = k * 10

Does that help?

The *p - '0' term adds the value of the next digit; this works because C requires that the digit characters have consecutive values, so that '1' == '0' + 1, '2' == '0' + 2, etc.

As for your second question (atof), that should be its own question, and it's the subject for a thesis, not something simple to answer...

回答2:

<< is bit shift, (k<<3)+(k<<1) is k*10, written by someone who thought he was more clever than a compiler (well, he was wrong...)

(*p) - '0' is subtracting the value of character 0 from the character pointed by p, effectively converting the character to a number.

I hope you can figure out the rest... just remember how the decimal system works.

Here is a specification for the standard function atoi. Sorry for not quoting the standard, but this will work just as fine (from: http://www.cplusplus.com/reference/clibrary/cstdlib/atoi/ )

The function first discards as many whitespace characters (as in isspace) as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many base-10 digits as possible, and interprets them as a numerical value.

The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.

If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed and zero is returned.

回答3:

#include <stdio.h>
#include <errno.h>
#include <limits.h>

double atof(const char *string);

int debug=1;

int main(int argc, char **argv)
{
    char *str1="3.14159",*str2="3",*str3="0.707106",*str4="-5.2";
    double f1,f2,f3,f4;
    if (debug) printf("convert %s, %s, %s, %s\n",str1,str2,str3,str4);
    f1=atof(str1);
    f2=atof(str2);
    f3=atof(str3);
    f4=atof(str4);

    if (debug) printf("converted values=%f, %f, %f, %f\n",f1,f2,f3,f4);
    if (argc > 1)
    {
        printf("string %s is floating point %f\n",argv[1],atof(argv[1]));
    }
}

double atof(const char *string)
{
    double result=0.0;
    double multiplier=1;
    double divisor=1.0;
    int integer_portion=0;

    if (!string) return result;
    integer_portion=atoi(string);

    result = (double)integer_portion;
    if (debug) printf("so far %s looks like %f\n",string,result);

    /* capture whether string is negative, don't use "result" as it could be 0 */
    if (*string == '-')
    {
        result *= -1; /* won't care if it was 0 in integer portion */
        multiplier = -1;
    }

    while (*string && (*string != '.'))
    {
        string++;
    }
    if (debug) printf("fractional part=%s\n",string);

    // if we haven't hit end of string, go past the decimal point
    if (*string)
    {
        string++;
        if (debug) printf("first char after decimal=%c\n",*string);
    }

    while (*string)
    {
        if (*string < '0' || *string > '9') return result;
        divisor *= 10.0;
        result += (double)(*string - '0')/divisor;
        if (debug) printf("result so far=%f\n",result);
        string++;
    }
    return result*multiplier;
}

回答4:

Interestingly, the man page for atoi doesn't indicate setting of errno so if you're talking any number > (2^31)-1, you're out of luck and similarly for numbers less than -2^31 (assuming 32-bit int). You'll get back an answer but it won't be what you want. Here's one that could take a range of -((2^31)-1) to (2^31)-1, and return INT_MIN (-(2^31)) if in error. errno could then be checked to see if it overflowed.

#include <stdio.h>
#include <errno.h>  /* for errno */
#include <limits.h> /* for INT_MIN */
#include <string.h> /* for strerror */

extern int errno;

int debug=0;
int atoi(const char *c)
{
    int previous_result=0, result=0;
    int multiplier=1;

    if (debug) printf("converting %s to integer\n",c?c:"");
    if (c && *c == '-')
    {
        multiplier = -1;
        c++;
    }
    else
    {
        multiplier = 1;
    }
    if (debug) printf("multiplier = %d\n",multiplier);
    while (*c)
    {
        if (*c < '0' || *c > '9')
        {
            return result * multiplier;
        }
        result *= 10;
        if (result < previous_result)
        {
            if (debug) printf("number overflowed - return INT_MIN, errno=%d\n",errno);
            errno = EOVERFLOW;
            return(INT_MIN);
        }
        else
        {
            previous_result *= 10;
        }
        if (debug) printf("%c\n",*c);
        result += *c - '0';

        if (result < previous_result)
        {
            if (debug) printf("number overflowed - return MIN_INT\n");
            errno = EOVERFLOW;
            return(INT_MIN);
        }
        else
        {
            previous_result += *c - '0';
        }
        c++;
    }
    return(result * multiplier);
}

int main(int argc,char **argv)
{
    int result;
    printf("INT_MIN=%d will be output when number too high or too low, and errno set\n",INT_MIN);
    printf("string=%s, int=%d\n","563",atoi("563"));
    printf("string=%s, int=%d\n","-563",atoi("-563"));
    printf("string=%s, int=%d\n","-5a3",atoi("-5a3"));
    if (argc > 1)
    {
        result=atoi(argv[1]);
        printf("atoi(%s)=%d %s",argv[1],result,(result==INT_MIN)?", errno=":"",errno,strerror(errno));
        if (errno) printf("%d - %s\n",errno,strerror(errno));
        else printf("\n");
    }
    return(errno);
}

回答5:

Here is my implementation(tested successfully with cases containing and starting with letters, +, - and zero's). I tried to reverse-engineer atoi function in Visual Studio. If the input string only contained numerical characters, it could be implemented in one loop. but it gets complicated because you should take care of -,+ and letters.

int atoi(char *s)
{    
    int c=1, a=0, sign, start, end, base=1;
//Determine if the number is negative or positive 
    if (s[0] == '-')
        sign = -1;
    else if (s[0] <= '9' && s[0] >= '0')
        sign = 1;
    else if (s[0] == '+')
        sign = 2;
//No further processing if it starts with a letter 
    else 
        return 0;
//Scanning the string to find the position of the last consecutive number
    while (s[c] != '\n' && s[c] <= '9' && s[c] >= '0')
        c++;
//Index of the last consecutive number from beginning
    start = c - 1;
//Based on sign, index of the 1st number is set
    if (sign==-1)       
        end = 1;
    else if (sign==1)
        end = 0;
//When it starts with +, it is actually positive but with a different index 
//for the 1st number
    else
    { 
        end = 1;
        sign = 1;
    }
//This the main loop of algorithm which generates the absolute value of the 
//number from consecutive numerical characters.  
    for (int i = start; i >=end ; i--)
    {
        a += (s[i]-'0') * base;
        base *= 10;
    }
//The correct sign of generated absolute value is applied
    return sign*a;
}

回答6:

about atoi() hint code from here:

and based on the atoi(), my implementation of atof():

[have same limitation of original code, doesn't check length, etc]

double atof(const char* s)
{
  double value_h = 0;
  double value_l = 0;
  double sign = 1;

  if (*s == '+' || *s == '-')
  {
    if (*s == '-') sign = -1;
    ++s;
  }

  while (*s >= 0x30 && *s <= 0x39)
  {
    value_h *= 10;
    value_h += (double)(*s - 0x30);
    ++s;
  }

  // 0x2E == '.'
  if (*s == 0x2E)
  {
    double divider = 1;
    ++s;
    while (*s >= 0x30 && *s <= 0x39)
    {
      divider *= 10;
      value_l *= 10;
      value_l += (double)(*s - 0x30);
      ++s;
    }
    return (value_h + value_l/divider) * sign;
  }
  else
  {
    return value_h * sign;
  }
}

来源：https://stackoverflow.com/questions/12791077/atoi-implementation-in-c