Self-implementation of atof function

Atof function for converting a string argument to a floating-point type

/*  system:linux  Self-implemented atof function, the main function is used to determine the legalization of parameters, if the law calls the sub-function, or exit   This program contains two algorithms atof implementation, are valid */#include  < stdio.h> #include  <string.h>float str_to_float (CHAR&NBSP;*STR); FLOAT&NBSP;STR_TO_FLOAT1 (char &NBSP;*STR); Int main (int argc, char* argv[]) {    /*     i  for string traversal     n  for recording the length of a string     flag_point  Used to mark whether the decimal point has occurred   default 0  occurs after the decimal point is 1    flag_return  the owning tag string is illegal   default is 0  illegal after 1     result  to store results without +/-    str  point to String to convert      */    int i, n, flag_point, flag_return;     Float result;    char *str;        if   (argc == 1)  {        fprintf (stderr,  "Usage:  %s string\n ",   argv[0]);        return 1;    }     flag_point = 0;    flag_return = 0;     n = strlen (argv[1]);    str = argv[1];     result = 0.0;    for  (i = 0;i < n;i++)  {     //determines if +/-is present in the string, if it is valid in the first place, or illegal         if  (str[i ] ==  ' + '  | |  str[i] ==  '-')  {             if  (i == 0)                  continue;            else {                 flag_return = 1;                 break;            }         }      //to judge that the decimal point can occur at most once, otherwise illegal          if  (str[i] ==  '. ')  {            if  (flag_point ==  0)                 if   (i == 0)  {                     flag_return = 1;                     break;                 } else&nbsP {                     flag_point = 1;                     continue;                 }             else {                 flag_return = 1;                 break;            }         }        //the rest of the word 符必 within 0-9 range   Otherwise illegal         if  (str[i] >=  ' 0 '  &&  str[i] <=&nbSP; ' 9 ')             continue;         else {             flag_return = 1;            break;         }    }    if  (flag_ return == 0)  {        result = str_to_float ( ARGV[1]);         printf ("The %s transform float is  %f\n ",  argv[1], result);        } else          printf ("the %s string is illegal\n",  ARGV[1]);     return 0;    }/* algorithm 1 traversing a string from left to right (from high to Low) example: integer part taken   123 =  (((1*10) +2) *10) +3 method The fractional part takes the 0.123 = 1/10 + 2/100 + 3/1000 method and finally adds the integer to the fractional part */float str_to _float (CHAR&NBSP;*STR) {    /*      m  saving results for small parts        k  Save the integer part of the result       o  save the fractional part of each bit        flag  Save sign       flag_point  standard decimal point has appeared, Change the conversion mode       p  Save the value that corresponds to the number of digits that are currently in the decimal point, such as the first bit  p 10  second bit  p 100     */    float m, o;    int n,  i, flag, flag_point, k, p;        n =  strlen (str);    flag = 1;    flag_point = 1;     k = 0;    m = 0;    o  = 0;    for  (i = 0;i < n;i++)  {         if  (i == 0 &&  (str[i] ==  ' + '  | |  str[i] ==  '-')  {             if  (str[i] ==  '-')                  flag = -1;             continue;        }         if  (str[i] ==  '. ')  {            flag_point = 0;             p = 1;             continue;        }         if  (Flag_point) {             k = k*10 +  (str[i] -  ' 0 ');         }        else {             p *= 10;             o = str[i] -  ' 0 ';             m += o/p;        }         }    return flag* (k+m);} /*  method 2  traversal string from right to left (from low to high)   example: 12.34  first get 1234  and record decimal position   re-use  1234/100 = &NBSP;12.34*/FLOAT&NBSP;STR_TO_FLOAT1 (CHAR&NBSP;*STR) {/*   flag  for marking decimal places     flag1  the positive and negative    result that are used to tag strings have stored results    p  is used to save the current to access how many bits    q  is used to save a total of several decimal */    int i, n,  p, flag, q, flag1;    float result;         n = strlen (str);    result = 0;     p = 1;    q = 1;    flag = 0 ;    flag1 = 1;    for  (i = n- 1;i  >= 0;i--)  {    //used to skip the extra 0         at the right end if  (i ==  (n -1))              while  (str[i] ==  ' 0 ')                  i--;        if  (str[i] ==  '. ')  {            flag = 1;             continue;        }         if  (flag == 0)              q *= 10;        if  (i  == 0 &&  (str[i] ==  ' + '  | |  str[i] ==  '-')     {             if  (str[i] ==  '-')                  flag1 = -1;             continue;        }         result += p* (str[i] -' 0 ');        p *= 10;    }     return flag1*result/q;}

Summarize:

Seemingly small functions, realized quite a lot of trouble, and practice.

1, the variable after the application, must be assigned the initial value before use

2, Float type variable has the precision loss in the assignment process

3. The problem of boundary judgment

Self-implementation of atof function