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  1. #include <stdio.h>
  2. #include <math.h>
  3. #include <stdlib.h>
  4.  
  5. // SVG specs for numbers :
  6. //
  7. // number ::= integer ([Ee] integer)?
  8. //           | [+-]? [0-9]* "." [0-9]+ ([Ee] integer)?
  9.  
  10. #define TRY(S) printf("%10s => %f (error is %g)\n", #S, my_atof(#S, NULL), my_atof(#S, NULL) - atof(#S)); 
  11. #define TRY_BAD(S) printf("%10s %f fails at:", #S, my_atof(#S, &end)); printf("%s\n", end);
  12.  
  13. float my_atof(const char *str, char ** last)
  14. {
  15.     char * end = (char*)str;
  16.  
  17.     float res = (float)strtoll(str, &end, 10);
  18. 	int badConv = end == str;
  19.     float sign = res < 0 || (*end == '-' && ++end) ? -1 : (end == str && *end == '+' && ++end) || 1; // Trim sign 
  20.  
  21. 	if (badConv && *end != '.') // Invalid input
  22. 	{
  23. 		if (last != NULL) *last = (char*)str;
  24. 		return 0;
  25. 	}
  26.  
  27.  
  28.     if (*end == '.')
  29.     {
  30.     	char* beg = ++end, *tmp = 0;   // So end is correct when returning below
  31.     	long long frac = strtoll(beg, &tmp, 10);
  32.         if (*beg < '0' || *beg > '9') 
  33.         {
  34.         	if (last != NULL) *last = badConv ? str : end;
  35.         	return res;
  36.         }
  37.         end = tmp;
  38.     	res += sign * frac/pow(10, end-beg);
  39.     }
  40.  
  41.     if (*end == 'e' || *end == 'E') 
  42.     {
  43.     	char * tmp = 0;
  44.     	res *= pow(10, strtol(end+1, &tmp, 10));
  45.     	if (tmp != end + 1) end = tmp; // Reject 34e as 34 followed by e
  46.     }
  47.  
  48. 	if (last != NULL) *last = end;
  49.     return res;
  50. }
  51.  
  52. int main(void) {
  53. 	TRY(-5);
  54. 	TRY(+5);
  55. 	TRY(-5.1);
  56. 	TRY(.5);
  57. 	TRY(.5e2);
  58. 	TRY(.5541e-2);
  59. 	TRY(.5e+2);
  60. 	TRY(+.57e-2);
  61. 	TRY(+.001e+1);
  62. 	char * end = 0;
  63. 	TRY_BAD(34f4);
  64. 	TRY_BAD(34.+44);
  65. 	TRY_BAD(34.-44);
  66. 	TRY_BAD(+.f)
  67. 	TRY_BAD(+e10)
  68. 	TRY_BAD(+.e10)
  69. 	TRY_BAD(34e)
  70. 	TRY_BAD(+33+4)
  71. 	return 0;
  72. }
  73.  
Success #stdin #stdout 0s 9432KB
comments (?)
stdin 
Standard input is empty
stdout 
        -5 => -5.000000 (error is 0)
        +5 => 5.000000 (error is 0)
      -5.1 => -5.100000 (error is 9.53674e-08)
        .5 => 0.500000 (error is 0)
      .5e2 => 50.000000 (error is 0)
  .5541e-2 => 0.005541 (error is -4.161e-10)
     .5e+2 => 50.000000 (error is 0)
   +.57e-2 => 0.005700 (error is 9.61125e-11)
  +.001e+1 => 0.010000 (error is 7.07805e-10)
      34f4 34.000000 fails at:f4
    34.+44 34.000000 fails at:+44
    34.-44 34.000000 fails at:-44
       +.f 0.000000 fails at:+.f
      +e10 0.000000 fails at:+e10
     +.e10 0.000000 fails at:+.e10
       34e 34.000000 fails at:e
     +33+4 33.000000 fails at:+4
https://ideone.com/NVJN2W
language:
C (gcc 8.3)
created:
8 years ago
visibility:
public

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