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  1. #include <stdio.h>
  2. #include <limits.h>
  3.  
  4. #if UINT_MAX >= 0xFFFFFFFF
  5. typedef unsigned uint32;
  6. #else
  7. typedef unsigned long uint32;
  8. #endif
  9.  
  10. #define C_ASSERT(expr) extern char CAssertExtern[(expr)?1:-1]
  11.  
  12. // Ensure uint32 is exactly 32-bit
  13. C_ASSERT(sizeof(uint32) * CHAR_BIT == 32);
  14.  
  15. // Ensure float has the same number of bits as uint32, 32
  16. C_ASSERT(sizeof(uint32) == sizeof(float));
  17.  
  18. double Ieee754SingleDigits2DoubleCheat(const char s[32])
  19. {
  20.   uint32 v;
  21.   float f;
  22.   unsigned i;
  23.   char *p1 = (char*)&v, *p2 = (char*)&f;
  24.  
  25.   // Collect binary digits into an integer variable
  26.   v = 0;
  27.   for (i = 0; i < 32; i++)
  28.     v = (v << 1) + (s[i] - '0');
  29.  
  30.   // Copy the bits from the integer variable to a float variable
  31.   for (i = 0; i < sizeof(f); i++)
  32.     *p2++ = *p1++;
  33.  
  34.   return f;
  35. }
  36.  
  37. double Ieee754SingleDigits2DoubleNoCheat(const char s[32])
  38. {
  39.   double f;
  40.   int sign, exp;
  41.   uint32 mant;
  42.   int i;
  43.  
  44.   // Do you really need strto*() here?
  45.   sign = s[0] - '0';
  46.  
  47.   // Do you really need strto*() or pow() here?
  48.   exp = 0;
  49.   for (i = 1; i <= 8; i++)
  50.     exp = exp * 2 + (s[i] - '0');
  51.  
  52.   // Remove the exponent bias
  53.   exp -= 127;
  54.  
  55.   // Should really check for +/-Infinity and NaNs here
  56.  
  57.   if (exp > -127)
  58.   {
  59.     // Normal(ized) numbers
  60.     mant = 1; // The implicit "1."
  61.     // Account for "1." being in bit position 23 instead of bit position 0
  62.     exp -= 23;
  63.   }
  64.   else
  65.   {
  66.     // Subnormal numbers
  67.     mant = 0; // No implicit "1."
  68.     exp = -126; // See your IEEE-54 formulas
  69.     // Account for ".1" being in bit position 22 instead of bit position -1
  70.     exp -= 23;
  71.   }
  72.  
  73.   // Or do you really need strto*() or pow() here?
  74.   for (i = 9; i <= 31; i++)
  75.     mant = mant * 2 + (s[i] - '0');
  76.  
  77.   f = mant;
  78.  
  79.   // Do you really need pow() here?
  80.   while (exp > 0)
  81.     f *= 2, exp--;
  82.  
  83.   // Or here?
  84.   while (exp < 0)
  85.     f /= 2, exp++;
  86.  
  87.   if (sign)
  88.     f = -f;
  89.  
  90.   return f;
  91. }
  92.  
  93. int main(void)
  94. {
  95.   printf("%+g\n", Ieee754SingleDigits2DoubleCheat("110000101100010010000000000000000"));
  96.   printf("%+g\n", Ieee754SingleDigits2DoubleNoCheat("010000101100010010000000000000000"));
  97.   printf("%+g\n", Ieee754SingleDigits2DoubleCheat("000000000100000000000000000000000"));
  98.   printf("%+g\n", Ieee754SingleDigits2DoubleNoCheat("100000000100000000000000000000000"));
  99.   printf("%+g\n", Ieee754SingleDigits2DoubleCheat("000000000000000000000000000000000"));
  100.   printf("%+g\n", Ieee754SingleDigits2DoubleNoCheat("000000000000000000000000000000000"));
  101.   return 0;
  102. }
  103.  
Success #stdin #stdout 0s 1832KB
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stdin
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Standard input is empty
stdout
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-98.25
+98.25
+5.87747e-39
-5.87747e-39
+0
+0
https://ideone.com/seYW5R
language:
C99 (gcc 8.3)
created:
10 years ago
visibility:
public

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