Convert double float to string

Question

I need to convert a floating point number to an equivalent string in decimal  or other base   Conversion at first needs to be done in the format xE 0 where x is the floating point number   The idea I have is to first truncate the floating point number into a temporary integer and then convert that integer into string  and then consider the fractional part  multiply it with 10 while the fractional part does not become 0  After the fractional part is transferred into the left side of the decimal point  apply the integer to string function again and convert the fraction part to string  Is there a better way  which will be faster than this  Will this method induce any kind of side effects   To convert the floating point number into exponential representation shall I do the same as above and then adjust the power  Or directly bitmask the IEEE nbsp 754 floating point representation and convert each part into string   Note  No other functions could be used  because I have access to absolutely no library functions  This code goes into a toy kernel

User · Answer

I know maybe it is unnecessary  but I made a function which converts float to string   CODE    include  lt stdio h gt       Number on countu      int n tu int number  int count        int result   1      while count--  gt  0          result    number       return result          Convert float to string      void float to string float f  char r          long long int length  length2  i  number  position  sign      float number2       sign   -1       -1    positive number     if  f  lt  0                sign    -           f    -1             number2   f      number   f      length   0      Size of decimal part     length2   0     Size of tenth         Calculate length2 tenth part        while   number2 -  float number     0 0  amp  amp     number2 -  float number   lt  0 0                   number2   f    n tu 10 0  length2   1             number   number2            length2                  Calculate length decimal part        for  length    f  gt  1    0   1  f  gt  1  length            f    10       position   length      length   length   1   length2      number   number2      if  sign     -                 length            position               for  i   length  i  gt   0   i--                if  i     length               r i      0           else if i     position               r i                 else if sign     -   amp  amp  i    0              r i     -           else                       r i     number   10     0               number   10

User · Answer

The only exact solution is to perform arbitrary-precision decimal arithmetic for the base conversion  since the exact value can be very long - for 80-bit long double  up to about 10000 decimal places  Fortunately it s  only  up to about 700 places or so for IEEE double   Rather than working with individual decimal digits  it s helpful to instead work base-1-billion  the highest power of 10 that fits in a 32-bit integer  and then convert these  base-1-billion digits  to 9 decimal digits each at the end of your computation   I have a very dense  rather hard to read  but efficient implementation here  under LGPL MIT license   http   git musl-libc org cgit musl blob src stdio vfprintf c h v1 1 6  If you strip out all the hex float support  infinity nan support   g  f  e variation support  rounding  which will never be needed if you only want exact answers   and other things you might not need  the remaining code is rather simple

User · Answer

Use snprintf   from stdlib h  Worked for me   double num   123412341234 123456789   char output 50    snprintf output  50    f   num    printf   s   output

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Go and look at the printf   implementation with   f  in some C library

User · Answer

sprintf can do this    include  lt stdio h gt  int main       float w   234 567    char x   SIZEOF FLOAT       sprintf x    g   w     puts x

User · Answer

See if the BSD C Standard Library has fcvt     You could start with the source for it that rather than writing your code from scratch   The UNIX 98 standard fcvt   apparently does not output scientific notation so you would have to implement it yourself  but I don t think it would be hard

User · Answer

Use this   void double to char double f char   buffer       gcvt f 10 buffer

[c] Convert double/float to string

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