Convert double to Char Array C++

Question

I'm trying to convert (and round) a double to a char array without converting with a std::to_string on the double first. However, I'm receiving random memory text instead. What am I doing wrong?

Here is my code:

double d = 1.0929998; 
d = std::round(d * 100) / 100;

char s[sizeof(d)];
std::memcpy(s,&d,sizeof(d));

Result:

s: q=×£pñ?

Intended value:

s: 1.09

Answer 1

You are translating the literal bytes of your double into chars. The double value is a binary representation (usually something like IEEE 754) while a char is a binary representation of a character (usually something based on ASCII). These two are not compatible.

Unfortunately, this means that you must do some kind of conversion process. Either the std::to_string() that you said you don't want to do, or a more complicated std::stringbuf (which will call std::to_string() under the hood anyway)

Answer 2

You are copying a double (which is a binary representation for a number) into a char array; there is no reason those bytes should correspond to digit characters.

Answer 3

This isn’t tested but you could try:

std::string to_string(double x)
{
    std::ostringstream ss;
    ss << x;
    return ss.str();
}

You could then put the returned string’s characters into a char array like this:

std::string str = to_string(doubleValue);
char digits[str.length()];

And then, thanks to the comment from Remy Lebeau, you can do either this:

std::strncpy(digits, to_string(doubleValue).c_str(), sizeof(digits))

or this:

to_string(doubleValue).copy(digits, sizeof(digits))

Answer 4

The conversion process by your self without using to_string may be as follow:

char* to_char_array(double num_double, int decimal_place)
{
    int num_int = std::round(num_double * pow(10, decimal_place));
    int sign = num_int < 0 ? 1 : 0;
    num_int = abs(num_int);

    if (num_int == 0)
    {
        char* s = (char*)malloc(decimal_place + 3);
        s[0] = '0';
        s[1] = '.';
        for (int i = 2; i < decimal_place + 2; i++)
            s[i] = '0';
        s[decimal_place + 2] = '\0';
        return s;
    }

    int digit_count = 1;
    int n = num_int;
    if (n >= 100000000) { digit_count += 8; n /= 100000000; }
    if (n >= 10000) { digit_count += 4; n /= 10000; }
    if (n >= 100) { digit_count += 2; n /= 100; }
    if (n >= 10) { digit_count++; }

    int size = digit_count + 1 + (decimal_place > 0 ? 1 : 0) + sign;
    char* s = (char*)malloc(size);

    for (int i = 0, integer = num_int; integer != 0; integer /= 10) {
        s[size - 2 - i++] = integer % 10 + 48;
        if (decimal_place > 0 && i == decimal_place)
            s[size - 2 - i++] = '.';
    }
    s[size - 1] = '\0';
    if (sign)
        s[0] = '-';
    return s;
}

void main()
{
    double d = -12.268904;
    char* s = to_char_array(d, 3);
    cout << "double: " << d << " - char array: " << s << endl;
    system("pause");
}

Answer 5

If you want to convert by hand without using to_string, you're going to have to pull digits out one at a time. A double isn't stored in memory as a set of characters, so you can't just copy over the memory and hope it all works. (for more on how doubles are interpreted, see here).

Answer 6

Just do this first:

std::ostringstream strs;
strs << dbl;
std::string str = strs.str();

This converts a double dbl to a string str.

Then you can convert it to an array of chars like this:

char *cstr = new char[str.length()+1];
str.copy(cstr, str.length());
cstr[str.length()] = '\0';
// use cstr as needed...
delete[] cstr;

Or, simply this:

const char *cstr = str.c_str();
// use cstr as needed...

Answer 7

just do this:

  #include <iostream>
  #include <string>
  #include <sstream>

  using namespace std;

  int main()
  {
      double a=23.25;
      char b[12];
      stringstream ss;

      ss << a;
      ss >> b;
      cout << b << endl;

      return 0;
  }

or this:

  #include <iostream>
  #include <string>
  #include <sstream>
  #include <stdio.h>


  using namespace std;

  int main()
  {


  double a=23.25;
  char b[12];
  stringstream ss  ;
  ss<<a;
  sprintf(b,ss.str().c_str());
  cout<<b<<endl;
  return 0;

or this:

here don't forget to
 #include<bits/stdc++.h>
  
  double a=23.25;
  char b[12];
  stringstream ss  ;
  ss<<a;
  strcpy(b,ss.str().c_str());
  cout<<b<<endl;
  return 0;

Answer 8

Use itoa applied on the double with the decimal plate moved to right by two then create a copy of the result by adding the decimal point where it should be : before the last to digits :

void convertToCharArray(double value, char *rezult)
{
  int k;

  char aux[10];

  itoa(value*100, aux, 10);
  int n = strlen(aux);
  k = 0;
  for(int i=0;i<n;i++)
  {
    if(i == n-1 - 1)
    {
      rezult[k++] = '.';
    }
    rezult[k++] = aux[i];
  }
  rezult[k++] = '\0';
}

usage :

int main()
{
    char aux[10];
    convertToCharArray(27.77, aux);
    cout << aux;
    return 0;
}