function gives back the same value

hi,

i wrote a small function that expects a double and gives back
a char pointer with the value as char-string.
the first time i use it in my program it works fine, but if i call the
function a second or third time, even if i just cout the results both times,
the results are always the same as the result of the first call.
where is my fault?

thanks,

toby

Code:

---

char *doubletostring(double number) {
        char numberstring[18];
        int i=0;
        double divisor=100000000000000000;
        while (i<18) {
        if (number < divisor) {
        divisor=divisor/10;
        numberstring[i]=(int)32;
        i++;
        } else {
        numberstring[i]=(int)(48+(number/divisor));
        number=fmod(number,divisor);
        divisor=divisor/10;
        i++;
        }
        }
        numberstring[i]='\0';
        string cut0s;
        i=0;
        while (numberstring[i]==(int)32) {
          i++;
        }
        while (numberstring[i]!='\0') {
          cut0s += numberstring[i];
          i++;
        }
          cut0s += '\0';
        char *returnchars =(char*)cut0s.c_str();
        return returnchars;
}

---

You are returning a pointer to a local variable, this is not valid - the variable's memory is "gone" by the time you return from the function. Either pass in (by reference) a string to store the text in, or allocate memory in the function (I would propose the former).

--
Mats

matsp is correct.

I would further add, however, if the block of memory will be static, or does not need to be resized, then have the caller pass it in, along with the size of the block of memory. If you need the memory to be dynamically sized with no way for the caller to have any idea of how big the buffer needs to be, then returning a pointer to dynamic memory is possibly a better solution. There are arguments for both approaches.

Using std::string would be the best way, however, since the dynamically sized buffer is handled for you automatically.

What the heck is that mess? That looks like tomato juice to me.
There's really a reason why we indent code - to make it readable! You should try it, too.
Indent each block one tab and don't mix spaces and tabs.

I made a few minor changes to make it use std::string consistently and saw that you have a long way to go:

Code:

---

#include <string>
#include <iostream>
#include <cmath>
using std::string;

string doubletostring(double number) {
    string numberstring;
    std::size_t i=0; //more proper to use for comparisons against string::size
    double divisor=100000000000000000.0; //too large for a long
    while (i<18) {
        if (number < divisor) {
            divisor=divisor/10;
            numberstring += ' '; //why use 32 and why cast int to int?
            i++;
        } else {
            numberstring +=(char)('0'+(number/divisor));
            number=fmod(number,divisor);
            divisor=divisor/10;
            i++;
        }
    }
    //numberstring[i]='\0';
    string cut0s;
    i=0;
    while (i != numberstring.size() && numberstring[i]==' ') {
        i++;
    }
    while (i != numberstring.size()) {
        cut0s += numberstring[i];
        i++;
    }
    //cut0s += '\0';
   
    return cut0s;
}

int main()
{
    double d;
    while (std::cin >> d) {
        std::cout << "As string: " << doubletostring(d) << '\n';
    }
}

---

A sample run:

Quote:

---

45
As string: 45
23.32
As string: 23
23.88
As string: 23
-1.1
As string:
1001
As string: 1 1

---

Apparently your code doesn't handle yet:
- negative values,
- decimal places,
- zeros in numbers (why would you convert them to spaces?!),
- and not shown here: very large values

Unless this is a school exercise and you are required to do this this way, you might want to take a look into stringstreams.

This functionality is also available in boost: lexical_cast

Edit: if you mean character constants like ' ' or '0', don't obscure your code with magic numbers 32 and 48.