I don’t know why my string can't save the value

Question

I am writing a C++ function to calculate Hex from Dec, but I don’t know why my string can't save the value. Can anybody help me?

string DecToHex(int a){
  int i=0,j=0;
  string d,c;
  while(a!=0)
  {
    i=a%16;
    switch(i)
    {
      case 10:
        d[j]='A';
        break;
      case 11:
        d[j]='B';
        break;
      case 12:
        d[j]='C';
        break;
      case 13:
        d[j]='D';
        break;
      case 14:
        d[j]='E';
        break;
      case 15:
        d[j]='F';
        break;
      default:
        d[j]=48+i;
        break;
    }

    a=a/16;
    j++;
  }

  cout<<"D="<<d;
  return d;
}

Please provide your snippets as text. You can't compile an image and not everyone is comfortable with your chosen color scheme. — François Andrieux, May 17 '17 at 14:07
@FrançoisAndrieux http://stackoverflow.com/questions/5508110/why-is-this-program-erroneously-rejected-by-three-c-compilers — Hatted Rooster, May 17 '17 at 14:08
@GillBates Thank you for that link, I was not aware that this existed. — François Andrieux, May 17 '17 at 14:09
Your function is actually trying to return a character...not a string. The data type for the function is string and the return is bringing back a character. Is that your intention? Change data type of the function to character OR return a string. — Dr t, May 17 '17 at 15:23

score 2 · Accepted Answer · answered May 17 '17 at 14:13

2

Change

d[j]='A';

To

d+='A'

And do the same for all the rest.

answered May 17 '17 at 14:13

Mohammad Asad

56
1
4

Or better, use an `std::ostringstream` to collect the characters, and then return the output of its `str()` method at the end. – Remy Lebeau May 17 '17 at 16:06
Thank you @RemyLebeau, I don't use string stream, is it better you mean faster or better for other reason? – Mohammad Asad May 17 '17 at 18:32

Remy Lebeau · Answer 2 · 2017-05-17T23:03:36.870

You are not actually allocating any memory for your string, so using d[j] is undefined behavior. You need to use d += ... or d.push_back(...) instead.

But even if you fix that error, your code still won't work correctly. It is not outputting any string value at all for a=0 when it should be returning "0". And it returns other strings in reverse order. For example, a=16 is returning "01" instead of "10", a=31 is returning "F1" instead of "1F", a=32 is returning "02" instead of "20", etc.

Try something more like this instead:

#include <string>

std::string DecToHex(int a)
{
    int i;
    std::string d;

    d.reserve(sizeof(a)*2);

    do
    {
        i = a % 16;
        if ((i >= 10) && (i <= 15))
          d.insert(0, 1, 'A'+(i-10));
        else
          d.insert(0, 1, '0'+i);
        a /= 16;
    }
    while (a != 0);

    return d;
}

Or:

#include <string>
#include <algorithm>

std::string DecToHex(int a)
{
    int i;
    std::string d;

    d.reserve(sizeof(a)*2);

    do
    {
        i = a % 16;
        if ((i >= 10) && (i <= 15))
          d += ('A'+(i-10));
        else
          d += ('0'+i);
        a /= 16;
    }
    while (a != 0);

    std::reverse(d.begin(), d.end());

    return d;
}

Or:

#include <string>
#include <sstream>
#include <algorithm>

std::string DecToHex(int a)
{
    int i;
    std::ostringstream oss;

    do
    {
        i = a % 16;
        if ((i >= 10) && (i <= 15))
          oss << ('A'+(i-10));
        else
          oss << ('0'+i);
        a /= 16;
    }
    while (a != 0);

    std::string d = oss.str();
    std::reverse(d.begin(), d.end());

    return d;
}

In any case, your function is not handling negative numbers at all, which requires some extra work (switching to 2s complement representation before then calculating the hex). I'll leave that as an exercise for you to work out.

I don’t know why my string can't save the value

2 Answers2