Thread: Need Help On an Assignment

An element cannot be NULL. Something either exists or it does not. For pointers, there is a special type of pointer called null pointer, which means that it points to nothing. But you don't have pointers here.

You should probably clarify with your instructor. Chances are, it is a typo error, i.e., your instructor probably meant "empty string" or equivalently in C++, "string of zero length".

How about you start with a simpler example of not using pointers and ensuring you're not running off the end of the array?
Hint: Use std::vector and .at() to catch programming errors. Switch to a C-array when everything is working.

Are you not providing some way to access an element by index? If you are, then you do not need to declare ToString as a friend. If you are not, then perhaps ToString should be a member function instead: you can always write a non-member wrapper function of the same name if it is desirable.

I think you could end up with something like this:

Code:

ostream& operator<< (ostream& os, const DynamicStringArray& arr) 
{
    // implementation ... the hard part
}

string DynamicStringArray::ToString() const
{
    ostringstream result;
    result << *this;
    return result.str();
}

To me at least, you end up with the same thing no matter where the hard part is.

What laserlight was saying still matters though. You need to implement a way to access all of the elements of the array. When you implement that, then you can focus on outputting a "comma separated list" with, say, a loop.

Originally Posted by Erik Ingvoldsen

Really quick, what's "*this" exactly?

this pointer - cppreference.com

I think I have an idea how to make this work...

Code:

for (i = 0; i < currentsize - 1; i++) {
  str = myarray[i];
  str = str + ",";
  myarray[i] = &str;
}

This should be able to access all the non null elements in the ray. And I believe subtracting 1 from "currentsize" in this for statement will remove the comma from the last element, giving us

A, B, C

instead of

A, B, C,

The problem, from my understanding, is this line:

Code:

&str = myarray[i];

When I tried compiling it, it wasn't allowed. Is there a way to set the string up so it will be the same as an array element? This is my full test code:

Code:

int main() {
  std::string * myarray[10];
  int i = 0;
  std::string str;
  std::string str1 = "A";
  std::string str2 = "B";
  std::string str3 = "C";
  std::string str4 = "D";
  while (i < 10) {
    myarray[i] = NULL;
    ++i;
  }
  int currentsize = 4;
  myarray[0] = &str1;
  myarray[1] = &str2;
  myarray[2] = &str3;
  myarray[3] = &str4;
  cout << "Array List 1:" << endl;
  for (i = 0; i < 10; i++) {
    if (myarray[i] != NULL) {
      cout << *myarray[i];
    }
  }
  for (i = 0; i < currentsize - 1; i++) {
    str = myarray[i];
    str = str + ",";
    myarray[i] = &str;
  }
  cout << "Array List 2:" << endl;
  for (i = 0; i < 10; i++) {
    if (myarray[i] != NULL) {
      cout << *myarray[i];
    }
  }
}

And this is the compile error:

Code:

$ make
/usr/bin/g++ -Wall -Wextra -g test.cpp  -o test
test.cpp: In function ‘int main()’:
test.cpp:36:20: error: invalid conversion from ‘std::string* {aka std::basic_string<char>*}’ to ‘char’ [-fpermissive]
     str = myarray[i];
                    ^
In file included from /usr/lib/gcc/x86_64-pc-cygwin/4.9.2/include/c++/string:52:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/4.9.2/include/c++/bits/locale_classes.h:40,
                 from /usr/lib/gcc/x86_64-pc-cygwin/4.9.2/include/c++/bits/ios_base.h:41,
                 from /usr/lib/gcc/x86_64-pc-cygwin/4.9.2/include/c++/ios:42,
                 from /usr/lib/gcc/x86_64-pc-cygwin/4.9.2/include/c++/ostream:38,
                 from /usr/lib/gcc/x86_64-pc-cygwin/4.9.2/include/c++/iostream:39,
                 from test.cpp:1:
/usr/lib/gcc/x86_64-pc-cygwin/4.9.2/include/c++/bits/basic_string.h:573:7: note: initializing argument 1 of ‘std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(_CharT) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’
       operator=(_CharT __c)
       ^
makefile:10: recipe for target 'test' failed
make: *** [test] Error 1

Consider a program like this:

Code:

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

int main()
{
    using namespace std;

    vector<string> words{"these", "are", "a", "few", "words"};
    if (!words.empty())
    {
        stringstream ss;
        ss << words[0];
        for (vector<string>::size_type i = 1; i < words.size(); ++i)
        {
            ss << "," << words[i];
        }
        cout << ss.str() << endl;
    }
}

The idea is that given the container of strings, assuming the container is not empty, you can start off a stringstream with the first string. Then, loop over the container and keep appending a comma followed by the current string. At the end, you can call the str() member function to get the resulting comma-separated value string.

I leave modifying this to ignore empty strings as an exercise. You probably need to be most careful to handle the case where the very first string is empty.

>>test.cpp:36:20: error: invalid conversion from ‘std::string* {aka std::basic_string<char>*}’ to ‘char’ [-fpermissive]
What does this compile error tell you? And what do you think it means? Can you spot an error in your code?